I intend to build a tachometer, which is capable of detecting rotations up to 1kHz (60 000 RPM). From the Internet I know that this is possible to do with a magnet and a Hall detector. However, I would prefer a non-invasive solution with a light and a photo-detector, but from Internet it seems that ordinary low-cost photo-diodes are just too slow for such a task (700 RPM in this video https://www.youtube.com/watch?v=WG_4iChrvR4).
I assume that I would need a photo detecting device, which response time is at least an order of magnitude lower than the period of the oscillation, that is less than 100 micro-s. Are there any photo-detectors (photo-diodes) capable of that? If possible, please suggest a product.
Best Answer
Never, ever, assume that a Youtube video presenter knows what he or she is talking about, unless he or she presents some credentials. From the video, it appears that the circuit being shown looks like
simulate this circuit – Schematic created using CircuitLab
Granted, backing out connections from a video is chancy, but that's what it looks like to me.
With this setup, it's the R1/C1 combination which limits the bandwidth, not the photodiode. I would suggest that the photodiode under discussion is probably good well into the MHz regime (since it's in a 5 mm package, which implies a small die area and low capacitance).
Assuming, of course, that you used a decent conditioning circuit and don't look at the wrong component as the culprit.
Here, for instance is a 50-cent IR diode in the same package. If you'll note the data sheet, the rise and fall times are under 10 nsec, which implies operation to something like 100 MHz. Granted, stray capacitance would make this a bit difficult to achieve, especially if you make the mistake of trying to do it on a breadboard (like the video), but that's the fault of the breadboard.
TLDR: The video is wrong.