This introduction of this device can be used to get an indoor or outdoor faint voice, tried to get somewhere as long as needed to install a snooping device, the 100M can clearly get an external FM radio sound somewhere.
Circuit as shown, it is highly sensitive microphone and wireless transmitter amplifier composed of amplifier consists of a quad op amp IC LM324 op amp in the two components.
When the electret microphone MIC picked up a weak voice signal, the signal will be generated across the voltage at the MIC, through capacitor C1 coupled to the first stage operational amplifier (3) feet to zoom in, zoom in up to 20 dB gain, after the first level amplification, from (1) pin output, coupled through capacitor C3, potentiometer RP control into (5) feet for the second stage amplification, after which two enlarged, weak enough to be any sound signal amplification to have sufficient range, C5 then coupled to the high-frequency transmitter circuit.
High-frequency oscillation circuit is composed of special high-tubes V and LC oscillator circuits, the design of its resonant frequency in the FM band range of 88 ~ 108MHZ, audio modulated carrier W by transmitting antenna to the surrounding space launch, with a FM radio you can listen to outside the 100M, of which R5 and R7 for the op amp's negative feedback resistor, the resistance can change the zoom to adjust its gain.
Component selection and commissioning
Selection of miniature electret microphone MIC to produce appropriate sound on the outside plus a poly pots, the MIC into a 25 mm diameter plastic tube, plastic tube in the inner layer of flannel surface acoustic affixed to improve the quality of the sound source . Launch tube V high-efficiency tubes using D-50 or D-40, etc., the magnification is greater than 100. L with a diameter of 0.31 mm enameled wire in diameter of 5 mm 6 laps around the drill bit, born out of the coil will be stretched to about 5 mm can be. Antenna can be used 40 cm of soft wire.
Good quality of all components in the circuit, and the welding is correct, you can power debug, adjust the potentiometer so that PR (7) feet, moderate-intensity output, high-impedance headphones can be connected across the C5 between anode and ground monitoring, adjusting circuit L spacing or C is the capacity of transmitting frequencies fall within the range 88 ~ 108MHZ, and modulation edge distance, can generally be heard clearly on the outside in the 100M Snoop is transmitting voice signals over the