The circuit schematics for our transmitter
The circuit schematics for our transmitter
Below is a block diagram of our circuit showing the functional elements of an FM transmitter.
RF Oscillator
The oscillator is the primary subsystem in the telephone bug circuit. It determines the FM band at which the audio signal is heard (the carrier frequency). The main components of the oscillator are the transistor (Q1), the inductor (L1) and capacitors (C2, C3, C4). The oscillator is L-C tuned meaning that the frequency of oscillation is controlled by the inductance and capacitance in the circuit. Specifically, inductor, L1, capacitors C2, C3, and the capacitance in the transistor control the natural resonant frequency of the oscillator. By fixing the inductor and using a variable capacitor, the oscillator can be made to resonate between 88Mhz and 108Mhz. Thus the two component values which radically affect our circuit are the inductor value, and the capacitance value of C2. We also note that the inductor is actually 6 inches of 30-gauge enamelled wire.
Audio Amplifier
The audio amplifier is used to increase the amplitude of the audio (voice) signal to boost the range of the transmitted signal. The amplifier is basically a common-collector transistor amplifier consisting of transistor, Q1, and a resistor, R1. It is also interesting to note that the load on the transistor is the oscillator circuit itself. (Smart!)
In order to amplify a signal it takes power. This power is supplied by the telephone line itself. When the telephone is off the hook a potential of 6-8 volts is found across the red and green wires of the telephone cable. This potential causes a current to flow through resistor R1 which biases the transitior Q2 into operation. Any audio frequencies on the telephone line will be amplified and appear on the collector of the transistor. Q2 is a PNP transistor since large currents will keep the transistor off. Since when the telephone is on hook, a potential of ~50V is found across the red and green wires. The green wire acts the positive supply and the red wire acts as a circuit ground.
Modulator
The modulator is the subsystem that combines the audio signal with the carrier signal from the oscillator to allow the audio signal to transmit over radio waves. As the audio signal fluctuates, the current into the base of the transistor fluctuates similarly, and changes the capacitance between the collector and emitter of the transistor Q1. As shown above in the oscillator circuit, a change in capacitance results in a change in frequency. Therefore, as the audio signal changes, the frequency changes or modulates.
Antenna
The antenna is used to transmit an electrical signal through the air waves. The size of the antenna determines the length of broadcast. Our antenna is a 6", 22-gauge wire. We could use a longer one to get a larger broadcast range but we didn't for a couple reasons. First, the FCC might get upset. Second, the longer wire would have some inherent capacitance and impedence that would have to be compensated for by changes to other parts of the oscillator circuit.
Population Diagram
This is the population diagram for the actual circuit board we ordered. It helps make sure the components are in the right orientation. This is the component side of the PC board.