This electronic circuit is an infrared microphone, this electronic circuit allows you to listen infrared radiation.
This circuit responds to infrared radiation, using common electronic components and has a very simple construction.
IR microphone consists of a photodiode connected to input terminals BP104 DC of an operational amplifier, whose gain is determined by R1. The device is quite effective if the surrounding noise sources such as incandescent lamps, are extinguished. A gas flame, such as that produced by a lighter, manifested as a slight breeze and a quiet fire burning home is heard as a hurricane. This circuit can be used as a sensor for the fire alarm sound.
If BP104 replaces the BPW34 photodiode sensitivity can be shifted from the infrared device in the visible spectrum.