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Optical rev counter

Discussion in 'Electronic Design' started by [email protected], Mar 5, 2007.

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  1. Guest

    Hi guys,
    I been given an electronic project to make an optical rev counter
    for a RC airplane. If anyone has any experience with makeing one i'd
    really appreciate any help or links to information or schematics.
    I really can't seem to find any information about this on the net.
  2. Phil Allison

    Phil Allison Guest

    **Groper Alert !!

    ** What the **** for ?

    You can but them for almost peanuts.

    Wanna make your own DVD player or GSM phone too ?

    ......... Phil
  3. 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

    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?
  4. DylanJ

    DylanJ Guest

    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.
  5. Rich Grise

    Rich Grise Guest

    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!
  6. 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

    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.
  7. Phil Allison

    Phil Allison Guest

    ** Groper Fool Alert


    No problem with LED displays ......

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

    ........ Phil
  8. We all know who you are Phil, no introduction is necessary.
    Try reading it again. LCD Are you using an LED display on your
  9. 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.
  10. 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.
  11. 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. ;-)
  12. 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.
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