The circuit shown here is a simple stereo FM transmitter that can transmit signals of high quality with a range of 70 feet. The circuit is based on BH1417 PLL Stereo Transmitter IC semiconductor Rhom. The IC has separate sections of audio processing for the left and right, the circuit of antenatal care to improve the signal to noise ratio, the control circuits of glass blocking the precise frequency, multiplex circuit for making the sum (left over right) and the difference (to the left without right) {See this article for a better understanding of the stereo decoder circuit} etc.
Another important feature of this circuit is that the transmission frequency can be set using a channel 4-switch DIP. The IC can be powered by a figure between 4 to 6 V DC and has an output of about 20 mW. At full power output of the circuit consumes only 20 mA and has a channel spacing of 14 possible frequencies 40dB.There transmission default, from 88.7MHz and increasing in steps of 0. 2MHz can be selected via DIP switch. The PLL circuit of the IC is so precise that there is virtually no frequency drift
Notes.
- Assemble the circuit on a good quality PCB.
- The circuit can be powered from anything between 4 to 6V DC. I recommend using a 6V battery pack.
- If you are using a battery eliminator for powering the circuit, it must be well regulated and free of any sort of noise.
- Antenna can be a 1 metre long insulated copper wire.
- Table for selecting the frequency is shown with the circuit diagram.
- Crystal X1 is 7.68MHz.
- For L1 make 2.5 turns of 1mm enamelled copper wire on a plastic former with an F29 ferrite slug inside it.
- Optional 10K POTs can be connected in series to the input pins (pins 22 and 1) of the IC for adjusting the channel balance.
- All electrolytic capacitors are rated at 10V DC.
