In this lab (ECE2k7), I built an optical heart rate sensor using an infrared emitter/receiver pair, two first order low and high pass filters, and a comparator connected to an LED indicator. The emitter and receiver were the IR204 and PT204-6B, respectively. The filters used op-amps from the LM324 chip. The comparator was made from the LM339 chip. The emitter/receiver pair measured infrared signal through our finger, and output a small amplitude, large offset wave to go through the bandpass filter. Once it reaches the filter, any signals with frequencies outside the regular heart rate range (40-200 bpm), cannot pass. Otherwise, the filter will amplify the signal with a linear gain of around 500X. Afterwards, it enters the comparator, configured as an analog to digital (A2D) signal converter so that our LED can only be on or off. The comparator is designed with hysteresis to remove as much noise as possible. The combination of all these will create a circuit that lights up an LED matching your heart rate.