As shown, the circuit due to the integrated amplifier, so the circuit is simple, small size. TDA4050 IC used here, a weak signal is received by the infrared photodiode VD, firs
t passes through the transistor amplifier. As a result of the converging lens (diameter of about 15mm, a focal length of about 30mm), and therefore the effective distance can be increased 20 to 30 times. Capacitance C can effectively reduce interfering signals in the low frequency range of 50 ~ 100Hz.