Electronic Circuits Zone

SWR Meter Circuits #01

2009-02-28T22:07:00.000-08:00

For Everyone involved with radio transmitters needs some instruments to assess basic antenna functionality. Among these instruments, the best-known and most-used one is the Standing Wave Ratio meter. Some radio amateurs develop a cult for these little gadgets, having them in line all the time and watching the needles bounce while they chat.

Wideband SWR meter - Transmission lines have a certain characteristic impedance, typically 50 or 75 Ohm for coaxial cable, and about 300 to 450 Ohm for open-wire balanced feedline. Such an impedance stating means that the cable is naturally suited to carry a ratio of voltage to current according to this impedance - for example, a 50 Ohm coaxial cable should carry 1A for every 50V applied to it, and the phase of voltage and current should be the same.
Switch-out SWR Bridge - The circuit uses a double pole changeover switch (Measure or Operate), a SWR C/O switch Forward & Reverse, three 50 ohm resistors rated at 1/4 RF power, one RF diode (OA81), two 1nF capacitors, a series meter calibration variable resistor (10K) and a meter (100uA).
Power and SWR Meter - The circuit uses bright LED's to display the power and SWR in binary coded format. This results in a very compact test instrument.The SWR/Power meter is powered by two internal "AAA" battery cells. The meter automatically powers itself down after a period of inactivity to conserve battery life.
A Simple SWR Meter For QRP - This circuit should work with QRP rigs in the HF bands, although I have made no attempt to try it on other than 20 and 40.
A Wifi (V)SWR Meter - The circuit on this page serves as a Voltage Standing Wave Ratio meter or VSWR meter for the VHF-UHF-SHF frequency range. This includes the Wifi (802.11b/g), 2m, 70cm and 13cm Radio Amateur bands.

Variable DC Power Supply 5-15V

2009-01-02T09:26:00.000-08:00

This is a variable DC power supply that is similar I used to power my FM Transmitter. After suffering long problems with mains hum, this design using a pi filtered C-L-C approach. This circuit offers excellent ripple rejection.

See at the above schematic, the specific inductance of the ferrite (core)is important. A core should be chosen to work within the specific frequency as stated by the manufacturer. L1 is a powder core and has 32 turns of 0.75mm wire.

The transformer of this Variable DC Power Supply has a 240V primary and has a secondary rated 24V at 2A. The bridge rectifier contains 4 diodes, their current rating needs to be high with respect to the transformers output current; if not the current may damage the diodes. I used MR751 which is rated 6 amps, but another good choice is 1N5400. C1 is the mainfiltering capacitor, the supply is further smoothed by the combination of L1 and C3. C2 and C4 are decoupling capacitors; their action further reduce ripple factor.

The regulator, U1 utilizes the action of zener diode ZD1 which is in parallel with the potentiometer, R1. The tuning action of R1 produces a variable regulator output. The output voltage is variable from the regulator output to the regulator output plus the zener voltage. E.G. A 7805 regulator and 10V zener give an output adjustable from 5 to 15 Volts. The regulator may be changed to provide different output voltages as may the zener. the zener should be rated a minimum of 1.3 Watts. All the parts should be available at local electronic shops.

Parts List of the Variable DC Power Supply:
T1 Transformer 10:1 Secondary 24V @ 2A
BR1 Bridge Rectifier 50V PIV 2A rating
C1 4700u (35V)
C2 0.001u
C3 2200u (35V)
C4 0.001u
C5 4.7u (25)
C6 0.01u
R1 10k potentiometer
L1 see text
U1 7805 N.B. This may be changed for different output voltages e.g. 7812 for higher output voltage
ZD1 15V zener @ 1.3W

More Variable DC Power Supply

Uninterruptible Alarm Power Supply

2009-01-02T09:14:00.000-08:00

Although this Power Supply was designed for the Modular Burglar Alarm - it has other applications. It provides an output of 12-volts - at a current of up to 1-amp. In the event of a mains failure - the back-up battery takes over immediately. And when mains power is restored - the battery recharges automatically.

The 7805 needs the larger heatsink because it has to dissipate a lot of energy - especially when called upon to recharge a flat battery. Its heatsink is at 9v1 - and must NOT be connected to ground. The 7812 never has to dissipate more than 2-watts - so its heatsink can be smaller.

Many of the components, which are shown lying flat on the board, are actually mounted standing upright. The links are bare copper wire on the component side of the board. The heatsinks are folded strips of aluminium, about 2mm thick. Use a well-insulated panel mounted fuse holder for the mains supply to the transformer - and fit it with a 1-amp fuse.

Use a genuine alarm type back-up battery. They are maintenance-free. Their terminals can be held at 13v8 for many years - with no apparent ill effects. They have a life expectancy of about five years. However, they tend not to recover from a very deep discharge. If you wish - you can use a smaller or larger capacity battery.

Uninterruptible Alarm Power Supply

24 VDC Power Supply with maximum Current 5A

2008-12-02T22:52:00.000-08:00

Here's a 24 VDC power supply circuit with voltage input 20-30 Vcc. This power supply produces voltage output 13.5 VDC maximum current 5A.

The features capability of this 24 VDC power supply are protection against short circuit current, over temperature, over output voltage, and inversion polarity.

See 24 VDC Power Supply schematic below.

Parts list of the 24 VDC Power Supply
C1 = 33uF 35V
C2 = 10 nf 50V
C3 = 10nF 50V
C4 = 33uF 35V
D1 = 1N5400
D2-D3 = 1N4004
D4 = Zener Diode 15V 5W
IC1 = IC2 = 7812
Fuse = 5A

Simple Rain Detector

2008-11-26T17:08:00.000-08:00

Here's a simple rain detector circuit. It uses a sensor made of a small piece of etched PC board and a simple SCR circuit to detect rain and sound a buzzer. The SCR could also be used to activate a relay, turn on a lamp, or send a signal to a security system.

Rain Sensor and Alarm
The sensor made of a small piece of PC board etched to the pattern showen in the schematic. The traces should be very close to each other, but never touching. A large spiral pattern would also work. A loud buzzer used as an alarm.

Rain Detector Parts List

R1 = 1K 1/4 W Resistor
R2 = 680 Ohm 1/4 W Resistor
D1 = 1N4001 Silicon Diode
BZ1 = 12V Buzzer
S1 = SPST Switch
SCR1 = C106B1 SCR 106CY
SENSOR = See Notes
MISC = Board, Wire, Case, PC Board (For Sensor)

More about rain detector

Coil Coupled Metal Detector

2008-11-24T18:06:00.000-08:00

Another metal detector circuit where coils overlap to produce tone. Position out of phase on search head. It is based on a standard transformer coupled oscillator (TCO) - hence the name Coil Coupled Operation (CCO) Metal Detector. Although requiring a BFO (in this case provided by a Medium Wave radio), it differs from a typical BFO detector in that its performance far outstrips that of BFO.

The metal detector is dependent on the balance of two coils to boost sensitivity. It also differs from IB, in that its Rx section is an active, rather than passive, component of the oscillator. Further, unlike IB, the design does not require critical placement of the coils. As with both BFO and IB, the design provides discrimination. Tune through MW radio or C3

More about Metal Detector Circuit

Beat Balance Metal Detector

2008-11-24T17:47:00.000-08:00

Here's a metal detector circuit that frequencies of the two oscillators are then mixed in similar fashion to BFO, to produce an audible heterodyne. On the surface of it, this design would seem to represent little more than a twinned BFO metal detector.

What makes the metal detector different above all else (check a simple metal detector), and significantly increases its range, is that each coil modifies the frequency of the adjacent oscillator through mutual coupling. This introduces the "balance" that is present in an IB metal detector, and boosts sensitivity well beyond that of BFO.

Beat Balance Metal Detector Source

Circular Polarized FM Broadcast Antenna 88-108 MHz

2008-11-23T01:53:00.000-08:00

Here's the diagram of Circular polarized FM antenna for FM broadcast band 88-108 MHz. If you want to stack the antenna use jumper cable as you can see at the diagram.

See more about FM antenna stacking. Welcome to experiment

FM Antenna Diagram in detail

Miniature FM Transmitter

2008-11-23T01:38:00.000-08:00

This simple circuit allows you to transmit audio signals over an area of about 100 meters radio. The signal emitted by this FM transmitter can be tuned anywhere in the Dial your FM radio, because their frequency of transmission can be easily located between the 88 and 108MHz.

Its uses of the fm transmitter are unlimited, can be used as a monitor for babies, such as wireless microphone for conferences, to transmit the audio from a PC to another point of the house.

The miniature FM transmitter has been designed in such a way that does not exceed the limits of its frequent swing that between 88 and the 130Mhz and the field generated by the radiation, no more than 50mV per meter, at a distance 15cm of the circuit.

Miniature FM Transmitter in detail

A Simple Digital RF Power Meter

2008-11-22T05:31:00.000-08:00

This electronic circuit is a simple digital power meter. It is designed to operate as an inline power meter and displays the results on a 16 digit LCD display. It measures, in one range, power from 100 nW to 100W ( 9 orders of magnitude ) covering all HF bands up to 2 metres. It is usable with correction to 70 cms.

The meter comprises two separate parts, an RF unit and a display unit. The interface between them is a DC voltage across a specified range. It is therefore possible to reuse either part in different ways - for example the RF unit may be built without the digital display either to drive an exiting meter or as an RF sniffer.

The unit operates over a 90 dB range from 100KHz to 500 MHz and displays its results on the LCD display in both dBm and Watts. The power source is 9 to 15 V DC consuming about 12 mA. Subject to calibration - and that is a big assumption - the meter is accurate to within +/- 1dB from 200 kHz to about 175 MHz, and the deviation from log law is +/- 0.5 dB. The frequency response is shown in Figure 1 Figure 2 and Figure 3. At 70 cms it reads about 5dB low.

The RF unit is based on the Analog Devices excellent log detector chip, the AD8307 [1]. The display is based on a single chip PIC processor, the PIC14C000 Mixed Mode Controller which requires programming. This can be achieved via a PC bi-directional parallel printer port using a small additional circuit if a programmer that will handle the 14000 is not available.

A Simple Digital Power Meter in Detail

FM Transmitter 2 Transistor

2008-11-20T22:16:00.000-08:00

A major improvement over the little FM transmitter 1-transistor circuit that we've discussed previously, this transmitter consist of two stages. The first transistor is used to amplify audio, which means that the microphone is now much more sensitive to sound. The second one acts as an oscillator.

All in all, this is an interesting project for beginners and more experienced hobbyists alike. Use any stiff wire or telescope whip antenna, just make sure that it's not too long. The range of the transmitter when running at 9V is about 300 feet. Running it from 12V increases the range to about 400 feet. This transmitter should not be used as a room or telephone bug.

More about FM Transmitter 2 Transistor

FM Transmitter 1 Transistor

2008-11-20T22:06:00.000-08:00

This is a truly minimalist circuit of FM Transmitter. Performance is relatively poor and stability is a problem. Q1 modulates the signal and acts as an oscillator. L1 and C5 determine the frequency. Decrease C5 for a higher frequency and vice-versa. It's suitable for beginners for their first fm transmitter project.

With one FM transmitter you won't get a long range transmission with this one. The author claims 50-500 feet. A lot depends on the antenna. It must be made of stiff wire or else the frequency will change as you move the antenna. Experiment with different lengths to get the maximum range. 50cm or less should work reliably most of the time. A longer antenna might give even better range but if you exaggerate, the circuit will begin to oscillate and performance will drop dramatically. A regular telescope whip should work, too.

If assembling the circuit on perf board, all connections should be kept short. You'll probably notice that this transmitter is not particularly stable. It might be difficult to tune a receiver to it as the operating frequency will vary a little with time and temperature or if someone steps close to the antenna. Here's Fm transmitter parts list and PCB..

Tiny 2-Zone Alarm

2008-11-20T21:48:00.000-08:00

This a really small alarm that could easily fit inside a pocket. However, I also demanded reliable operation, simple construction and very low power consumption. I started with CMOS logic gates, but was soon forced to abandon the concept after a few unsuccessful (and far too complicated) takes.

As you can clearly see from the schematics, the circuit is utterly primitive and consists of two identical transistor switches. Each has its own alarm LED and they're coupled to a neat 82dB buzzer. The two 1N4148 diodes are used to prevent a signal from one sensor from triggering both LEDs. The sensors are connected to the jumpers. Use either loops of very thin (like 0.1mm) enamel coated wire or normally closed reed switches or even a combination of both. Tiny 2-Zone Alarm in detailed

Regulated Power Supply 78xx

2008-11-20T21:38:00.000-08:00

Here's an universal circuit of all the voltage regulator ICs, the 78xx family is by far the most established. Not surprisingly! It's difficult to get performance this good with so few outside components necessary.

The 78xx family just like the famous LM317, incorporates automatic shutdown in case of overheating. You can get up to 1.5A of current with a suitable heat sink. No fuse is required.

Regulated Power Supply 78xx

100 MHz Ground Plane FM Antenna

2008-11-20T21:29:00.000-08:00

Here's a simple fm antenna plan you can find. This simple ground plane antenna works well in the FM Band 88-108 MHz. It's built of a small plastic disk (around 5cm in diameter, not critical) and stiff copper wire (preferably enamel coated) some 3mm thick. It's omnidirectional, therefore it works equally well in all directions. This makes it suitable for most applications.

100 MHz Ground Plane FM Antenna Plan

Variable Power Supply 1-25V LM317

2008-11-20T21:18:00.000-08:00

This is a variable power supply circuit uses LM317. LM317 is a versatile and highly efficient 1.2-37V voltage regulator that can provide up to 1.5A of current with a large heat sink. It's ideal for just about any application. Since LM317 is protected against short-circuit, no fuse is necessary.

Although LM317 regulated IC is capable of delivering up to 37V, the circuit pictured here is limited to 25V for the sake of safety and simplicity. Any higher output voltage would require additional components and a larger heat sink.

Metal Detector

2008-11-20T21:01:00.000-08:00

The main idea of metal detector is really quite simple: build two identical oscillators and adjust them to the same frequency. One of the oscillators uses the search coil while the second one incorporates a variable inductor. When both are operating at the same frequency, the output is zero. If the search coil moves near any metal, however, frequency of the first oscillator shifts and an audible tone is heard in the headphones.

More About Metal Detector Circuit

FM Transmitter 4W

2008-11-20T20:45:00.000-08:00

This FM transmitter circuit is fairly small and operates in the FM band 88-108 MHz. The last two transistors dissipate quite a lot of heat. Make sure to protect them with sufficiently large heatsinks or else you'll have a nasty meltdown. All off the coils are not all that critical, but should still be winded carefully to ensure efficient operation. Use enamel-coated wire at least 1mm thick. As with all RF circuits, work slowly and pay attention to details.

More about FM Transmitter 4W

Linear Power Supply 13.8V 20A

2008-11-20T11:38:00.000-08:00

This linear power supplycircuit has a transformer that steps down the line voltage to some voltage that is higher than what will be required at the regulated output. Then a rectifier and a filter capacitor transform the low voltage AC into a moderately filtered DC that still is unregulated and has some ripple. Finally, a regulating circuit "burns off" the excess voltage, leaving only the exact amount desired at the output, typically 13.8V for communication equipment.

More about Linear Power Supply 13.8V 20A

Switching Power Supply 13.8V 40A

2008-11-20T11:09:00.000-08:00

This power supply circuit produces 13.8V regulated to better than 1%, at a continuous load current of up to 40A. It has current limiting, making it appropriate for direct connection to a 12V backup battery. If the current limit potentiometer is turned up, the power supply can deliver up to 60A on an intermittent basis, while maintaining regulation. No minimum load is required. The ripple on the output is about 20mV, and the efficiency is 88%.

More Switching Power Supply

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2008-11-17T10:04:00.001-08:00

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