Simple DIY light mater using photodiode with resistor and oscilloscope demonstration circuit

Simple DIY light meter using photodiode resistor voltage divider learning electronics lesson 0061
Simple DIY light meter using photodiode resistor voltage divider learning electronics lesson 0061

Putting a reverse biased photodiode in series with a 1,000 ohm resistor will result in the resistor having 1 volt across it for every mA of current that the photodiode allows to pass through them.

The light coming from the lamp I was using in my video, adjusts it’s brightness by turning on and off rapidly. To smooth out the voltage I added a 0.47µF capacitor parallel to the photodiode.

An Oscilloscope connected in parallel to the resistor measures the voltage across the resistor. When the light level is low, the voltage stays close to 0 volts as the photodiode is allowing almost no current to flow through.

When my lamp is at it’s brightest setting, the voltage across the resistor gets around 2.5V, depending on how close it is to the photodiode, and how well the photodiode is pointed at the light. 2.5V across the resistor means that the photodiode is allowing 2.5mA to flow.

My headlamp light focuses a lot more light directly on the photodiode, so it makes it go right to 5V.

5 volts seems to work well for powering this circuit. The oscilloscope I use is powered by a 120VAC t0 9C DC adapter.


The photodiodes in the affiliate link ad above have mostly good reviews and probably act like the ones used in my video but are a bit larger. Some reviewers mentioned that they are actually phototransistors, which I suspect is true based on what they said, and my earlier testing of the photodiodes that I bought elsewhere, and have gotten the same results that they did.  Regardless, I will still call them photodiodes for now since that is what they are being sold as and I can’t prove that they aren’t at this time.


Oscilloscope including power adapter from a seller with OK reviews. Affiliate link ad.

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