ENGINEERS BLOG

OVER- / UNDER-VOLTAGE PROTECTION OF ELECTRICAL APPLIANCES

noreply@blogger.com (Anonymous) — Wed, 7 May 2014 07:52:00 +0530

C.H. VITHALANI

This circuit protects refrigerators as well as other appliances from over and under-voltage. Operational amplifier IC LM324 (IC2) is used here as a comparator. IC LM324 consists of four operational amplifiers, of which only two operational amplifiers (N1 and N2) are used in the circuit.

The unregulated power supply is connected to the series combination of resistors R1 and R2 and potmeter VR1. The same supply is also connected to a 6.8V zener diode (ZD1) through resistor R3.Preset VR1 is adjusted such that for the normal supply of 180V to 240V, the voltage at the non-inverting terminal (pin 3) of operational amplifier N1 is less than 6.8V. Hence the output of the operational amplifier is zero and transistor T1 remains off. The relay, which is connected to the collector of transistor T1, also remains de energised. As the AC supply to the electrical appliances is given through the normally closed (N/C) terminal of the relay, the supply is not disconnected during normal operation.

When the AC voltage increases beyond 240V, the voltage at the non-inverting terminal (pin 3) of operational amplifier N1 increases. The voltage at the inverting terminal is still 6.8V because of the zener diode. Thus now if the voltage at pin 3 of the operational amplifier is higher than 6.8V, the output of the operational amplifier goes high to drive transistor T1 and hence energise relay RL. Consequently, the AC supply is disconnected and electrical appliances turn off. Thus the appliances are protected against over-voltage. Thus the appliances are protected against over-voltage.

Now let’s consider the under-voltage condition. When the line voltage is below 180V, the voltage at the inverting terminal (pin 6) of operational amplifier N2 is less than the voltage at the non-inverting terminal (6V). Thus the output of operational amplifier N2 goes high and it energises the relay through transistor T1. The AC supply is disconnected and electrical appliances turn off. Thus the appliances are protected against under-voltage. IC1 is wired for a regulated 12V supply.

Thus the relay energises in two conditions: first, if the voltage at pin 3 of IC2 is above 6.8V, and second, if the voltage at pin 6 of IC2 is below 6V. Over-voltage and under-voltage levels can be adjusted using presets VR1 and VR2, respectively.

VOICE RECORDER AND PLAYBACK SYSTEM

noreply@blogger.com (Anonymous) — Sat, 19 Apr 2014 19:24:00 +0530

There are several types of voice recorder and playback systems available in the market but most of them are expensive and their circuits are also very complex to assemble. Here is a simple circuit for recording and playback of voice messages. You can leave a voice message for your family or friends whenever you go out, which they can hear by pressing the ‘play’ button.

The circuit is built around a recording and playback chip that supports voice recording for 16 to 30 seconds and reproduces it clearly. It can be used in different types of applications such as door bells, railway announcement systems and automatic telephone answering devices.

Fig.1

Fig. 1 shows the circuit of voice recorder and playback system. The circuit is built around voice recording and playback IC APR9301-V2 (IC1), voltage regulator 7806 (IC2), npn transistor BC547 (T1), 8-ohm, 0.5W speaker (LS1), electret microphone (MIC1) and a few other components.

IC APR9301-V2 is a high-quality voice recording and playback IC. The length of message recording depends on the value of external resistor R1 connected to its pin 7. The operation modes are described below.

Recording mode. When switch S1 is pressed, LED1 glows to indicate that recording has started. Now you can speak close to microphone MIC1 in order to record your message. You may have to vary VR1 to adjust for different microphones. IC1 remains in recording mode as long as switch S1 is pressed and pin 27 of IC1 is grounded. Recording stops after 20 seconds (selected by 52-kilo-ohm resistance in this case), pin 25 of IC1 becomes ‘high’ and LED1 stops glowing.

The recording time duration can be increased or decreased depending on the value of resistor R1 as follows:

1. 38 kilo-ohms for 16 seconds

2. 52 kilo-ohms for 20 seconds

3. 67 kilo-ohms for 24 seconds

4. 75 kilo-ohms for 30 seconds

WINDOW CHARGER

noreply@blogger.com (Anonymous) — Fri, 18 Apr 2014 12:20:00 +0530

Keep away intruders with this compact electrified window charger. The charger produces non-lethal shocks that are strong enough to threaten intruders.

The circuit uses IC CD4047 as a free-running astable multivibrator. Capacitor C1 and preset VR1 are timing components. The pulse repetition rate is determined by the value of 4.4C1×VR1. The frequency can be varied with the help of preset VR1.

The IC generates complementary squarewave signals at pins 10 and 11. Transistors T1 and T2 serve as drivers for the following push-pull amplfier stage. A high-voltage generator, realised using step-up transformer X1 and medium-power transistors T3 and T4, follows the astable multivibrator. The stepdown transformer is used for reverse function (step-up) and its output is rectified by diode D1, filtered by capacitor C3 and then given to window (made of metal frame).

ELECTROLYTIC CAPACITOR TESTER

noreply@blogger.com (Anonymous) — Fri, 18 Apr 2014 11:41:00 +0530

Using this electrolytic capacitor tester you can detect leaky and dead (open) electrolytic capacitors. It operates based on the time constant (T) of the capacitor when it is charged up to 63 percent of the applied voltage via a known resistor. The time constant is calulated as follows:
T=C×R

Where ‘T’ is in seconds, ‘C’ is in microfarads, and ‘R’ is in mega-ohms.

Two NE555 timer ICs are used. IC1 is wired in the monostable mode. Initially, when the power is applied, the low output of IC1 causes LED1 to glow. When IC1 is triggered by pressing switch S3, the capacitor under test starts charging via the selected resistor (R1, R2, R3, or R4) and its output jumps to high state, causing LED1 to go off. It remains high for a time duration (in seconds) depending on the RC time constant and then returns to the original low state, which causes LED1 to glow again.

The monostable time period (=1.1×R×C) can be measured by a stop-watch. By comparing this time period (delay time) with that of a good capacitor, we can find the value of the capacitor.

IC2 is connected in the astable mode. Two red LEDs (LED2 and LED3) are connected to its output pin 3. When the output of IC1 jumps to high state, LED1 goes off and the power is applied to pins 4 and 8 of IC2, causing LED2 and LED3 (connected to IC2) to start flashing. Using VR1 adjust the flashing rate of LED2 and LED3 to one flash per second. After the monostable time period is over, LED2 and LED3 stop flashing and LED1 glows again. The number of flashes counted is the time period in seconds.

Connect the capacitor under test at the indicated position with polarity as shown in the figure. Close switch S1 to apply power to the tester. LED1 glows immediately to indicate that power is applied to the tester. Set selection switch S2 to low-resistance range position. On pressing switch S3, LED1 goes off and LED2 and LED3 start flashing. Count the flashes carefully until the LED stops flashing.

Now connect a good capacitor of the same value to the tester and note its delay period. If the delay period of the capacitor under test is almost equal to that of the good capacitor, it is in good condition. In case LED2 and LED3 flash indefinitely without stopping or no flashing, the capacitor under test is leaking or dead short.

To calculate the approximate value of the capacitor under test, multiply the delay time by an arbitrary factor. The arbitrary factor is different for different resistance ranges (refer Table I).

Example 1: For a 10µF capacitor, delay time is 126 seconds in the 10-mega-ohm range. On multiplying 126 by 0.09, we get 11.34 µF as the measured value of the capacitor.

Example 2: For a 1000µF capacitor, detail time is 130 seconds in the 100-kilo-ohm range. On multiplying 130 by 9.0, we get 1170 µF as the measured value.

The delay times and measured values of the capacitor are given in Table II.

FAN ON/OFF CONTROL BY LIGHT

noreply@blogger.com (Anonymous) — Fri, 18 Apr 2014 10:41:00 +0530

This circuit lets you turn on/off a fan by just directing torchlight or other light toward its light-dependent resistor (LDR). The circuit is powered from a 5V power supply.

Preset VR1 and a light-dependent resistor (LDR) work as the potential divider. Normally, the LDR’s resistance is high (20 kilo-ohms) in darkness and low (2 kilo-ohms) in light. This value of high and low resistances varies for other LDRs. Preset VR1 is used for setting the intensity of light, while preset VR2 is used for setting the output time period of IC1.

When light falls on the LDR, the monostable (IC1) triggers at pin 2, making its output at pin 3 from low to high. This low-to-high transition forms a clock for D flip-flop. The D flip-flop is operated in toggle mode by connecting its Q output to D point. The flip-flop output goes to an inverter (N1). The inverter output is fed to the relay driver transistor.

When the inverter output is low, diode D1 conducts and the current is diverted into the inverter. Hence the relay does not energise. When the inverter output is high, diode D2 conducts and the current is diverted into transistor T. Hence the relay energises.

One terminal of the fan is connected to the normally-open (N/O) contact of the relay, while another terminal is connected to the neutral (N) of mains. The mains live (L) is connected to the pole of the relay. When the relay energises, the fan turns on. Otherwise, the fan remains off.

Switches S1 and S3 are for initial resetting of the monostable (IC1) and D flip-flop (IC2), respectively, and switch S2 is used for setting the D flip-flop. Paste a piece of paper on the face of the LDR so that it doesn’t get activated by ambient light. Use a torch to light the LDR.

After initial resetting of the monostable and D flip-flop, the inverter output goes high and the fan turns on via the relay. When light falls on the LDR, the fan goes off. If torchlight is again directed toward the LDR, the fan turns on. The sequence repeats.

Initially if switch S2 is used to set the D flip-flop, the fan is held ‘off’. The relay does not energise as the Q output of D flip-flop goes high to make the inverter output low. Directing the light towards the LDR at this moment turns the fan ‘on.’

KEY CHAIN LIGHT

noreply@blogger.com (Anonymous) — Fri, 18 Apr 2014 10:36:00 +0530

A key chain with a built-in white LED comes in handy to help you at your front door or search your valuables in the dark. The intensity of white LED is 4000 to 5600 mcd (millicandela) at forward voltage of 3.6V and forward current of 20 mA.

Here’s such an LED light circuit for key chains. It comprises a toroidal transformer and two complementary transistors, and is powered by a single AAA cell. Transistors T1 (BC547) and T2 (BC558) form a relaxation oscillator with capacitor C2 (0.01 µF) in the feedback loop. The feedback is controlled by the time constant of timing components R1 and C2, which controls the frequency of operation.

The toroidal transformer steps up the oscillator output to a sufficient value to flash the white LED. The values of R1 and C1 need not be precise. Use of surface mount devices will make the unit more compact.

A single 1.5V AAA cell gives enough brightness. For more brightness, connect two such cells in series. A good-quality white LED from a reputed manufacturer is highly recommended.

Caution. The white LED beam, when viewed directly, can harm the eyes.

CELL PHONE DETECTOR

noreply@blogger.com (Anonymous) — Thu, 17 Apr 2014 23:34:00 +0530

This handy mobile bug or cell phone detector, pocket-size mobile transmission detector or sniffer can sense the presence of an activated mobile cellphone from a distance of one and-a-half metres. So it can be used to prevent use of mobile phones in examination halls, confidential rooms, etc. It is also useful for detecting the use of mobile phone for spying and unauthorised video transmission.

The circuit can detect both the incoming and outgoing calls, SMS and video transmission even if the mobile phone is kept in the silent mode. The moment the bug detects RF transmission signal from an activated mobile phone, it starts sounding a beep alarm and the LED blinks. The alarm continues until the signal transmission ceases.

An ordinary RF detector using tuned LC circuits is not suitable for detecting signals in the GHz frequency band used in mobile phones. The transmission frequency of mobile phones ranges from 0.9 to 3 GHz with a wavelength of 3.3 to 10 cm. So a circuit detecting gigahertz signals is required for a mobile bug.

Here the circuit uses a 0.22μF disk capacitor (C3) to capture the RF signals from the mobile phone. The lead length of the capacitor is fixed as 18 mm with a spacing of 8 mm between the leads to get the desired frequency. The disk capacitor along with the leads acts as a small gigahertz loop antenna to collect the RF signals from the mobile phone.

Op-amp IC CA3130 (IC1) is used in the circuit as a current-to-voltage converter with capacitor C3 connected between its inverting and non-inverting inputs. It is a CMOS version using gate-protected p-channel MOSFET transistors in the input to provide very high input impedance, very low input current and very high speed of performance. The output CMOS transistor is capable of swinging the output voltage to within 10 mV of either supply voltage terminal.

Capacitor C3 in conjunction with the lead inductance acts as a transmission line that intercepts the signals from the mobile phone. This capacitor creates a field, stores energy and transfers the stored energy in the form of minute current to the inputs of IC1. This will upset the balanced input of IC1 and convert the current into the corresponding output voltage.

Capacitor C4 along with high-value resistor R1 keeps the non-inverting input stable for easy swing of the output to high state. Resistor R2 provides the discharge path for capacitor C4. Feedback resistor R3 makes the inverting input high when the output becomes high. Capacitor C5 (47pF) is connected across ‘strobe’ (pin 8) and ‘null’ inputs (pin 1) of IC1 for phase compensation and gain control to optimise the frequency response.

When the cell phone detector signal is detected by C3, the output of IC1 becomes high and low alternately according to the frequency of the signal as indicated by LED1. This triggers monostable timer IC2 through capacitor C7. Capacitor C6 maintains the base bias of transistor T1 for fast switching action. The low-value timing components R6 and C9 produce very short time delay to avoid audio nuisance.

Assemble the cell phone detector circuit on a general purpose PCB as compact as possible and enclose in a small box like junk mobile case. As mentioned earlier, capacitor C3 should have a lead length of 18 mm with lead spacing of 8 mm. Carefully solder the capacitor in standing position with equal spacing of the leads. The response can be optimised by trimming the lead length of C3 for the desired frequency. You may use a short telescopic type antenna.
Use the miniature 12V battery of a remote control and a small buzzer to make the gadget pocket-size. The unit will give the warning indication if someone uses mobile phone within a radius of 1.5 meters.

LIGHTING OF FUSED FLUORESCENT LAMP

noreply@blogger.com (Anonymous) — Thu, 17 Apr 2014 23:25:00 +0530

Here, a mechanism is presented by which fused fluorescent lamp can be enlighted:

In this model, only healthy filament of fused fluorescent lamp can emit the sufficient electrons and collected by other one to glow up. In the circuit a DC voltage is provided across two filaments with fixed polarity through a Full Wave Bridge Rectifier. In which one port inputs are two different tube-end-pins connection. And other’s port inputs are Supply Line and Choke input point connection. Choke is connected between supply line and bridge rectifier input terminal to control the current flow through back emf process. Starter is also connected across the tube to develop striking voltage. Here Starter and Choke functions same as in a healthy fluorescent lamp.

MOBILE CHARGER WITHOUT TRANSFORMER

noreply@blogger.com (Anonymous) — Thu, 17 Apr 2014 23:09:00 +0530

Here is a simple battery charger circuit diagram:

Click on the image to enlarge it:

You can see the values of the components used below:

R1: 56 giga ohms resistor

R2: 220 Mega ohms resistor

C1: 105 Kilo pico farad , 250 voltage capacitor

D1: IN 4007 Diode

D2: Light emitting diode indicator

D3 : IN 4007 Diode

If you use the values of the components stated above the circuit can recharge a 3 voltage rechargeable battery , You can change the value of R1 and C1 to get recharge battery of more voltage.

USB CHARGING DEVICE

noreply@blogger.com (Anonymous) — Thu, 17 Apr 2014 22:58:00 +0530

Nowadays mobiles can also be charged using the USB outlet of PC. The mobile charger circuit presented in this project can give 4.7V of synchronized voltage for charging the phone. As USB outlets can give 5V DC and 100mA of current. It is sufficient for slow charging of mobile phones so they can be used to charge the mobile phones. USB stands for Universal Serial Port. It is one of the latest methods to exchange information from PC to the real world. The USB port offers power to the external devices. +5V of power is available at pin1 and -5V of it is available at pin4.

WIRE TESTER

noreply@blogger.com (Anonymous) — Sun, 31 Mar 2013 15:35:00 +0530

Here is a circuit that helps you test cable continuity without requiring any physical contact with the bare cable. This circuit detects AC signal frequencies and gives an LED indication if the cable is conducting. This circuit is highly sensitive and can detect signals from the surface of the cable itself and thus no direct contact with the bare cable is necessary. The circuit can be used to test other wires, including modem, audio/video and dish antenna cables to name a few.

Fig. 1: Circuit of cable tester

TP0- GND

TP1- 9V

TP2- Amplified output corresponding to the input signal

AUDIO AMPLIFIER- 15WATTS

noreply@blogger.com (Anonymous) — Thu, 7 Mar 2013 08:32:00 +0530


Amplifier Circuit

The circuit described here is of a Class-B audio amplifier based on operational amplifier TL082, transistors TIP41 and TIP42. LM833 is a dual operational amplifier with very high slew rate and low noise distortion particularly designed for audio applications. This audio amplifier circuit can delivers upto 15 watt audio output into an 8 ohm speaker at +12/-12V DC dual supply. Both operational amplifiers in the IC(LM833) are used here. IC1a is connected as a buffer and capacitor C3 decouples the input. Ic1b is connected in the inverting mode and it provides negative feedback. Complementary power transistors TIP41 and TIP42 are connected in the Class B push pull scheme and they drives the loud speaker. Diode D1 provides 0.7V bias voltage for the push pull pair and capacitor C2 protects the 0.7V bias voltage across D1 from heavy voltage swings at the IC1b’s output.

Key Points:

Assemble the audio amplifier circuit on a good quality Board and Use a holder for every IC using here. Variable Resistor R2 can be used for controlling the volume.
+12/-12V dual supply must be use for powering the amplifier.
TIP42 and 41 can supplied maximum of 6A.
Maximum supply voltage for IC1 is +16/-16 V DC.

DIGITAL THERMOMETER

noreply@blogger.com (Anonymous) — Thu, 28 Feb 2013 23:37:00 +0530

Digital Thermometer generally used in wide variety of scientific and engineering applications, especially measurement systems.here we present a highly reliable digital thermometer which can be used in aquarium and measuring temperatures around +85°C.This project was born from the need to easily check the water temperature at a glance. It features three 2.2” large LED digits that are easy to read from across the room and a precision DS1822 temperature sensor. Temperatures can be selected to display in Centigrade, as well as Fahrenheit.Of course, a digital thermometer with a large LED readout and a remote temperature probe is not just limited to aquarium owners. This project will appeal to anyone that wants accurate digital temperature measurements. Home beer brewers, hydroponic gardeners, amateur weather watchers, or folks just interested in energy management will all find this to be a very useful device.

Download this project as PDF

MINI UPS FOR DC CIRCUITS

noreply@blogger.com (Anonymous) — Thu, 28 Feb 2013 23:32:00 +0530

“ This article describes a simple UPS circuit that you can incorporate into the design of your own low power DC project to ensure continued operation during short term power failures “.

An uninterruptible power supply (UPS) ensures the continuous operation of critical electronic equipment.They are especially necessary if you live in an area where there are frequent power failures.They are manufactured to meet a wide range of power requirements, from backing up your personal computer to keeping your entire home office (or workshop) going during a power failure. Most UPS systems are designed to transparently maintain AC power to your equipment.They provide for a smooth transition from main power to backup power and back again. There are a number of applications that have relatively low power requirements and run on DC rather than AC voltage but must also remain operational in the event of a main power failure.These include small security sensor modules, data acquisition, and status monitoring devices among others.

Download FREE PDF of this project

REMOTE-CONTROLLED MAINS SWITCH

noreply@blogger.com (Anonymous) — Thu, 28 Feb 2013 23:30:00 +0530

Want to switch mains appliances on and off remotely? This UHF Remote Mains Switch can do it for you. It’s operated using a handheld UHF transmitter, and an in-built timer also enables the unit to turn off automatically after a preset period.

There are many instances when it would be convenient to switch an appliance on or off remotely, rather than switching it manually. Such circumstances include switching on pathway lights when you arrive home, switching garden and/or pool lighting on or off, and switching power to water pumps. remote switching can also be very convenient for appliances that are difficult to access, eg, in a factory. This unit was originally designed to switch mains-powered water pumps on and off in response to signals transmitted by a water tank level meter base station. however, we soon realised that by adding a separate hand held transmitter to control the unit, it could also be used as a stand-alone unit for lots of other applications. Commercial remote control mains-operated switches are readily available for switching appliances rated up to about 1000W. however, if you want to switch devices rated over 1000W, or control water pumps, then you need the UhF remote Mains Switch described here. It can switch devices rated at up to 2500W over a range of up to 200m. That’s 10 times the range typically available from the low-cost commercial units!

Main Features

• Switches loads of up to 1875W (or 2500W using 10A mains wiring)
• Up to 10 units can be used with the transmitter, each with a separate
identity
• 16 encoder selections
• Over 200m range
• Unit is operated using a separate handheld UHF transmitter
• On and off switching via remote transmitter or local switch
• Timer operates from one minute to four hours in 15 ranges,
plus a continuously on selection
• Brownout detection switching
• Optional power-on variation
• Not suitable for security or safety-critical applications.

Download Free PDF here

FREEZER ALARM

noreply@blogger.com (Anonymous) — Thu, 28 Feb 2013 23:24:00 +0530

SOME modern freezers contain alarms which sound if you leave the door open and allow the internal space to warm up. However, they do not work if the freezer suffers a power failure, which is a bit of a drawback. Making a temperature-sensitive circuit which can sound an alarm is not too difficult but what is required here is a lowcost circuit which can run on batteries for a very long time. This design uses a circuit based on a PIC, using a feature about which little has been written, namely the ability to send it to sleep! The circuit is extremely simple, and the software uses several techniques which could be useful in other projects.

Circuit Description

If you are the sort of person who enjoys the challenge of constructing complex circuits, you will be disappointed! The complete circuit contains only five components, as shown in Fig.2. The clever stuff, of course, is provided by the PIC. The temperature sensor used is a lowcost disc thermistor, R1, which can be attached via a length of 2-core cable. A small preset variable resistor, VR1 is used to set the operating point, the temperature threshold at which the alarm sounds. Capacitor C1 is used to make the input circuit time-dependant, as described in the next section. For the alarm, a piezo sounder (WD1) is used because it can make a relatively large amount of noise whilst using a very small amount of electrical power. The whole circuit will conveniently run off a 6V battery.

Construction

Construction is very simple. The suggested stripboard component layout and track cut details are shown in Fig.10. The thermistor can be soldered to a short length of wire such as thin audio coax. An improvement would be to waterproof the thermistor connections by dunking it in polyurethane varnish. The wire can be fed into the freezer via the door seal. It is important to resist the temptation to add a light emitting diode as a battery indicator – the l.e.d. would take about a thousand times more power than the rest of the circuit! The PIC should be plugged into the board via an 8-pin d.i.l. socket. The circuit and batteries can be housed in a plastic box to sit outside the freezer, a small hole being provided to glue the piezo sounder behind. You should not need to replace batteries very often.

Testing

The circuit will work quite happily at room temperature. Once the batteries are connected (it seems to work well on 6V although this is higher than the maximum recommended). Gently rotate preset VR1 until the threshold is found between the alarm bleating or not. Set it so that the alarm is just off. Then hold the thermistor in your fingers to warm it up, and the alarm should sound; let go to allow the thermistor to cool again to room temperature, and the alarm should stop. Once you are convinced all is well, put the thermistor in the freezer, and after allowing time for the temperature to stabilise, increase the resistance on the preset so that the alarm threshold is set where you would like it. In fact, the best way to find out if the batteries are OK is to let the thermistor warm up a bit when you open the freezer – if it is working and the alarm sounds, the batteries are fine!

Downlad as PDF ( detailed description + program code)

SUPERFAST RECHARGABLE BATTERY

noreply@blogger.com (Anonymous) — Thu, 28 Feb 2013 23:21:00 +0530

” It just takes 20 seconds to recharge !!! “

Here is an interesting project which uses capacitors to store energy instead of chemical,sit uses an different type of capacitors called Goldcap capacitors,GoldCap capacitors offer an interesting alternative power source when compared to conventional disposable or even rechargeable batteries. They can be charged very rapidly and can also deliver a high peak output current. Their voltage rating however is quite low so a little electronic assistance is necessary to raise the output voltage to a more useful level.PP3 (6F22) type 9 V batteries are often used in small portable equipment that require very little current and may only be used intermittently. Under these conditions its often the case that the battery is flat just when you urgently need to use the equipment. NiCd rechargeable cells are not a good choice in these applications because their self-discharge characteristics are much worse than dry cells and often there is no charge left after a long time in storage

Download Free PDF here.

AUTOMATIC PLANT WATERING REMINDER

noreply@blogger.com (Anonymous) — Thu, 28 Feb 2013 23:14:00 +0530

House plants in general often have a pretty hard time of it compared to their garden bound cousins, which seem to get more than their fair share of watering, even if their owner forgets, thanks to the British weather. With so many other things to think about, the first reminder that many people get to water their plants is when it is noticed that one or two are wilting or the leaves are turning brown and dropping off! Modern central heating also ensures that the soil in pots dries out much faster, making regular watering more important, so that a little electronic help in remembering to do so should be most welcome.

Circuit Diagram

The circuit suggested here, and shown in Fig.20, drives a piezo sounder, WD1, to
provide a timely warning that the soil in the plant pot is almost dry. Hopefully, the plants will be watered regularly so the alarm will remain off but it may become active at any time and it is unlikely that the plants will be watered immediately as the owner may be out. It may therefore continue to sound all day before the plants are watered. To avoid having to replace the battery too often, it is important to ensure that the current drain in either condition is as low as possible. To minimise the current drain during the alarm condition, a complementary astable circuit built around transistors TR2 and TR3 is used. Its operation is beyond the scope of this article, but it oscillates with a frequency determined by resistor R2 and capacitor C1. With the component values given the frequency will be about 2kHz, producing a fairly loud sound from piezo sounder WD1. This device has a very high impedance and so a load resistor, R3, is provided for TR3. Since both transistors switch on and off together and remain off for a relatively long period (dependant on the value of R2) compared to the time when they are on, the average current drawn from the battery is very low, at about 1mA. The output consists of short positive going pulses which turn on the piezo sounder WD1. The operation of the oscillator is controlled by TR1. When this transistor is on, the base of TR2 is held low and the circuit cannot oscillate. The circuit relies on sensing the resistance of the soil between two metal probes which are inserted into the pot close to the plant. Completely dry soil will have a relatively high resistance but this will fall as the moisture content is increased.

The series resistance of the probes, resistor R1 and potentiometer VR1 form a potential divider across the supply. With the soil moist, the resistance of VR1 can be adjusted so that the voltage at the base of TR1 is at 0·6V, ensuring that this transistor is switched on and so disabling the oscillator. As the soil dries out, the base-emitter voltage of TR1 falls to a point at which it switches off sufficiently to allow the oscillatorto function, producing an audible warning.
As described earlier, this circuit produces short output pulses and therefore draws only a small current when it is oscillating (about 1mA). In the stand-by condition when the oscillator is switched off, the current drain on the battery is only 10mA, so the battery should last a long time.

Construction

The circuit is built on a piece of stripboard having 7 strips × 15 holes, as shown in Fig.21. Only one strip cut is required and there are no link wires. Care should be taken to ensure that the transistors and the sounder are connected the correct way around. The probes consist of two stiff metal wires the length of which is not
particularly important and will depend to a large extent on the size of the pot into which the unit is placed. Copper is perhaps the easiest wire to get hold of (and to solder). In the prototype, two 10cm lengths of 2·5mm diameter rigid wire of the type used in house wiring were used. These were soldered directly to the tracks at the positionsshown, the wire being too thick to pass through the holes in the board.Since these are liable to break off if the probes are pushed into hard earth, it is probably best to solder the wires directly to the copper tracks straddling several holes. This may then be strengthened by covering the joints and an adjacent area of the board with epoxy glue. Alternatively, the wires may be mounted a few millimetres apart on an insulating surface, such asthe plastic box in which the unit is to be placed, and connected to the board by flying leads.

Soundless Alarm

When completed, place the probes in moist soil close to the roots of the plant. Set VR1’s wiper to a fully anti-clockwise position, and then adjust it until the circuit just fails to oscillate. Should the alarm sound as the soil dries out but it is still judged to be too moist to require watering, VR1 should be turned further clockwise. In some situations, an audible alarm may not be desirable, in which case the sounder can be omitted, and an l.e.d. plus ballast resistor of about 470 can be wired in place of R3, with the anode (a) connected to transistor TR3’s collector, and the other side of the 470ohm resistor on the 0V line. Omit R3 itself. Do not use the l.e.d. without the ballast resistor as the current through it cannot be guaranteed to be within its limits, even though the current is pulsed. The sounder and l.e.d. may both be fitted, although this will result in a slightly increased current consumption and a slightly reduced sound output, but should still be adequate for most situations.

TOUCH LIGHT

noreply@blogger.com (Anonymous) — Thu, 28 Feb 2013 23:12:00 +0530

There are many places around the house where a small light would be useful but running a mains cable to the location is impractical. Corners of dark cupboards, over the telephone to light a note pad, by the front door to help find the keyhole at night, are just some of the applications which come to mind. None of these require very much light and high brightness light emitting diodes (l.e.d.s) can not only provide the illumination needed, but can also be readily fitted with a time delay circuit so that they switch off automatically, so saving battery power. The circuit described here offers such a solution. It is shown in Fig.1 To ensure that the circuit switches off after use and prevent having to change the battery too often, a timing circuit is required. For this a monostable configuration is used. A monostable has one stable state, in this case the off state.
When triggered into its on state, it will remain in that state for a preset period before switching off again. Some circuits of this type use two transistors (npn or pnp types) configured so that in the stable state one transistor is on while the other remains off. Following a trigger pulse, both transistors change state. A disadvantage of this circuit is that during the off state, one of the transistors is always turned on, and so consuming power. An alternative configuration is used here in which all transistors remain off when the circuit is in its stable state, so consuming virtually no current.

Touch Circuit

In the circuit diagram shown in Fig.1, transistors TR2 and TR3 form the monostable circuit, with capacitor C1 and resistor R2 determining the time for which the transistors remain on once the circuit has been triggered. This occurs when finger contact is made with touch pad TP1. The 50Hz mains “hum” normally present in all households will be induced into the circuit through the finger, causing transistor TR1 to turn on. This provides base current to TR3, turning it on, together with the l.e.d. (D2), whose negative-going pulse is generated acrosscurrent is buffered by resistor R3.When the collector of TR3 goes low, a capacitor C1, causing TR2 to turn on and provide more current to the base of TR3. When the contact with TP1 is broken, TR1 ceases to conduct, but TR3’s base continues to be held on via TR2. However, C1 starts to charge via resistor R2. Eventually, its charge rises to within less than 0·6V of the positive power supply, turning off TR2 and thus TR3 and the l.e.d. as well. Diode D1 inhibits any positive-going pulse generated across C1 when TR2 switches off. With the component values shown, the l.e.d. will remain on for about three minutes.Touch Down

It is worth noting that touch pad TP2 may be needed if the 50Hz mains “hum” introduced by finger contact with TP1 is not strong enough, or non-existent, as in a garden shed for example. Making finger contact between TP1 and TP2 causes a
small current to flow from the positive ine, though the finger and into the base of TR1. It is advisable to insert resistor R4 between TP2 and the positive line to prevent damage to TR1 should the two pads be shorted accidentally by an object with a low resistance. If the unit is found to be too sensitive, a high value resistor of about 10M can be connected from the base of TR1 to the battery negative. This will prevent the circuit from switching on inadvertently, especially in areas where the mains field is high.

Construction

The circuit is built on a small piece of stripboard having 12 holes by 8 strips, as shown in Fig.2. Only two track cuts need to be made and no wire links are required. Apart from the resistors, all other components must be inserted the correct way round. Power to the circuit should be supplied by a 9V battery. As the stand-by current is extremely low (basically the leakage current of the transistors), the expected life should be almost the shelf life of the battery, depending of course on how often it is switched on. Consequently, an on/off switch is not required. The finished unit should be mounted in an insulated plastic box of a size suitable for the battery and circuit board. The touch contact(s) can be made from any piece of metal such as a bolt or nail, but a drawing pin pushed through a suitable hole in the box and connected to the board via a short length of wire provides a neater, more attractive finish.

LED Considerations

When on, the total current is 6mA with the l.e.d. accounting for about 5·8mA. White l.e.d.s exhibit a forward voltage drop of around 4V, so two could be used in series to provide more light. Resistor R3 would then need to be reduced to 470ohmto maintain the l.e.d. current at around 5mA. The brightness of the l.e.d.(s) can be increased by reducing the value of R3 to increase the current flow. Do not allow the current to be greater than that permitted by the l.e.d., which should be stated in its data sheet and supplier’s catalogue. There appears to be little apparent increase in brightness beyond about 10mA. Data sheets normally quote an l.e.d. viewing angle and this describes the “off axis” brightness of the device. Unlike the filament in a light bulb, an l.e.d. chip emits light only from its surface, rather than all around, so the light comes mainly from the front of the device. This is modified to some extent by the plastic package and l.e.d.s are available with a more or less focused light beam. Depending on the use, a wider angled light pattern may be preferred.

HEART RATE MONITOR

noreply@blogger.com (Anonymous) — Thu, 28 Feb 2013 23:09:00 +0530

The simple but very reliable monitor shown in Fig will be an asset to those who have difficulty finding their pulse in their wrist. It is also useful for checking the pulse rate immediately after exercise, which should be well above the normal rate of 60-80 beats per minute if any benefit from the exercise is to be derived.

Light Finger The device depends for its operation on variations in light intensity. When a finger is placed on a light dependent resistor R2, the l.d.r. detects the minute changes in light level caused by variations in blood flow as the heart pumps. These light changes are translated into minute voltage fluctuations that are subsequently amplified through a two-stage amplifier, a non-inverting op.amp (IC1a) and an inverting op.amp (IClb), by a gain of approximately 800 as determined by resistors R5, R7 and R10. At the output (pin 7 of IC1b), each heartbeat is reflected in the rhythmical swing of a meter needle across the dial of a milliammeter (ME1) or other suitable panel meter. No special lighting is needed as the l.d.r. is able to “see” through a finger tip in normal daylight.

The gain of the first op.amp is fed into the second and the overall gain is sufficient to obtain a healthy swing of the meter needle. Almost any moving coil meter can be pressed into service because we are not concerned with voltage or current measurement, only the needle deflections across the dial. However, be sure to fit a series limiting resistor R11 to suit the meter and prevent damage. A miniature button-type l.d.r. is preferred to the bulkier ORP12 so that the finger can completely cover the sensor surface and prevent stray lighting from reaching it. Two discrete 741 op.amps could be used in place of the LM358N if more readily available. Although not shown here, the prototype also housed a 30-second timer, using a 555 with an l.e.d. indicator.

When the timer is initiated, the needle movements are counted during the 30 second period, then doubled to obtain pulses per minute. The circuit could also be adapted as a front-end to more advanced monitoring systems. In use, after the unit is switched on, allow several seconds for the meter needle to stabilise somewhere about mid-scale. Place the fleshy part of the middle finger tip on the l.d.r. and rest the hand comfortably while keeping it still, then monitor the meter needle movement. If the meter needle responds by only a small amount, it is probably because your hand is excessively cold and the circulation is sluggish.

BARGRAPH USING LED

noreply@blogger.com (Anonymous) — Thu, 28 Feb 2013 23:07:00 +0530

The circuit shown in Fig was devised as a cheap alternative to a moving coil meter when I wanted to monitor the output current of a power supply. Its operation is as follows: As the output current increases from zero, it flows through l.e.d. D1 and resistors R1 and R2 until the voltage across R2 is about 0·6V. At this point transistor TR1 starts to conduct, shunting current round D1 and through D2/R3. When this current produces 0·6V across R3, TR2 begins to conduct, shunting the extra current via D3/R4. This continues until finally D5 is illuminated, showing “full scale deflection”. The purpose of resistor R1 is to provide sufficient voltage across the circuit to cater for:

0·6V across the 33ohms resistor
The voltage across the l.e.d.
0·3V across the transistor in saturation (this is also the voltage across R1)

Bargraph using LED's

The minimum voltage required across the circuit is about 3V, and this must be taken into account when considering the power supply voltage, which must be connected before any voltage regulator. Using the resistor values shown, the maximum current for each l.e.d. is about 17mA, so “full scale deflection” is about 85mA, though the display is not perfectly linear – the second l.e.d. starts to illuminate above about 15mA, and the other l.e.d.s start in approximately20mA increments.
I tried building a display using 10 l.e.d.s but the circuit was unstable; as soon as the 6th l.e.d. started to illuminate, the circuit oscillated in the Megahertz region, and the l.e.d.s lit before they were supposed to (this situation was worse when I left the decoupling capacitors out), so five l.e.d.s. seems to be the limit.

TENSION METER

noreply@blogger.com (Anonymous) — Thu, 28 Feb 2013 23:04:00 +0530

If you, like so many other people in this day and age, arrive home from work stressed out and with the problems of the day still lingering,this simple little instrument will go a long way to relieving nervous tension. Of the various types of feedback devices, probably the best approach for the amateur experimenter is the Galvanograph, better known as the Galvanic Skin Response Monitor. The instrument described here relies for its operation on changes in skin resistance in sympathy with changes in emotional state. An increase in tension level reduces skin resistance and, conversely, a decrease in tension is accompanied by an increase in skin resistance.
The correlation between emotional stress and skin resistance is still not fully understood. What is known, though, is that minute changes in the permeability of the skin produce corresponding voltage variations across two electrode pads attached to two fingers on the same hand.

Tension Meter

Tension Monitor meter circuit

These signal fluctuations are amplified and fed to an oscillator to produce an audible tone. A decrease in pitch therefore signifies a decrease in tension, and vice-versa. A visual indicator in the form of a panel meter also aids the user in monitoring tension levels. The monitor is quite sensitive to fluctuations. During use, a sudden moment of stress, even a deep sigh, will increase the pitch and cause a shift of the meter needle. Circuit Details In the circuit diagram of Fig.1, IC1 is configured as an astable multivibrator to drive an 8-ohm miniature speaker LS1 via capacitor C3, resistor R6 and volume control potentiometer VR2. The latter allows users to set a desired level and avoid it becoming a distraction.
Whereas the trigger input of IC1 is normally connected to the positive rail via a resistor in a conventional 555 oscillator, here it is connected via resistor R4 to the emitter of transistor TR1. The base of TR1 is connected between one electrode pad and the voltage divider formed by potentiometer VR1 and resistor R1. It will be seen that with the pads fitted to the fingers, the tone level will be dependent on the setting of VR1 and skin resistance. Resistor R2 in the transistor base is necessary should the pads be accidentally touched together. A 1mA meter is fitted in the collector line, along with R3, as a visual indicator. Although not essential or intended to measure current levels, it does help to emphasize fluctuations in emotional level.
The design of the pads is not critical. For the prototype, stripboard was used. The tracks were wired together at one end and connected to a 30cm length of twin lighting flex. The pads were then glued to Velcro straps. When the unit is first switched on, a highpitched tone should be heard, rapidly diminishing and ceasing. Turn the Sensitivity control VR1 to the minimum setting. Attach the electrodes to the fleshy pads of the first two fingers on the less-dominant hand with the Velcro straps, firmly but not tight. Rest the hand comfortably and keep it reasonably still, allowing half a minute for the pads to “bond”. Normally, at the minimum setting, the oscillator will hardly tick over, unless the user is in a high state of anxiety. Keep in mind that any form of stimulant, and that includes tea, coffee, alcohol and cigarettes, will reduce one’s capacity to relax. Rotate the control until a medium pitched tone is obtained and apply your relaxation technique. The monitor does not teach any method of meditation or relaxation; it only monitors the effectiveness of the technique applied. The tone should slowly diminish, with fluctuations as unconscious thoughts flit across the mind.When the sound ceases altogether, repeat the above procedure by increasing VR1. Twenty minutes is considered by therapists to be an adequate relaxation session.

SIMPLE LED TESTER CIRCUIT

noreply@blogger.com (Anonymous) — Thu, 28 Feb 2013 23:00:00 +0530

This simple LED tester consists of a current source with a potentiometer that can be used to adjust the current. The current source is implemented using a type TL081 opamp. The output current of the opamp flows through the diode and R2. The voltage drop across R2 is fed back to the inverting input and compared with the reference voltage, which is set with R1 and applied to the non-inverting input. The adjust- ment range is approximately 0–30 mA, which is suitable for testing all normal LEDs. If you wish, you can connect a multi- meter across the LED to measure the voltage on the LED. For the power source, a good option is to use a small laboratory power supply with the output voltage set to 5 V. It is convenient to fit the potentiometer with a scale so you can see directly how much current is flowing through the LED. In order to calibrate the scale, you can temporarily connect an ammeter in place of the LED.

schematic

INTERACTIVE TRAFFIC LIGHTS MODEL

noreply@blogger.com (Anonymous) — Thu, 28 Feb 2013 22:56:00 +0530

In this project we are going to extend the previous project to include a set of pedestrian lights and a pedestrian push button to request to cross the road. The Arduino will react when the button is pressed by changing the state of the lights to make the cars stop and allow the pedestrian to cross safely. In this project we are able to interact with the Arduino and cause it to do something when we change the state of a button that the Arduino is watching (i.e. Press it to change the state from open to closed).

Connect it up

Connect the LED?s and the switch up as in the diagram. You will need to shuffle the wires along from pins 8, 9 and 10 in the previous project to pins 10, 11 and 12 to allow you to connect the pedestrian lights to pins 8 and 9.

Enter the code

Enter the code on the next page, verify and upload it. When you run the program you will see that the car traffic light starts on green to allow cars to pass and the pedestrian light is on red. and if so passes code execution to the function we have created called changeLights(). In this function the car lights go from green to amber then red, then the pedestrian lights go green. After a period of time set in the variable crossTime (time enough to allow the pedestrians to cross) the green pedestrian light will flash on and off as a warning to the pedestrians to get a hurry on as the lights are about to change back to red. Then the pedestrian light changes back to red and the vehicle lights go from red to amber to green and the traffic can resume. The code in this project is similar to the previous project. However, there are a few new statements and concepts that have been introduced so let?s take a look at those. When you press the button, the program checks that at least 5 seconds have gone by since the last time the lights were changed (to allow traffic to get moving),

// Project  Interactive Traffic Lights
int carRed = 12; // assign the car lights
int carYellow = 11;
int carGreen = 10;
int pedRed = 8; // assign the pedestrian lights
int pedGreen = 9;
int button = 2; // button pin
int crossTime = 5000; // time allowed to cross
unsigned long changeTime; // time since button pressed
void setup() {
pinMode(carRed, OUTPUT);
pinMode(carYellow, OUTPUT);
pinMode(carGreen, OUTPUT);
pinMode(pedRed, OUTPUT);
pinMode(pedGreen, OUTPUT);
pinMode(button, INPUT); // button on pin 2
// turn on the green light
digitalWrite(carGreen, HIGH);
digitalWrite(pedRed, HIGH);
}
void loop() {
int state = digitalRead(button);
/* check if button is pressed and it is
over 5 seconds since last button press */
if (state == HIGH && (millis() - changeTime) > 5000) {
// Call the function to change the lights
changeLights();
}
}
void changeLights() {
digitalWrite(carGreen, LOW); // green off
digitalWrite(carYellow, HIGH); // yellow on
delay(2000); // wait 2 seconds
digitalWrite(carYellow, LOW); // yellow off
digitalWrite(carRed, HIGH); // red on
delay(1000); // wait 1 second till its safe
digitalWrite(pedRed, LOW); // ped red off
digitalWrite(pedGreen, HIGH); // ped green on
delay(crossTime); // wait for preset time period
// flash the ped green
for (int x=0; x<10; x++) {
digitalWrite(pedGreen, HIGH);
delay(250);
digitalWrite(pedGreen, LOW);
delay(250);
}
// turn ped red on
digitalWrite(pedRed, HIGH);
delay(500);
digitalWrite(carYellow, HIGH); // yellow on
digitalWrite(carRed, LOW); // red off
delay(1000);
digitalWrite(carGreen, HIGH);
digitalWrite(carYellow, LOW); // yellow off
// record the time since last change of lights
changeTime = millis();
// then return to the main program loop
}

TRAFFIC LIGHT MODEL USING ARDUINO

noreply@blogger.com (Anonymous) — Thu, 28 Feb 2013 22:51:00 +0530

We are now going to create a set of traffic lights that will change from green to red, via amber, and back again, after a set length of time using the 4-state system. This project could be used on a model railway to make a set of working traffic lights or for a childs toy town.we have connected 3 LEDs with the Anode of each one going to Digital Pins 8, 9 and 10, via a 220ohm resistor each.

We have taken a jumper wire from Ground to the Ground rail at the top of the breadboard and a ground wire goes from the Cathode leg of each LED to the common ground rail.

Enter the code

Enter the following code, check it and upload.

int ledDelay = 10000; // delay in between changes
int redPin = 10;
int yellowPin = 9;
int greenPin = 8;
void setup() {
pinMode(redPin, OUTPUT);
pinMode(yellowPin, OUTPUT);
pinMode(greenPin, OUTPUT);
}
void loop() {
// turn the red light on
digitalWrite(redPin, HIGH);
delay(ledDelay); // wait 5 seconds
digitalWrite(yellowPin, HIGH); // turn on yellow
delay(2000); // wait 2 seconds
digitalWrite(greenPin, HIGH); // turn green on
digitalWrite(redPin, LOW); // turn red off
digitalWrite(yellowPin, LOW); // turn yellow off
delay(ledDelay); // wait ledDelay milliseconds
digitalWrite(yellowPin, HIGH); // turn yellow on
digitalWrite(greenPin, LOW); // turn green off
delay(2000); // wait 2 seconds
digitalWrite(yellowPin, LOW); // turn yellow off
// now our loop repeats
}

In upcoming posts we will see about how to make this project more interactive.