I'm planning to build a laser tripwire tachometer that measures the frequency of beam breaks to calculate rpm. The device will break the beam at ~30,000 times/second at peak. Planning to use a 650nm laser with an LDR sensor module (xc4446) for detection, but it appears LDRs have a ~10ms latency. Will triggering multiple times during a previous latency period be a throw off readings? Will I need to use IR, or is there a better way?
Thanks!
You should use a sensor that is at least 10x faster than that, that is, with a response time of less than 3 microseconds. A photodiode will work.
In this case, would I be able to just use an LED as the detector? (sorry I'm a bit new to electronics)
LEDs work as a photodiode, but they are not optimized for speed. I have no idea if an LED would be fast enough for your project.
LEDs also respond only to light in a rather narrow range of wavelengths, typically 50 nm "bluer" than the emission wavelength.
Photodiodes are cheap -- buy them from any distributor (Mouser, Digikey, etc.). Or you can buy them as line and obstacle sensors from robotics suppliers like Pololu, Sparkfun, Adafruit, DFRobot, etc.
alright cheers thanks for your help
Sorry mate made a typo, it was actually ~30,000/min, so about 500/second. Would you still recommend a photodiode for this situation?
Again, the rule is to use a sensor at least 10x faster than the transition interval, so the sensor response time has to be less than 200 microseconds.
An LDR will obviously not work.
ah got it thanks for clarifying
Photodiodes are better performers than photoresistors - repeatable, fast, temperature-insensitive, no toxic components. There are also many light sensor chips that make digital interfacing directly possible.
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