Category Archives: Radio

FM Beacon Transmitter (88-108 MHz)

This circuit will transmit a continuous audio tone on the FM broadcast band (88-108 MHz) which could used for remote control or security purposes. Circuit draws about 30 mA from a 6-9 volt battery and can be received to about 100 yards. A 555 timer is used to produce the tone (about 600 Hz) which frequency modulates a Hartley oscillator. A second JFET transistor buffer stage is used to isolate the oscillator from the antenna so that the antenna position and length has less effect on the frequency. Fine frequency adjustment can be made by adjusting the 200 ohm resistor in series with the battery. Oscillator frequency is set by a 5 turn tapped inductor and 13 pF capacitor. The inductor was wound around a #8 X 32 bolt (about 3/16 diameter) and then removed by unscrewing the bolt. The inductor was then stretched to about a 3/8 inch length and tapped near the center. The oscillator frequency should come out somewhere near the center of the band (98 MHz) and can be shifted higher or lower by slightly expanding or compressing the inductor. A small signal diode (1N914 or 1N4148) is used as a varactor diode so that the total capacity in parallel with the inductor varies slightly at the audio rate thus causing the oscillator frequency to change at the audio rate (600 Hz). The ramping waveform at pins 2 and 6 of the timer is applied to the reversed biased diode through a large (1 Meg) resistor so that the capacitance of the diode changes as the ramping voltage changes thus altering the frequency of the tank circuit. Alternately, an audio signal could be applied to the 1 Meg resistor to modulate the oscillator but it may require an additional pullup resistor to reverse bias the diode. The N channel JFET transistors used should be high frequency VHF or UHF types (Radio Shack #276-2062 MPF102) or similar.

Copyright 2006 [url=http://www.bowdenshobbycircuits.info]Bill Bowden[/url]

Simple FM transmitter with a single transistor

Mini FM transmitters take place as one of the standard circuit types in an amateur electronics fan’s beginning steps. When done right, they provide very clear wireless sound transmission through an ordinary FM radio over a remarkable distance. I’ve seen lots of designs through the years, some of them were so simple, some of them were powerful, some of them were hard to build etc.

Here is the last step of this evolution, the most stable, smallest, problemless, and energy saving champion of this race. Circuit given below will serve as a durable and versatile FM transmitter till you break or crush it’s PCB. Frequency is determined by a parallel L-C resonance circuit and shifts very slow as battery drains out.

Technical data:

Supply voltage : 1.1 – 3 Volts
Power consumption : 1.8 mA at 1.5 Volts
Range : 30 meters max. at 1.5 Volts

Main advantage of this circuit is that power supply is a 1.5Volts cell (any size) which makes it possible to fix PCB and the battery into very tight places. Transmitter even runs with standard NiCd rechargeable cells, for example a 750mAh AA size battery runs it about 500 hours (while it drags 1.4mA at 1.24V) which equals to 20 days. This way circuit especially valuable in amateur spy operations 🙂

Transistor is not a critical part of the circuit, but selecting a high frequency / low noise one contributes the sound quality and range of the transmitter. PN2222A, 2N2222A, BFxxx series, BC109B, C, and even well known BC238 runs perfect. Key to a well functioning, low consumption circuit is to use a high hFE / low Ceb (internal junction capacity) transistor.

Not all of the condenser microphones are the same in electrical characteristics, so after operating the circuit, use a 10K variable resistance instead of the 5.6K, which supplies current to the internal amplifier of microphone, and adjust it to an optimum point where sound is best in amplitude and quality. Then note the value of the variable resistor and replace it with a fixed one.

The critical part is the inductance L which should be handmade. Get an enameled copper wire of 0.5mm (AWG24) and round two loose loops having a diameter of 4-5mm. Wire size may vary as well. Rest of the work is much dependent on your level of knowledge and experience on inductances: Have an FM radio near the circuit and set frequency where is no reception. Apply power to the circuit and put a iron rod into the inductance loops to chance it’s value. When you find the right point, adjust inductance’s looseness and, if required, number of turns. Once it’s OK, you may use trimmer capacitor to make further frequency adjustments. You may get help of a experienced person on this point. Do not forget to fix inductance by pouring some glue onto it against external forces. If the reception on the radio lost in a few meters range, than it’s probably caused by a wrong coil adjustment and you are in fact listening to a harmonic of the transmitter instead of the center frequency. Place radio far away from the circuit and re-adjust. An oscilloscope would make it easier, if you know how to use it in this case. Unfortunately I don’t have any 🙁

Every part should fit on the following PCB easily. Pay attention to the transistor’s leads which should be connected right. Also try to connect trimmer capacitor’s moving part to the + side, which may help unwanted frequency shift while adjusting. PCB drawing should be printed at 300DPI, here is a TIFF file already set.

[img:9473329737]http://www.circuitdb.com/download.php?fileID=110[/img:9473329737]
PCB design for the FM Transmitter

The one below is a past PCB work of mine, which was prepared to fit into a pocket flashlight. Since it was so crowded, use the new computerized PCB artwork instead, yet very small. Take a look at my [url=http://tacashi.tripod.com/elctrncs/pcbworks/pcbworks.htm]PCB design page[/url] to get information on my work style.

[img:9473329737]http://www.circuitdb.com/download.php?fileID=111[/img:9473329737]
Transmitter PCB compared in size with an AA battery

Here is a completed and perfectly running circuit, mounted in a pocket light, taking the advantage of the 1.5V AA cell slot near it. Microphone is fixed into the bulb’s place and antenna is made out of a 30cm soft cable. When cover is placed, it becomes very handy!

[img:9473329737]http://www.circuitdb.com/download.php?fileID=112[/img:9473329737]
Transmitter placed in a pocket flashlight

Do not forget, restrictions on radio frequency transmitting devices may differ in your local area. This circuit has a power output that should be less than 1mW so have to be safe under many kinds of legal conditions but particular attempts such as listening to other people’s private life will always be disapproved everywhere.

Copyright 2006 [url=http://tacashi.tripod.com/elctrncs/elctrncs.htm]Burak[/url]

Op Amp Radio

Here is a simple radio that is easy to build and inexpensive. In fact, you probobly have all the parts you need in your junk box. You’ll be suprised at the great reception with this little set.

[b:f4287f9cad]Notes:[/b:f4287f9cad]
1. The antenna can be a piece of wire or a telescoping antenna. 18 inches is a good length for in the city.
2. The tuning capacitor is a regular broadcast band tuning capacitor. I got mine from a junked AM radio. I got the loopstick antenna from that same radio.
3. You can change L1 and C1 to recieve different bands (eg. Shortwave). To recieve shortwave, try this: Make L1 30 turns of 30 guage wire wound on a film can and make C1 a 10-365pf capacitor.
4. Combine this circuit an amplifier for a really neat radio.

Source: [url]http://www.aaroncake.net/circuits/radio.htm[/url]Parts:1 x Tuning Capacitor (C1)
1 x 0.1uf Disc Capacitor (C2)
1 x 10 Meg 1/4 W Resistor (R1)
1 x 1N34 Germanium Diode (D1)
1 x 741 Or Similar Op Amp (U1)
1 x “Loopstick” Antenna (L1)
MISC: IC Socket, Crystal Earphone, Wire, Antenna