photo transistor -an analog component?

Thread Starter


Joined Apr 17, 2018
have anyone here have experience with phototransistor?. my question is that are we gonna get different current produced by different light intensity through the phototransistor?


Joined Jul 5, 2008
It's just like a say a NPN the more light the harder it turns on.

You have to read the data sheet to see what it can do tho most can't sink a lot of current.
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Joined Feb 8, 2018
If your requirements aren't too demanding, a phototransistor can be a very useful device. If you need good performance in terms of linearity over a wide range or higher speed, a photodiode is a better choice but requires more elaborate circuitry. A PIN (P, Intrinsic, N, referring to the doping of the silicon) photodiode can be very linear and very fast, but is even more demanding of the circuitry to gain the speed benefit.

An ap note on phototransistors:


Joined May 4, 2013
When I was a kid, we would file the tops off of 2N3055 power transistors and use them as photo detectors.
I've filed the tops off many transistors and IC's. The danger is that some power transistors can have beryllium in them. Beryllium is quite toxic.

I once filed the top from a signal MOS-FET. It was real sensitive to light. I suspect the sensitivity was limitted by the value of the load resistor needed to discharge the gate capacitance to get it to turn off.


Joined Feb 8, 2018
Most semiconductors are light sensitive to some degree. Glass-packaged signal diodes can make for some very odd behavior in precision circuitry.


Joined Sep 24, 2015
Back around 1990 I got my hands on a bunch of "Pump Stroke Counter/Totalizer" units. Their function was simply to count once every time a switch was struck by a cam on a pump. It was LCD and powered from a small coin cell battery. They were in stock for over 5 years and the company decided to get rid of them because they could prove to be unreliable. I managed to get quite a few. (lost in the divorce - dang it!) They also had a lot of other stuff they decided to scrap. Among them were bags and bags of photo transistors, also many of which I got a hold of. (and lost) I put a PT (photo transistor) on a counter and stuck it in the refrigerator. Every time the door was opened and the light came on the counter would advance by 1 count. I was amazed to see that my refrigerator was being opened on a daily basis as many times a 60 to 70 times a day. Wanting to make sure it wasn't an aberration caused by an outside light source I stood there and opened it 10 times in the morning, 10 times in the afternoon and 10 times in the evening. It counted all 30 times I opened the door. That lead me to putting restrictions on when the door could be opened by anyone in the house. And you think I'm crazy!

Another experiment I did with a PT and a counter was to place it in the window in the evening during a thunder storm. Sometimes with a flash of lightning the counter would advance anywhere between 5 and 20 counts. Other times it would count as many as 120 counts from what I perceived to be a single flash of lightning. So even though we see a single flash that may be modulated a few times, my experiment drew the hypothesis that a single flash of lightning actually consisted of many flashes from a dozen to 10 times that. Like I said, this is a hypothesis based on what I saw from my cheap little counter and the transistor acting as a switch across the counter input.

I also made a dual light sensor. Each sensor fed a comparator. One circuit was direct to a single input while the other was filtered with a capacitor. The reasoning was that when the light levels were remaining largely unchanged both sensors would set up a voltage at each input. When light levels changed due to a reflection or shadow or even a light source, one input would change more rapidly than the other. The comparator would then output a voltage that would drive a sonic alert and I would know that someone or something was passing within the field of vision. And it worked. Not great, but it did work. And with the changes of temperature the balance would drift and trigger a false alarm. But back then I was messing with stuff I got for free and didn't fully understand how they work. Today things are - um - well, I WAS going to say "Different". But honestly, I left electronics for a long time (caught up in a cult). I'm free of that for several years now and have been getting back into electronics. So today my understanding of electronics, better than before, is largely unchanged. I wish I knew more about these things than I do. But I DO know more today than I did back when I had my hands on a bunch of free stuff.