FM Transmitter, Power : 12 - 15 VDC 500mA 5 X 1.2 Mm

FM Transmitter is used for wireless listening to TV sound or a short range repeater. This low power transmitter for the VHF FM band has many applications such as a wireless babysitter or a short-range repeater. The transmitter may also be used for wireless listening to TV sound.
About the Circuit :

• As shown in the circuit diagram, microphone MIC receives its bias voltage via resistor R1. The microphone signal is amplified by transistor T1. The amplified microphone signal may be taken from C2. When the transmitter is used for line signals, the microphone preamplifier is not used. Since these signals are going to the capacitor C6.
• The voice- operated switch receives at the preset P1. Preset P1 determines the input level. The signal at the wiper of P1 is fed to one of the inverter of IC1 at pin9 through capacitor C3. The next inverter at pin11, functions as a limiter. The rectified signal obtained through diodes D1/D2, resistor R7 and capacitor C5 raised to a suitable level for controlling switching transistor T2 through three inverter gates of IC1.
• Depending on the signal level at pin2, transistor T2 is off or on. Thus it controls the power supply to the RF oscillator, T3. Choke L1 prevents the RF signal developed by the oscillator from being short circuited by the power supply lines.
• LED D5 lights when T3 powers. The AF signal received at C6 is taken through a pre-emphasis network composed of C6/C7/R11 before it arrives on the modulation level control, preset P2. Frequency modulation of the oscillator is achieved by C8 superimposing the modulation signal on the bias voltage of dual varicap D3/D4. Transistor T3 oscillates at a frequency determined by tuned circuit L2/C10/D2, with C11 providing positive feedback the collector and the emitter.
• The oscillator frequency can be set between 88MHz and about 108MHz by adjusting trimmer C11. Capacitors C9 and C10 decouple the RF and the AF component respectively at the base of T3. The transmitter has given an aerial connection of 5cms wire from the capacitor C14. Its RF signal is radiated by L2..
• Power Supply : The circuit requires a DC supply of 9V.

Calibration :
Initially set P2 at its center and apply an audio signal to the transmitter. Tune FM receiver at 105MHz. Switch the transmitter on. Adjust trimmer C11 until the signal is received. Adjust P2 until the optimum volume setting is obtained without running into distortion.