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Introduced a wireless FM transmitter

2016-04-02 07:40  
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 This article is a wireless FM transmitter introduction. To more effectively grasp this principle, the proposal combine text read schematics. The circuit is simple but effective.

 

 

 

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.

You can replace the coin cells, typically found in calculators and watches, with regular 1.5V AA, C or D-cells. The coin cells however take less room and can solder onto the protoboard.

 

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.

Make sure you correctly identify the 2N2222A’s pinout and correctly wire the base, collector and emitter in the schematic. Often, circuit malfunctions because the pins were mis-wired.

 

The leads for the variable capacitor do not fit in normal 0.1 inch protoboards. You can dremel-drill into the protoboard to make the leads fit. Alternatively you can solder wire to the leads, but if you do, keep wires as short as possible in order to avoid stray capacitances.

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 anair 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.

The component values in the circuit are derived to better understand how this FM transmitter will work. The underlying math is rather simple and can be found in most undergraduate university physics textbooks.

Your self-made inductor has a value determined by its radius , length and number of wire turns .

For your McDonald’s soda straw inductor, inches, inches and turns and results in microHenry or 0.000000171 Henry.

The specific frequency generated is now determined by the capacitance and inductance measured in Farads and Henry respectively:

 

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:

Referring to the schematic, and act as decoupling capacitors and typically (or ) are used. attempts to maintain a constant voltage across the entire circuit despite voltage fluctuations as the battery dies.

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).

The capacitor across the 2N2222A transistor serves to keep the tank circuit vibrating. In theory, as long as there is a supply voltage across the parallel inductor and variable capacitor, it should vibrate at the resonant frequency indefinetely. In reality however, the frequency decays due to heating losses. is used to prevent decay and the 2N2222A spec sheet suggests a capacitance between to .The spec sheet for the Jameco #136573 electret microphone says the maximum current is . When battery powered at , then the voltage drop across is . The resulting current through the microphone is below the rated maximum since.The 2N2222A transistor has rated maximums thus demanding a voltage divider made with and and emitter current limiting with .The 2N2222A’s maximum rated power is . This power ultimately affects the distance you can transmit. Overpowering the transistor will heat and destroy it. To avoid this, one can calculate that the FM transmitter outputs approximately and is well below the rated maximum. The mathematical details are given in rfMath.pdf.

 

 


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