Optical rev counter

You can but them for almost peanuts.

Wanna make your own DVD player or GSM phone too ?

........ Phil

The standard way seems to be to bounce infra-red light off the prop and count the pulses coming back over a defined period of time and then dividing the pulses by the number of blades on the prop (2,3 or 4 usually). Since most RC engines run at 20,000RPM or less, you have to be able to count pulses at the rate of up to neary 1500 pulses per second for a four blade prop.

You could also take an all analog approach by cleaning up the pulses and turning each into a fixed width. You would then low-pass filter this signal using an RC circuit to create an analog voltage proportionate to the pulse rate and use this to swing an analog meter.

What are your accuracy requirements? What kind of display or output device did you want to use?

Thanks man, the accuracy I was asked for was +- 5%, and I was thinking of using a LCD display, the displays not too much of a problem though.

Do you want to buy or build? A google for "optical tachometer" with the quotes gives "about 20,100 hits".

If you're building it, just get a reflective opto sensor and count pulses. :-)

Good Luck! Rich

I have designed one that works very well, but I am not going to ruin your assignment by giving you the complete solution. I'll tell you what I did, though, so you have something to start with:

The optical pickup turned out to be easier than expected: A photo transistor with a suitable amplifier and filter was all that was needed. No need for active lighting. Ambient light is plenty.

Nowadays, I'd simply use a micro controller to read the pulses and drive the display. Back then, I didn't have the knowledge to do micro controllers.

I used three decade counters, each connected to an LCD driver with latch. The counters would count propeller pulses directly off the photo transistor amplifier. A crystal driven timer (a counter which resets itself when it reaches a certain number) would then latch the counter values into the LCD drivers and reset the counters at a certain interval, depending on how many blades the propeller had. The crystal would also drive the LCD drivers, but off a low frequency in the timer frequency divider.

Good luck.

No problem with LED displays ......

Except for the act you cannot read one in bright daylight !

....... Phil

We all know who you are Phil, no introduction is necessary.

Try reading it again. LCD Are you using an LED display on your computer?

I don't know how hard that would be to obtain using an all analog method, but getting that accuracy with a micro should be easy. The first problem to solve is how to get pulses from the prop. Another poster pointed out (from personal experience) that a light source was not necessary. He says that enough ambient light will be reflected from the prop to do it. At this point, I would start experimenting with a phototransistor and an op-amp. Something like this might be a good starting point:

I just quickly googled up that page, I have no idea if the circuit works or has fast enough response time for your needs, but it is a good example of amplifying and detecting pulses from a phototransistor. The output of the op-amp could be fed to a LM-393 comparator to further clean up the pulses so they can be fed into digital logic of some sort if you like. Personally this is the approach I would take if it were my project.

Or, as I said before, you could take those pulses and use them to trigger a one-shot timer to set the pulse width to a consitent size and then integrate these to come up with a voltage proportionate to the pulse rate. I would think this could be calibrated to within 5%, just.

I never put a scope on the counter while measuring an actual propeller, but I strongly believe that the effect is opposite. The propeller is actually blocking some of the ambient light when it passes in front of the photo transistor.

Yes, that looks like a good start (omit the light source, though). It has a problem though: The potentiometer essentially adjusts comensation for ambient light level. Since the ambient light varies greatly (this is a handheld device), you'd drive yourself nuts twiddling the knob while trying to hold perfectly still.

I solved the problem by taking a low pass filter (simple R/C type) and putting the output of that into the other input of the op amp.

I even used a Schmitt-trigger here. It turned out I also had to put a low pass filter between the first op-amp and the Schmitt-trigger, because the photo transistor would pick up the white lettering on the black propeller and cause incorrect and unstable readings.

Sorry about that, it is me that added the reflected part to what you said. Sorry again. That does make sense.

Sounds like you refined it pretty well. You wouldn't happen to have a schematic laying around would you? :-) I would start out like I described and then let it spiral out of control from there letting my old oscilloscope lead the way. ;-)

I do, but if I tell you where to find it, I am worried I'll spoil Dylan's project :-)

Besides, the solution is very old-fashioned. A microcontroller would reduce the chip count by 80%. If you have a uC with A/D, you might be able to connect the phototransistor almost directly to the uC and do all the filtering and calibration in software.