[Dhananjay Gadre] happened across a useful little trick the other day. Take any old 1N4148 or 1N914 glass-package signal diode and wire it up right, and you’ve got yourself a nifty little IR detector.
The trick is to treat the diode just like you would a proper IR photodiode. The part should be reverse biased with a resistor inline, and the signal taken from the anode side. Point an IR remote at your little diode and you’ll readily see the modulated signal pop up on a scope, clear as day.
The phenomenon is discussed at length over on Stack Exchange. Indeed, it’s a simple fact that most semiconductor devices are subject to some sort of photoelectric effect or another. It’s just that we stick the majority of them in opaque black packages so it never comes up in practice. In reality, things like photodiodes and phototransistors aren’t especially different from the regular parts—they’re just put in transparent packages and engineered and calibrated to give predictable responses when used in such a way.
Is this the way you’d go if your project needed an IR detector? Probably not—you’d be better served buying the specific parts you need from the outset. But, if you find yourself in a pinch, and you really need to detect some IR signals and all you’ve got on hand is glass-package signal diodes? Yeah, you can probably get it to work.
While this trick is well known to many oldheads, it’s often a lightbulb moment for many up-and-coming engineers and makers to realize this. Glass-packaged diodes aren’t the only light-sensitive parts out there, either. As we’ve explored previously, certain revisions of Raspberry Pi would reboot if exposed to a camera flash, while you can even use regular old LEDs as sensors if you’re so inclined. If you’ve got your own secret knowledge about how to repurpose regular components in weird ways, don’t hesitate to notify the tipsline!
Back in the CRT monitor days we had a particularly tricky repair at the depot. This unit worked fine until the cover was slid on. First guesses were some sort of bending of the main board and a questionable solder joint, but we couldn’t find one. Then we noticed the shadow of a hand passing over one section of the circuit board would cause the failure. Ended up being a glass zener diode in a voltage regulator changing value when dark.
Oof! I remember an old repair technician article from decades ago where the TV would work fine when the technician was looking at it, and fail when he wasn’t. He took a picture of his face and stuck it inside the case looking at the PCB and the TV started working consistently.
Apart from the obvious “written for amusement purposes rather than factual accuracy” thing, having a light sensitive component’s behaviour change because the paper covered a case hole would also be a potential explanation.
Reminds me of a Light Activated SCR that I got from Radio Shack in the 70’s. It was just a normal SCR in a metal can with the top sawn off, and potted.
Hehe yep and they called them LASCRs and they found use in photographic flash “slaves” which triggered when the main camera flash went off.
Reminds me of the striking voltage of neon lamps changing with exposure to light.
So why are 1N4148s etc packaged in glass? Is glass really cheaper than the opaque black stuff small signal transistors are packaged in?