What is it?

This is a pulse width modulation (PWM) visualizer for your oscilloscope or multimeter. An OSRAM/Vishay BPW34s PIN photodiode is used to capture light from a source like a flashlight. A precision resistor is used to measure the output voltage of the reverse biased diode. This lets you visualize PWM on an oscilloscope or multimeter.

Why did you make it?

I'm a bit of a flashaholic and needed a way to test my driver circuits and flashlights that I purchased. A flashlight that drives an LED at a PWM frequency 256Hz is visible while running, hiking, or walking. This is obviously undesirable behaviour, so being able to test a product that you purchase is ideal!

How do I use it?

Run it off a battery. It can accept any low voltage. I run it off of a 4.2V lithium or 9V alkaline. The BPW34s has a breakdown voltage of 60V. Be safe.

The 2.54mm header is the battery input.

The 4mm holes are used for binding posts in an enclosure, or banana plug jacks to your oscilloscope or multimeter.

The output to your measuring tool will be between 0V and battery voltage. If your light source utilizes PWM with a DC bias, i.e., it doesn't hit 0V, this may pose a problem for inexpensive multimeters. Use an oscilloscope for better visualization of the output!

I've measured up to 500kHz PWM with this board.