This is a Phototransistor heartbeat Sensor board, phototransistor used to obtain the flux emitted, when the blood pressure pulse by the finger when the resistance of the phototransistor will be slight changed. We chose a very high resistance resistor R1, because most of the light through the finger is absorbed, it is desirable phototransistor sensitive enough. Resistance can be selected by experiment to get the best results. The most important is to keep the shield stray light into the phototransistor and faint heartbeat will add considerable noise. Specification: Color: Black Material: PCB Working Voltage: 5v Length: 19mm Width: 15mm.