This module consists of an IR-LED and a photo transistor which can be used to read a pulse when a fingertip is positioned between the LED and photo transistor. The module requires additional external circuitry. A 330 ohm series resistor for the LED is included. The 5 V supply connects to the centre pin, ground to the ‘-’ pin. The photo transistor signal is available on the ‘S’ pin which has a built-in pullup resistor.