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Show wireless FM transmitter circuit

2016-04-11 23:21  
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The following show is a wireless FM transmitter circuit. The circuit is very simple, and very practical. Recommend Duokanjibian The schematic in order to master this principle. I hope to be helpful to your work.

The photo shows a wireless FM transmitter, pocket radio and yellow disk for size comparisons.





A combination of wirewrapping and soldering was used to construct the FM transmitter. Jameco’s prototyping card provides enough room for (non-critical) part placement. You should try to keep all parts close together and keep wire leads short. The photos below illustrate possible part placement (left) and the solder side (right).


An electret microphone has two pins which connect to the positive and negative leads of a battery. As shown in the drawing below, one looks at the bottom of the electret microphone. The pad that physically touches the microphone’s casing connects to the battery’s negative lead.

The 2N2222A  is a very common NPN transistor. The one used here (Jameco #38236) is the metal can type (TO-18 casing). Its three pins are for the transistor’s base (B), collector (C) and emitter (E). There is no standard pinout for transistors. As such, request the transistor’s spec sheet when ordering it to identify the pinout, or if you own a multimeter with a transistor tester, use it.The 2N2222A also comes in a black plastic casing (TO-92 style) which you can use if you want. The T0-18 is preferred because the can has a small tab that? represents the emitter pin.

An inductor is just a coil of wire and you need to wind one for this circuit. An inductor is characterized by its length, radius and the number of turns of wire in the coil. Magnet wire (Radio Shack part 278-1345) was used to build the inductor but you can use standard solid strand 22 AWG gauge copper wire.

Some on-line and printed articles describe winding the wire around a pencil. Unfortunately, pencils come in different diameters and hence a McDonald’s soda straw was used; the yellow-red-white striped straw, found in every McDonalds in the world, is the same size. The straw’s radius is exactly 0.1325 inches (diameter = 0.2650 inches) and 1/4 inches was snipped off the straw.

Next, a straight piece wire was wound around this 1/4 inch snippet six times and then soldered on the prototyping board. The end result is an inductor (also known as an?air corecoil) with an 0.1325 inch radius. If you wish, you can apply some womens’ clear fingernail polish to permanently keep the wire on the straw snippet.

A 30 inch long piece of 22 gauge solid stramd copper wire is a suitable antenna. However when carrying the transmitter, you risk tangling the wire. As such you can screw a telescopic antenna, like ones found a radios, into the prototyping board.

The variable capacitor and your self-made inductor will vibrate at frequencies in the FM radio band (88 to 108 MHz). The electret microphone has a resistance that depends on how loudly you speak into it. This microphone is battery powered and according to the? Ohm’s Law, changes in resistance for fixed voltage will result in proportional changes in current. This current feeds into the base of the 2N2222 NPN transistor which is connected to your variable capacitor, inductor and antenna. The net effect is that depending on your variable capacitor’s value, your voice will be modulated to transmit at a frequency between 88 and 108 MHz. If a nearby pocket FM radio is tuned to this frequency, you’ll be heard when speaking into your transmitter.



You built your antenna either with a piece of solid strand 22 gauge wire 30 inches long or used a telescopically extendable antenna. Its length should be approximately 1/4 the FM wavelength; recall that multiplying frequency and wavelength equals the speed of light. You’ll most probably be operating your transmitter near 108 MHz, as such:

A capacitor can be thought of as a frequency-dependent resistor (called reactance). Speech consists of different frequencies and the capacitor? impedes them. The net effect is that? modulates the current going into the transistor. Using a large value for? reinforces bass (low frequencies) while smaller values boost treble (high frequencies).

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