Circuit capacitance three-point oscillator, simple, reliable, easy start-up. BG1 amplifier circuit composed of a small capacitor from the microphone to detect the faint sound signal amplification, up to a certain extent to the BG2 after the three-point oscillator capacitor formed on the BG2 generated carrier signal modulation, the final output from the antenna. Diode regulator circuit technology, which makes the circuit more stable work. Circuit as shown below:
BG1 with 2SC9014 equivalent power transistor. BG2 use high-frequency high power transistor 2SC9018, requires β ≥ 100, prone to instability amplification factor is too high, too low, vibration will not be conducive to the circuit device. MIC selected micro-thin condenser microphone. L1 enameled high strength with Φ51mm Shangrao 8 laps in the pen core is obtained, with the C4 around the parallel resonant and 100MHZ. Antenna with a cord around 80mm long.
The entire circuit can be used in a size of about 3 × 2mm board, with ordinary FM radio in the 100M away the microphone should be able to clearly monitor the sound around the small less than 5M.
For a long time stability, 3V battery-powered circuit, the whole consumption of the current 8-10MA.