Thread for random DIY-related questions

[crochambeau]

Could be an IR LED or sensor, similar to those used in old TV remote controls. Semiconductors do react to light, and the saw waveform could be a result of a diode blocking conduction in one direction. I would expect it to be rather inefficient as an LDR, but gain corrects many shortcomings.

One of those automated component testers (like the Peak or many of the mega328 based knockoffs) can answer this sort of question at the push of a button. I have only speculation to offer.

[FAP]

Talked to one of my [vastly more knowledgeable] coworkers and they basically came to the same conclusions you did: they even tested it with one of those mega328-type devices (though results were inconclusive).

I’m leaning towards photodiode, but without a way to emit infrared to test it, I can’t rule out IR sensor, either. I’m also totally clueless about IR to begin with so perhaps there’s another way to test it…?

I kind of like these compared to traditional LDRs: they’re far less sensitive to light (ambient or otherwise) though I can totally see how this would be a problem in many applications.

[crochambeau]

Any standard/classic remote control can quickly determine if the parts are sensitive to infra-red, just aim and shoot while the circuit is under operation. Could be a super cool application.

[FAP]

CONFIRMED they are, in fact, IR sensors. Thanks!

Using these in an anti-parallel pair in lieu of a proper LDR yields some intriguing results indeed. The TV remote interference is an obvious plus, but I was kind of surprised at how similar the waveforms of a proper LDR vs. an IR sensor anti-parallel pair could look at times. See attached screenshots: the first one compares both methods with a static frequency (not exactly the same frequency for both, but close enough), while the second highlights an example drop in frequency from high to low (again, the control in these experiments was anything but consistent, so analyze these screenshots with that in mind).
EDIT: the source signal is a square wave oscillator running from a range of about 30Hz–3kHz (via a 40106 chip).

The one major drawback (IMO) is a proper LDR has a more "natural" sounding transition from high to low frequencies: in practice, said transition sounds like a nice, smooth bass drop. In contrast, the same transition in an IR pair lacks that same impact. Obviously an IR pair necessitates twice the number of components as well, though this isn't an issue for me since I have a bag of like 50 of them.

Having just one IR sensor simply gives half of a waveform, though certain conditions (e.g. exposure to visible light) can pull it closer to zero; up close, it looks more akin to a series of pulses than it does a sawtooth wave.

I just keep on tripping into new rabbit holes, each one deeper than the last...

[FAP]

I'm trying to add a control to an LFO that'll allow me to adjust how much light from the LED will "bleed" into the low state of a square wave function (and eventually the triangle wave, if possible).
For context, this is the circuit I'm using:

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And here's a visual aid of what I'm trying to achieve:

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As you can see, the sine wave output already has a trim that can set how "deep" the waveform swings; rotating the trim fully counter-clockwise essentially yields a solid light (LED is always on), while rotating to the other side gradually decreases the brightness of the low state until the low state is simply off.

I tried to replicate the same trim circuit for the square and triangle waves (near U1b-pin 5 and U1a-pin 2, respectively), but it didn't work: this makes sense given how the op amps are cascaded/daisy-chained together, as adding a similar trim circuit earlier in the "chain" would affect every op-amp in it thereafter.

Also, the circuit in the bottom right adjusts the brightness of the high state: what I'm trying to do is essentially the inverse of that.