Here is a simple and very easy to build alarm siren producing gadget capable of producing five different types of siren tones. The gadget is built around a single IC 556 which is nothing but a dual IC timer 555.That is, it has two independent 555s   within the same package. The type of siren can be selected from an appropriate combination of switches to be closed given in the accompanying table. The gadget could also be constructed using two 555s in place of one 556.

CIRCUIT DESCRIPTION

For a clear understanding of this circuit functioning, it is very important to identify those pins in IC556 that represent two independent 555’s. IC555 has eight terminals namely the COMMON (Pin-1), TRIGGER (Pin-2), OUTPUT (Pin-3), RESET (Pin-4), CONTROL (Pin-5), THRESHOLD (Pin-6), DISCHARGE (Pin-7) and +Vcc (pin-8). IC556 which is a 14-pin IC has two fully independent 555s with only a common supply and a common ground point. lts pin connection diagram as shown in Fig. 22.2 tells about the pin numbers for the different terminals of two 555s inside this IC. As is clear from this diagram, one of the 555s is represented by pins 1 to 7 and 14 with pin-7 being the ground pin and pin-14 the supply pin The second 555 is available on pin numbers 7 to 14.

SIMPLE SINGLE TONE ALARM

With switches S10, S13 and S14 closed and all other switches open, the given circuit is reduced to a simple astable multivibrator configuration feeding the speaker. The astable multivibrator produces a waveform at the output that has a high time depending upon R1, R4 and C6 and a low time depending upon R4 and C6. The values for these components are so chosen that the frequencies the output waveform is about 4.5kHz. You would notice that the second 555 remains disconnected from the output side.

FACTORY SIREN

With switches S6, S7, S10, S12 and S14 closed and other switches open, the given circuit is once again an astable multivibrator built around oneof the 555s The high time this time depends upon the resistance R1 , the resistance offered by potentiometer P3 and capacitance: C6. The low time depends upon potentiometer P3 setting and capacitor C6.You would notice that there is an RC network connected from supply to ground with the capacitor connected across control pin (Pin- 3) of the 555 being used. The RC time constant is variable upto a maximum of 10 seconds. Thus as the circuit switched on, voltage at the control pin rises towards Vcc with the time constant depending upon P1 setting and C7. This rising voltage changes the output frequency.Thus what we hear is a frequency modulated audio tone resembling that of the one from a factor siren.The second 555 is again not coming into figure.

AMBULANCE SIREN

With switches Sl, S4, S5, S9 and S10 closed, the

given circuit is reduced to a configuration where the first 555 wired as an astable multivibrator drives the control pin of the second 555 again wired as the astable multivibrator. The frequency of the first astable multivibrator is decided by R1, P4 setting and C5 while that of the second depends upon R2, P2 setting and C3 in addition to voltage. being fed externally at its control pin (Pin-1 1). The second astable multivibrator operates on two different frequencies corresponding to two different voltage amplitudes present at its control pin due to the low and high potions of the waveform appearing at the output of first 555. Both frequencies of the second astable are much higher than the frequency of the first astable. The final result is therefore a siren with two different tones repeating alternately.

The second astable sets the frequencies of these different tones arid the first astable decides the durations of these tones.

POLICE SIREN

With the switches S2, S4, S5, S9, S10 and S1 1 closed and other switches open, the circuit that we get is again similarly what we have just seen in case of an ambulance siren with the only difference that the control pin of the second astable instead of being driven from the output of the first astable is fed from the trigger-threshold junction point of the first astable multivibrator. The more or less triangular waveform appearing at this junction continuously frequency modulates the output frequency of second astable output which feeds the speaker.The sound like a police siren is the result.

BEEPING SIREN

The circuit configuration is again similar to what we have seen in the two immediately preceding cases. In the present case, the output of the first astable multivibrator feeds the reset terminal of the second astable. The second astable output finally feeds the speaker. Wheh the output of the first astable is high, the second astable functions normally and produces a tone. As the output of the first astable goes low, the second astable is reset and its output goes to zero. Thus result is a siren comprising of a repetitive seqence of presence and absence of tone.Figs. 22.3 and 22.4 respectively show the PCB layout and the components layout.

PARTS LIST

Resistors and Capacitors

R1, R2 : IK, 1/4W

R3 : 68K, 1/4W

R4 : 15K, 1/4W

P1 : 4.7K ,preset

P2 : 100K, preset

P3   : 100K , preset

P4 : 4.7K, Preset

C1 : 100µF, 25V (Electrolytic)

C2 : 10µF, 25V (Electrolytic)

C3, C4, C9 : 0.01µF (Ceramic Disc)

C5 : 1000µF, 25V (Electrolytic)

C6 : 0.01µF, 25V (Polyester)

C7 : 1000µF, 25V (Electrolytic)

C8 : 1000µF, (Electrolytic)

Semiconductors and ICs

IC-1 : IC 556C

Miscellaneous

S1 to S9, S11 to S14: DIP switches, S10: Toggle: switch, 8 ohm speaker

+9V battery Solder wire, multistrand wires etc.

TESTING GUIDELINES

1. The alarm gadget should be tested for its functioning in all the five different alarm settings separately by closing the appropriate set of switches and leaving the rest open. In all individual settings, check that you get an overall sound effect from the alarm that it is supposed to generate.

2. You would notice that S10 has got to be closed in all the five alarm settings. It is rightly so as it is the switch in series with the 9V battery it is suggested that this switch be closed last of all after you have closed all other relevant switches when trying to test a particular alarm setting.

3. In case of those alarm settings which have potentiometers in the frequency determining portions, the effect of varying the potentiometer resistance on the output should be seen and the resistance should be set for the best overall effect.

OBJECTIVE

Wide fluctuations in the AC mains voltage is a very common problem in India. You should not be surprised if some one tells you that the voltage fluctuation could be as much as from 150 volts to 290 volts. Although, majority of our electrical and electronic appliances and gadgets have some kind of voltage stabilization internals built-in, yet more than 90 percent of the faults in these gadgets occur due to these power fluctuations. This simple test gadget can give you real time monitoring of the AC mains voltage in the form of glowing LEDS. It can give you the visible indication of AC mains voltage over a range of 170 volts to 280 volts in steps of 10 volts.

There are 12 LEDS numbered from LED-! to LED-12. For input AC mains voltage of less than 170 volts, all LEDs remain OFF. LED-1 glows when the voltage reaches 180 volts, LED2 glows when the voltage reaches 190 volts. The number of LEDS that glow keeps increasing with every additional 10 volts increase. When the input voltage reaches 280 volts, all the 12 LEDS glow.

CIRCUIT DESCRIPTION

The circuit basically comprises of 12 voltage comparators built around opamp comparator IC type number LM 339. Each of the LM 339 has four comparators inside the IC. One of the inputs of all the comparators (the inverting input) is fed from the unregulated DC output where as the other inputs (the non-inverting inputs)  are applied  reference  DC voltages. Resistors R13 to R25 are so chosen that the reference voltages at points 1 to 12 are respectively 0.933V, 1 .866V, 2.80V,3.732V, 4.665V, 5.598V, 6.531V, 7.464V, 8.397V,9.33V, 10.263V and  11.196V. P1 is so adjusted that when the input voltage is 230VAC the DC voltage at the junction of R26-P1 series combination and R27 is 6.531 volts. All reference voltages have been generated from a regulated voltage of 12 volts. In all these comparators, whenever the voltage at their inverting input exceeds the voltage at their non-inverting input, the LED connected at the output glows.

CONSTRUCTION GUIDELINES

Figs. 21.2 and 21.3 respectively show the PCB layout and components layout.

PARTS LIST

Resistors and Capacitors

R1 to R12   : 100 ohms, 1/4W

R13 to R24  : 1OK, 1/4W

R25         : 8.59K, 1/4W (Use a series combinational 8.2K and 390

ohms instead)

R26         : 15K, 1/4W

R27         : 1OK, 1/4W

P1           :10K Preset

R28 to R39  : 1K, 1/4W

R40 to R51  : 100K, 1/4W

C1           : 100gF, 50V (electrolytic)

C2, C3       : 0.1gF (Ceramic disc)

Semiconductors and ICS

LED-1 to LED-12   : miniature LEDS

Bridge Rectifier,B-1 : Type 1B2 (The bridge can also be made using four rectifier      diodes of the type IN 4001) .

VR-1               : Three terminal regulator, type 7812 lC-1 , lC-2, 1C-3 : LM             339

Miscellaneous

Transformer,T-l : Mains transformer: primary 230V Secondary-15V Secondary    current-250mA

Fuse : 500mA rating

SW-1 : Mains Power ON/OFF switch  Solder metal, Wires etc.

TESTING GUIDELINES

The test procedure has more or less been described during circuit description.

Following steps should however be followed for the calibration of the circuit.

1. Connect the input of the mains transformer to an auto-transformer (VARIAC). Set the auto-transformer voltage at precisely 170 volts. Verify the set voltage with the help of millimeter as the auto-transformer dial may not be properly calibrated.

2. Adjust the resistance of the preset P1 till only LED-! glows. Remember that you have to gradually increase the resistance starting from the minimum and stop increasing it further the moment LED-1 glows.

3. The calibration can be funder checked by increasing auto-transformer voltage beyond 170 volts. Gradually increase the voltage upto 280 volts and observe different LEDS glowing at 10 volt intervals as explained earlier.

The objective here is to construct an emergency fluorescent light circuit that requires neither a choke nor a starter unlike conventional mains   operable fluorescent light. Such emergency lighting units having different lighting power

capabilities are available in abundance in the market. The circuit given here is simple and highly| efficient. It operates from a 6V rechargeable battery (Lead-Acid type). The battery is trickle charged when the mains is present. The battery with a capacity of 6Ah (this is the battery used in this project) is capable of providing uninterrupted lighting for four hours in the absence of mains. The switch over is fully automatic i.e. When the mains goes OFF, the battery comes into the lighting circuit automatically and there is no requirement of manually switching on the light.

CIRCUIT DESCRIPTION

The emergency fluorescent lights are based on the principle of high frequency lighting. Typically, the frequencies the applied pulse train is around 30 to 35kHz and the initial peak amplitude (when the light is not yet on) of the pulses is around 750 to 800 Volts. When the tube lights up, the peak amplitude falls to a value depending upon the output power delivery capability of the inverter circuit supplying high frequency pulse train to the tube. The circuit operation can be described as follows: In the presence of AC mains power, the AC/DC power supply arrangement consisting of a step down transformer with center tapped secondary (T-1), the two diode conventional full wave rectifier constituted by diodes (D1) and (D2), the filter capacitor (C1) and the three terminal regulator (VR-1) generate regulated even which charges the battery through diodes (D3) and (D4). Since the emitter potential of transistor (Q) is less than its base potential by two diode drops, (Q1) is surely in cut-oft As a result, no voltage appear? at the collector of (Q1) and hence the drive (circuit for the DC/AC inverter portion remains without its DC supply.The fluorescent tube (a 6Watt, 9 inch tube in this case) stays OFF. To sum up uphill now, when ever AC power is ON, the battery is getting charted but is not supplying any powered the DC/AC inveter circuit feeding the fluorescent tube. LED-1 is ON and it indicates that mains is present. LED-2 is OFF and it tells that transistor (Q1) is in cut-off.   When the mains power is OFF, diodes (D3) and (D4) are reverse biased, tsansistor (Q1) conducts and the DC   voltage is now available at the collector terminal of (Q1) and hence for the drive circuit of DC/AC inverter. Case of transistor (Q2) gets drive pulses and it is switched ON and OFF alternately Basically, transformer (T-2), transistor (Q2) and the drive circuit constitute an externally driven DC/AC inverter of the fly back type. During every conduction time (when the output of 555 timer is LOW) of the transistor (Q2), energy is stored in the primary of the inverter transformer and during every OFF time (when the output of   555 timer is HIGH), the energy stored in the previous cycle is transferred to the secondary circuit. The inverter transformer has been so designed here that it produces a train of pulses with a frequency of about 30kHz and a peak amplitude of 750 volts. When such a pulse train appears across the tube it lights up and the peak pulse amplitude drops to about 150 volts.

Capacitor (C8) limits the tube current. The transformer and the drive circuit have been designed to feed a 6 Watt, 9 inch tube.

CONSTRUCTION GUIDELINES

The PCB layout the components layout are respectively shown in Figs. 20.2 and 20.3 respectively. Transformer (Tl), Transformer(T2), transistor (Q2) and the battery are not the part of the PCB. These are mounted separately. Transformer (T2) (the inverter transformer) is wound on a good quality ferrite rod (10mm diameter and room long) usually seen in transistor sets. These are easily available in the market. The dots shown in the transformer windings indicate start of windings assuming that both primary and secondary are wound in same direction. Other details such as number of primary and secondary turns, the gauge of the wire to be used etc. are given in the parts list.

PARTS LIST

Resistors and Capacitors

R1 : 470Ω, 1/4W

R2 : 3.3K, 1/4W

R3 : 1 .5K, 1/4W

R4 : 100 Ω, 1/4W

R5 : 10 Ω, 5W (Wire wound)

R6 : 4.7 Ω,2W

R7, R8 : 4.7K, 1/4W

C1 : 470µF, 16V (Electrolytic)

C2 : 0.01µF(polyester)

C3 : 0.01µF   (Ceramic disc)

C4, C6 : 0.01µF (Ceramic disc)

C5 : 10µF, 16V (Electrolytic)

C7 : 2.2µF, 16V (Electrolytic)

C8 : 0.01µF, 1KV (Polyester)

Semiconductor Device: and ICs

D1 to D5 : 1N4001 or Equivalent

LED-1, LED-2 : Preferably two different colour LEDS

Q1 : SK100 or equivalent

Q2 : 2N3055

Q3 : 2N2222

VR-1 : Three terminal regulator type 7809

IC-1 : Timer 555

Other Components

1. S1 : Mains power ON/OFF switch

2. T-1 : Mains transformer, Primary : 230VAC,

Secondary 9-0-9, 500mA 3. T-2 Inverter transformer, primal:

Secondary:

Core : 7.5cm long ferrite rod (Fig. 20.4)

Miscellaneous

6 Watt, 9 inch fluorescent tube with appropriate holder, 8-pin IC base, LED holders, multistrand wires, solder metal, mains power cord, suitable mounting cabinet etc.Note: The photograph offering rods shown in Fig.20.4 is only a representative photograph to give you an idea of how this component typically may look like.

Objective

It is an ideal project for those who want to build something that not only teaches them something but also gives them lot of fun. As the title suggests, this gadget can be used to measure quantitatively the reaction time of an individual. The reaction time can be measured in two different settings: In the first setting, the reactiontime can be measured upto a maximum of 99ms with a resolution of 1ms. In the second setting, the reaction time can be measured upto a maximum of 990ms with a resolution of toms. You can try this gadget with your friends and relatives and find out how quickly they react to a given situation.You can, try this with yourself too at different times and when you are In different states of mind and see the difference yourself.

Circuit description

The circuit is basically a counter clocked by a clock generator having two different frequency selections of 100Hz and 1000Hz. There are two push buttons represented by micro switches  SW1 and SW2. One of the push buttons to be held by the examiner is used to start the counter and the other push button to be held by the examinee is used to stop the counter. The display reading obviously gives the time interval between the start and stop operations and hence the reaction time. The display reads the reaction time in milliseconds directly if the clock is set at 1ms period and it reads one tenth of the reaction time in milliseconds if the clock is set at toms period. The functioning of counter type 4510 has been explained in detail in the project DIGIKIT-III. The present circuit is nothing but a cascaded arrangement of two such counters thus extending the capability of the counter up to a maximum count of 99.clock pulses are continuously applied to the counters’ clock inputs. The RESET points of the counters are fed from the output of a J-K flip flop wired as a T-flip flop. Initially, the flip flop is kept in the HIGH output state so that the counter is reset to 00. Also, in this condition, LED-2 is OFF. The flip flop is made to toggle when the examiner presses the push button thus sending a pulse to the clock input of the flip flop. This removes the RESET condition from the counter and it starts counting. The flip flop is again made I to toggle and thus reset the counter by the examinee when he or she presses the push button provided to him or her. It may be mentioned here that the person under test has to press his push button (only in response to the lighting of LED-2. LED-2 lights when the person who is the judge presses his hush button.

The clock generator is a simple circuit built around three NAND gates in cascade The frequency of this clock generator is given by   f=0.56/RC

where R = R5 = R6 and C = C1 or C2

PARTS LIST

Reslstors and capacitors

R1 to R4 : 22K 1/4W

R5, R6 : 560K, 1/4W

R7 to R20 : 680 ohms, 1/4W

R21, R22 : 3.9K, 1/4W

C1 : 0.1µF (Polyester)

C2 : 0.01µF (Polyester)

Semiconductors and ICs

LED-2 and LED-2 : Miniature LED

IC-1 : CD 4011

IC-2 : CD 4071

IC-3 : CD 4027

IC-4 : CD 4011

IC-5, IC-6 : CD 4510

IC-7, IC-8 : CD. 4511

IC-9, lC-10 : LT 543 (Common cathode type seven segment display)

Switches

SWI, SW2 : Microswitches

SW3 : SPDT

SW4 : Miniature toggle switch

Miscellanies

9V battery, solder metal, wires, IC bases

Figs. 19.2 and 19.3 respectively show the PCB layout and components layout.

TESTING GUIDELINES

The test gadget can detested asper the following procedure:

1. Initially, keep the switch SW3 in position-1 so as to select 100Hz clock frequency. Check  that the LED-2 is OFF. lf it is not so, pressing release SW1 once.

2. Once the LED-; is OFF, this ensures that the counter is in RESET mode. Now you are ready to check the reaction time.Hold the microswitch SW1 in your hand and ask the other person (whose reaction time is to be determined) to hold the switch SW2 Instruct the other person to concentrate on the LED-: and press the switch SW2 immediately when he sees the LED-: glowing.

Now, press the switch SWl without telling the other person. The reading on the display gives his reaction time in tense of milliseconds.

That is, the actual reaction time is 10 x display reading.

5. lf the person is too quick and the reaction time comes out to be less than or equal to 99ms, you can repeat the test by changing over the clock frequency to 1000Hz. This will give you a more accurate value.

OBJECTIVE

The objective here is to build a 3l/2-digit digital voltmeter with a standard instrument sized LED display. It may be mentioned here that the IC type number ICL7107 is almost universally used for digital voltmeter application and the cirduit shown in Fig. 18.1 is basically what you would discover inside any digital panel meter with a 3l/2-digit LED display. The said voltmeter has beep designed to operate from AC mains to generate regulated +5VDC and -5DVC for the circuit.

Digital panel meters with an LED display usually operate from AC mains due to significant current drive requirement of LED displays. There is a separate power supply section (Fig. 18.2) that generates regulated +5VDC and -VDC It is also worthwhile   mentioning here that this circuit can also be used for the digital display of any electrical or non-electrical quantity that can be converted into a proportional DC voltage. Digital pH-meters, Pressure meters, Digital Thermometers etc. are all built around the basic circuit shown in Fig. 18.1. The digital   voltmeter circuit shown in Fig. 18.1 along with the   power supply section of Fig. 18.2 can be used to   measure DC voltages over a range of 0 to 1000 VDC It also had a provision of measuring the AC mains voltage RMS value.

CIRCUIT DESCRIPTION

IC lCL7107 from INTERSIL is the heart of the system. Without going into the internal circuit details of this IC, it would suffice to mention here that the said IC is basically a dual slope integrating A/D converter with its own on-chip   oscillator (that serves as the clock), reference,  decoder and driver and it is capable of driving directly an instrument ’sized LED display of the common- anode type. The display reading depends upon the analog input voltage and the reference voltage.

The reference voltage is so adjusted that the       display reads the analog input directly. In the   circuit shown in Fig. 18.1, switch S2 can be used to select DC or AC measurement. When the switch is on AC position, what is actually fed to the analog input terminals of the 1C7107 is the rectified (full wave) AC input after the voltage divider arrangement constituted by resistor (R5) and potentiometer (P2). We shall discuss the adjustment of (P2) under the heading of CALIBRATION. Similarly, when the AC/DC select switch   (S2) is on DC, what gels applied to the analog input terminals of the 1C7107 is the DC input divided by the voltage divider arrangement constituted by (R6) and (P3). Adjustment of (P3) will be discussed when we discuss the calibration procedure. The power supply section (Fig. 18.2) is a conventional AC/DC power supply using a step down center tapped mains transformer (T-1), full wave rectifier circuits (D1 and D2 for positive output, D3 and D4 for negative output) and capacitor filters (C1 for positive output and C2 for negative output). Regulation is achieved using three terminal regulators (VR-1) for posltive output and VR-2 for negative output). C3′ to C6 are decoupling capacitors.

CALIBRATION PROCEDURE

The calibration procedure is as follows:

1. Select the AC/DC switch (S2) to be on DC position, feed 100 VDC from a high voltage power supply at DC IN terminals. Adjust potentiometer (P3) to get 2VDC at point-! of the SPDT switch (S2). If 1000VDC is not available, a lower DC voltage can also be used and in that case, the divided voltage should also be proportionately reduced. For instance, for 100 VDC input, the divided voltage will be 200mV. In other words, (P3) should be so adjusted that (R6) and (P3) give a voltage division by a factor of 500.

2. Now, as a second step, adjust potentiometer (P1) so that the meter directly reads the DC input voltage.

3. Change the AC/DC select switch to AC position. Feed AC mains voltage at the AC IN terminals. Adjust potentiometer (P2) so that display directly reads RMS value of the AC input. Before feeding the AC mains, the RMS value of the AC mains should be checked independently with another voltmeter or multimeter with AC voltage measuring capability. – 4. The voltmeter is now calibrated and ready for else.

5. The calibration of this voltmeter should be checked at regular intervals as it may get disturbed due to inherent change in the resistance values of voltage divider resistors.

You may be required to readjust different potentiometers to restore calibration.

CONSTRUCTION GUIDELINES

Figs 18.3 and 18.4 respectively show the PCB layout and the Components layout.

PARTS LIST

Resistors and Capacitors

R1      :470K,1/4W

R2      : IM, 1/4W

R3      :22K 1/4W

R4      :100K, 1/4W

R5      :1M, 15000V (You can also use three 330K 1/2W resistors in series)

R6      :2.2M 1W 1500V resistor

P1      :1 K Preset

P2, P3  :10K Preset

C1, C2  :100µF  16V (Electrolytic)

C3 to C4 :0.1 µF(Ceramic disc)

C7      :0.2 µF (polyester)

C8      :0.47 µF (Polyester)

C9      :0.01 µF(Ceramic disc)

C10     :0.1 µF (Ceramic disc)

C11    :100 µF (Polyester)

Semiconductor Devices and ICS

D1 to D8 : 1N4007 or equivalent

VR-1     :7805 (Three terminal regulator)

VR-2     :7905 (Three terminal regulator)

IC-1      : ICL 7107

Display

(DL-I to DL-4) : Common anode display type no. LTS-542

Other Components

S1 : Mains ON/OFF switch

S2 : SPDT switch, 230V, 1A F1 : Fuse, IA rating with fuse holder

T-1 : Mains transformer (Primary: 230VAC Secondary: 7.5-0-7.5,

500mA)

Miscellaneous

Solder metal, multistrand wires, suitable mounting cabinet.

Pin Connection Diagram of Display

Fig. 18.5 shows the pin connection diagram of   seven segment display type no. LTS-542.

The project under construction here is a DIGITAL STOPWATCH that could count time up to a maximum of 99.9 seconds with a resolution of 0.1sec (or in steps of 0.1 sec) or up to a maximum of 999 seconds with a resolution of 1 second.

This has been possible by having two clock frequency options, a 10Hz clock and a 1Hz clock. This gadget can be used to accurately measure short time intervals. It is portable and operates from four rechargeable Ni-Cd cells.

CIRCUIT DESCRIPTION

IC1 (MM5369) is a popular IC used for generating clock pulses in digital time pieces. lC-1 along with resistors (R1), capacitors (C1) and (C2) and crystal generates a 60 Hz clock signal at its output (Pin-1). This 60Hz clock signal is passed on to the clock input of IC-3 (CD4018B) via an arrangement of NAND-gates. It is easy to see that whenever the START switch is pressed momentarily, pin-4 and thus pin-: of IC-2 goes HIGH which enables the NAND-gate designated .IC-2A with the result that the clock signal is

passed onto the IC-3 clock input. When STOP switch (S2) is pressed momentarily, the NAND gate designated IC-2A is disabled due to pin-4 of IC-2 going LOW with the result that the clock pulses are not allowed to reach lC-3. IC-3 and IC4 (CD4018B) are presentable divide by (N) counters where (N) could be 2, 3, 4, 5, 6, 7, 8, 9 or 10. IC-3 here has been wired as a divide byes counter by feeding (Q3) output to Data input of the IC. IC-4 has been wired as a divide-by-10 counter by feeding (Q5) output back to the Data input. With 60Hz clock input to IC-3, we get a 10Hz clock signal at IC-3output which when fed to the lC-4 as the clock signal results in a 1Hz clock signal at the output of IC-4.IC-5, lC-6 and 1C-7 (CD4510B) are presentable UP/DOWN BCD decade counters. These counters are connected in cascade arrangement and can count up to a maximum of 999 clock pulses. When fed with a l Hz clock, the maximum attainable time delay between start and stop operations is thus 999 seconds. Similarly, when the .clock frequency is chosen to be 10Hz, the maximum attainable time delay is 99.9 seconds. The RESET switch can be used to reset the counter to all zeros before start. Usually, the reset terminal is at GND, it is momentarily connected to +5V to reset the counter.IC-8, lC-9 and lC-10 (CD4511B) are BCD to seven segment latch decodes drivers. IC CD4511B   can directly drive seven segment LED displays of the common cathode variety.

IC-11, IC-12 and lC-13 (LT543) are seven segment displays of common cathode type. R5 to R25 are current limiting resistors.

CONSTRUCTION GUIDELINES

Figs. 17.2 and 17.3 receptively show the PCB layout and the components layout. Pin connection diagrams of CD4510B, CD4511B, CD4011B and LT543 are respectively shown in Figs. 17.4, 17.5,

17.6 and 17.7.

PARTS LIST

Resistors & capacitors

R1 : 22M, 1/4W

R2, R26 : 1K, 1/4W

R3, R4 : 22K 1/4W

R5 to R25 : 680Ω, 1/4W

C1, C2 : 33pF (Ceramic)

C3 : 0.0022µF (Ceramic)

Semiconductor Devices and lCs

D1 : LED

IC-1 : MM5369

lC-2 : CD4011B

IC-3, IC-4 : CD4018B

IC-5, lC-6, IC-7 : CD4510B

IC-8, IC-9, IC-10 : CD4511B

IC-11 , lC-12, IC-13 : LT543 (Common cathode seven segment display) Other comments X-TAL : 3.5795 MHZ crystal

S1, 52 : Miniature push button micro switches

S3, S4 : SPDT switch

S5     : ON/OFF switch

Miscellaneous

IC Bases (14-pin, 16-pin), Nickel-cadmium Cells (4 Nos.), Multistrand wires, solder metal, suitable mounting cabinet.

A simple, easy to build portable all electronic resistance meter that can be used to accurately measure resistances up to a maximum of 1MΩ is the project under construction here. The circuit operates from twin 9V batteries. The resistance meter has a linear scale and the zero resistance point appears on the extreme left end of the scale. Another significant feature of this meter is that it can be used to measure resistances without   actually taking them out of the circuit.It has six different selectable ranges that permit resistance measurement in decades of (i) 1Ω to 10 Ω (ii) 10 Ω  t0 100 Ω  (iii) 100 Ω  to 1K (iv) 1K to 10K (v) 10K to 100K and (vi) 100K to 1M

CIRCUIT DESCRIPTION

The measurement of resistance in this meter is in terms of voltage developed across the unknown resistance due to a constant current flowing through it. The voltage is directly proportional to the unknown resistance value if the current is

constant. Different resistance ranges have been achieved by changing the amplitude of constant current i.e. the current remains constant only for a given resistance range. Also, the constant current amplitude is so chosen that in a given resistance range, it produces a voltage of 0.5Vfor the highest value of resistance in that range. That is, if you have selected a range of say 1K to 10K, then the amplitude of constant current for this range Will be 50 µA. This enables the user to use the meter without taking the components out of the circuit as 0.5V would not operate any of the silicon semiconductor devices (Diodes, Transistors etc.)

The constant current source is constituted by transistor (Q1), diode: (D1) and (D2), Zener diode (VZ1), resistors (R1) to (R7). Resister (R1) provides bias current for the zoner diode (VZ1) and diode (Dl). The amplitude of constant current is given by VZ1 divided by the resistance appearing in the emitter lead of (Q1) as the drop across (D1) cancels the (VBE) drop of (Q1).VZ1is a 5V zener diode and the resistors (R2) to (R7) are so chosen that the amplitudes of constant currents in six different range settings of switch (S1) are 50mA, 5mA, 500 µ A, 50 µ A, 5 µ A and 500nA. Six different ranges are (i) 0 to 10 ohms (ii) 10 ohms to 100 ohms (iii) 100 ohms to 1000 ohms (iv) lKto 10K(v) 10Kto 100K and (vi) 100K to IM. That is, 10Ω, 100Ω, 1000Ω, 10K, 100K and 1M resistors produce 0.5V across them and give full scale deflection in six different range settings. Resistors (R2) to (R7) should be precision resistors (±1% or better) as the accuracy of the instrument largely depends upon precision of these resistors.IC-1 if an opamp and is wired as a non-inverting amplifier with a gain of (1+R9/R8). The gain has been chosen to be (11) to produce 5.5 V at the opamp output for a full scale deflection in the meter. (C1) (C2), C3 and C4 are power supply decoupling capacitors. (P2) is the offset adjust potentiometer for the opamp. This potentiometer is used to calibrate the meter to read zero When a zero (or a short) resistance is connected between the unknown resistance terminals. (P1) is used to calibrate the full scale deflection end of the meter.

CALIBRATION

Connect a small piece of wire between the unknown resistance terminals to simulate a zero resistance. Select any of the resistance ranges.

Potentiometer (P2) is Adjusted to get a zero reading on the meter. Choose 10K full scale deflection range and connect a 10K, ±0.5% or ±1% resistor in place of shorted wire. Adjust (P1) to get full scale deflection on the meter. The meter is calibrated. If you have any difficulty in getting a precision 10K resistor for calibration, you could choose one from a general purpose resistor lot that is within ±0.5% of (OK.

CONSTRUCTION GUIDELINES

Figs. 16.2 and 16.3 respectively show the PCB layout the components layout.

PARTS LIST

Resistors and Capacitors

R1 : 330Ω, 1/4W

R2 : 100 Ω, 1/4W (Metal film)

R3 : 1 K, 1/4W (Metal film)

R4 : 10K, 1/4W (Metal film)

R5 : 100K, 1/4W (Metal film)

R6 : 1M, 1/4W (Metal film)

R7 : 10M, 1/4W (Metal film)

R8 : 1K, 1/4W

R9 : 10K, 1/4W

R10 : 10K , 1/4W

R11   : 1 K, 1/4W

P1, P2 : 10K, Multiturn trimmer potentiometer

C1 , C2 : 0.1µF (Ceramic disc)

C3, C4 : 10µF, 16V (Electrolytic)

Semiconductor Devices and its

D1, D2 :   1N4001 or equivalent

VZ1 : 5V, 400mW zener diode

IC-1 : opamp741 Q1 : 2N2907

Other components

Battery : Two 9V batteries

S1 : Rotary switch with one pole and at least six throws

S2 : DPDT switch

Meter : Meter with 1mA f.s.d.

Miscellaneous

8-pin D.I.L. IC base (1 No.), Multistrand wires, solder metal, leads with crocodile clips, suitable mounting cabinet.

Believe it or not, there are numerous occasions when you have switched. on your TV set to watch a specific program of your interest and you found out that there are still a few more minutes to go. The options before you are to either remain glued on to the TV get and wait for your program or switch off the TV set and then switch it on again after sometime. The first option is boring and with the second, you always run the risk of missing a part of the program if you get busy in something else and forget to switch it on in time. Here is a very simple and interesting gadget that could be very easily mounted on your electrical switchboard and which can be used to perform the auto switch on function for your TV set (or fortran matter any other mains operated system) at a time programmed by you. With this gadget, you could set a timing of 99 minutes maximum with a resolution of one minute. For instance, if you want that your TV set should automatically switch on fifteen minutes from now, all you have got to do is to set l5 in the BCD switches, reset the gadget and press start. Remember that you have to keep the ON/OFF switch on your TV set in the On-position. The gadget can also be bypassed lf you so desired with the help of a switch provided on the gadget itself.

CIRCUIT DESCRIPTION

The circuit shown in Fig. 13.1 is identical to the one described in case of Electronic Reminder gadgetry the previous project except for addition of a power supply section that generates +l2V DC for circuit operation and the output of lC-2B driving an SCR to energise a relay rather than a microbuzzer and an LED as in case of reminder gadget. The ICs used are the same. IC-1 is a Quad 2-input NAND wired as two independent denouncing circuits, 1C-2 is a dual D-type flip flop, IC-3 is a 555 timer generating clock pulses at a rate of one clock pulse per minute, IC-5 and IC6 are programmable Binary/Decade UP/DOWN counters, IC-4 is a Quad 2-input AND gate. All these ICs along with their pin designations and functions have already been discussed at length in project-l2 and shall not be repeated here. The auto switch on gadget operates as follows: Initially, both MASTER RESET (MR) and START outputs are LOW. Set the time in minutes from the BCD switches S4 and S5 after which you want your gadget (TV, VCR etc.) to switch on.

Having programmed the time delay, press and release (Sl) to apply the reset pulse. The positive going reset pulse resets flip flop IC (IC-2) and also loads the programmed time delay information into counter ICs, IC-5 and IC-6. Since the output of IC-2A is initially LOW, the AND-gate is disabled and the clock pulses are not allowed to reach the clock input terminal of counter lC-5 when we press and release (S2), IC-2A is clocked and its output goes HIGH, the AND-gate is enabled and the clock pulses are allowed to the clock input of IC-5, This is the time when the time delay begins. Also, as the power to the TV set or VCR is through the normally open relay contact, the system remains off. We shall not go into the detailed operation of counter ICs but it would suffice to say that (TC) output of IC-6 is normally HIGH and that it would go to LOW state and then HIGH again when the preset time delay has elapsed. This pulse appearing at (TC) output of IC-6 clocks IC2B whose output goes from LOW to HIGH. This LOW to HIGH transition triggers the SCR thus energising relay coil RL-1. The normally open relay contact closes and the system is switched on. The output of IC-2B also forces the Clock Enable (CE) input of IC-5 to go HIGH and disable the clock. The system remains on unless you reset the system.The counters have been wired in the DOWN count mode as it is only then that the counter IC in LSB position completes its count cycle in a number of clock pulses equal to the number set in the LSB BCD switch. The counter connected in MSB position then completes its count cycle in a number of clock pulses equal to ten times the number set in the MSB BCD switch. This gives a total time delay equal to the time period of (N) clock pulses where (N) here is the decimal number preset with the help of BCD switches (S4) and (S5). As the clock period is one minute, the time here can be set in steps of one minute. In essence, maximum time delay that is achievable from the gadget is 99 clock cycles’ Period with a time resolution equal to period of one clock cycle.The power supply section is a conventional step- down transformer, two-diode fpll wave rectifier (using a transformer With a center tapped secondary winding) and a capacitor filter configuration followed by a 12V output three     terminal regulator.

CONSTRUCTION GUIDELINES

Figs. 13.2 and 13.3 respectively show the PCB layout and the components layout.

PARTS LIST

Resistors and Capacitors

R1, R2, R3, R4 : 22K 1/4 Watt

R5 : 2.2M 1/4 Watt (Metal film)

R6 : 680K, 1/4 Watt (Metal film)

R7 : 5.6M, 1/4 Watt (Metal film)

R8 : 220K 1/4 Watt (Metal film)

R9 :  1.5K, 1/4 Watt

R10 : 220K, 1/4 Watt

P1 : 100K multitude trimmer potentiometer

C1 :  (Ceramic Disc)

C2 :  16V tantalum)

C3:  (Ceramic Disc)

C4:  Rev (electrolytic)

C5,C7:  (Ceramic Disc)

C6 :  16V (Electrolytic)

Semiconductor Device: and ICs

D1 to D4 : 1N4001 or equivalent

IC-1 : CD4011B

IC-2 : CD4013B

lC-3 : 555

IC-4 : CD4081B

IC-5, IC-6 : CD4029B

SCR-1   : OE101 or equivalent

VR-1 : Three terminal Regulator, type 7812

OTHER COMPONENTS

S1 , S2 : Miniature push button microswitches

S3 : Mains power ON/OFF switch

S4, S5 : BCD switches (Thumb- wheel switches)

S6 : ON/OFF switch (230VAC, 1A)

RL-1 : 12DC Relay with at least one normally open contact

T-1 : Mains transformer (Primary: 230VAC Secondary: 14-0-14 250mA)

F-1 : Fuse (0.5A) with holder

Miscellaneous

IC Bases (8 pin, 14pin, 16pin), Multistrand wires, Solder metal, Suitable mounting cabinet, etc.

CLOCK PERIOD ADJUSTMENT, PINCONNECTION DIAGRAMS

Clock period should be precisely adjusted to one minute. The adjustment guidelines are the same as explained in case of Project: 12. The pin connection diagrams of different ICs being used have already beeh given in Project: 12. Pin connections for the SCR are shown in Fin. 13.4.

Here is a simple battery charger circuit, the one that can be used to charge 12 volt batteries of both the usual automobile type as well as the maintenance free sealed lead-acid type. The charger circuit being described here is quite   compact can be placed right on the top of the battery required to be charged. In the event of your car battery misbehaving or having become weak, what not require In addition to this gadget. Is only the |230VACmains.230VAC point is quite conveniently available even if you are stuck somewhere on the roadside or – elsewhere far away from a place where you can get your battery charged. This gadget could be one of the most important tools in your tool-kit and you could make use of this gadget to keep your battery healthy. Another significant point about this charger circuit is that it provides you swell regulated source of   constant charging voltage. And that Is what   is recommended for charging of lead-acid     batteries. The gadget also provides to you   metering of both voltage across the battery being charged as well as the charging current being drawn by the battery throughout the charging process. This tells you the status of the battery being charged as the time progresses.

CIRCUIT DESCRIPTION

The circuit is nothing but an AC/DC powersupply that generates a regulated DC voltage of 13.2volts from AC mains. Transformer T-l Diodes D1 and D2 and Capacitor C1 constitute the unregulated power supply portion with D1 and do alongwith transformer providing voltage transformation and rectification and C1 providing the filtering action.IC-l is a three terminal regulator of the type 7812. The common terminal of this regulator has been lifted to a potential of about 1.2 volts by the forward biased diodes D3 and D4   so as to give a regulated output voltage of 13.2volts instead of 12V which would be to: case if the common terminal was grounded. C2, C3 are decoupling capacitors. Diode do provides the charging path for the battery. The charging current flows from the power supply to the battery. The diode D5 also prevents the current to flow in the opposite direction i.e. from battery towards the power supply.Meter M-1, which is a current meter, gives a continuous reading of the charging current. The current reading becomes zero when the battery is fully charged. Meter M-2 gives the reading of the voltage across the battery as it is being charged. A fully charged battery typically has an open circuit voltage of 12.5 volts or so.

CONSTRUCTION GUIDELINES

Refer to Fig.10.2 for PCB layout and Fig.l0.3 for components layout. Lead identification of lC-1 is given in Fig. 10.6.

PARTS LIST

Resistors and Capacitors

R1                    : 2.2 K, 1/4 W

C1                    : 1000  25V (electrolytic)

C2, C3              : 0.1  (ceramic disc)

Semiconductor Devices and ICs

Diodes D1, D2, do : BY 127

Diodes D3, D4 : 1N4001 or equivalent IC-1   : 78T12 (lt is 7812 in TO-3 package) LED-1 : LED (Any Colour)

Miscellaneous

Meter M-1 : Ammeter 0-3A (Fig.10.4)

Meter M-2 2 DC Voltmeter 0-15VDC (Fig.10.5)

Transformer T-1 : 15-0-15, 2A   Fuse F-1 : 1A tubular type with holder Power supply terminals, solder metal, wires, mains power ON/OFF switch

TESTING GUIDELINES

Testing the assembled gadget is straight forward.

Switch on the AC power. Measure the regulated DC output voltage across C3.13.2 to 13.4 volts appearing across C3 shows that the gadget is    ready for use. Additionally, the load delivering capability of the charger can be ascertained by temporarily connecting a 6 to 10 ohms (25 watt) resistance across the output in place of the battery. Keep the gadget ON for at least ten minutes and see that there is no change in the regulated DC output voltage and also that there is no excessive heating of the transformer and regulator.

LEAD IDENTIFICATION OF MAJORCOMPONENTS

Fig.10.6 shows the pin connection diagram of IC 78T12.

Note: Photographs shown in Figs. 10.4 any 10.5are only representative ones to give an idea as to how the meters may look like.