BPW34 Photodiode Pulse Detector
Shine an IR LED through your fingertip onto a photodiode and watch your heartbeat appear on the scope.
Difficulty: Through-hole. Estimated build time: about 60 minutes. Estimated parts cost: about US$4.00. 5-line bill of materials. Compare supplier offers when available. A photoplethysmogram (PPG) front end built from a BPW34 silicon photodiode, an LM358 transimpedance amplifier, and an infrared LED.
Build the transimpedance amplifier
Use one half of an LM358. Connect the photodiode reverse-biased between V_CC and the inverting input (−). The non-inverting input (+) goes to a Vref of V_CC/2 (a 2.2 V divider works for 5 V supply). A 1 MΩ feedback resistor between the output and the (−) input converts photocurrent to voltage: 1 nA in = 1 mV out. A 10 pF cap in parallel with the feedback resistor keeps it stable.
Add the IR illuminator
Wire a TSAL6200 (940 nm IR LED) in series with a 100 Ω resistor from V_CC to ground. It glows invisibly. Place it about 5 mm from the BPW34, oriented so that your fingertip will sit between them in transmissive mode (light passes through tissue) or reflective mode (light bounces off the back of your finger).
AC-couple and amplify further
The TIA output sits near Vref with tiny variations on top. Couple it through a 1 µF cap into a second LM358 stage with gain ~50 and a high-pass corner at 0.5 Hz (so DC drift is rejected). The final output should swing ±500 mV around 2.5 V — the live heart-rate waveform.
See the heartbeat on a scope
Probe the final stage output. Press your fingertip gently between the LED and photodiode. After a few seconds the waveform stabilises into the classic PPG shape: a sharp upstroke (systole), a small notch (dicrotic), and a slow decay back to baseline. Count the peaks in 15 seconds and multiply by four — that’s your bpm.