Good circuit for measuring speed on RC car?

Discussion in 'General Electronics Chat' started by Popnfrsh24, Sep 13, 2010.

  1. Popnfrsh24

    Thread Starter New Member

    Sep 7, 2010
    Hey everybody, I want to measure the velocity on my RC car. I was thinking about using an hall effect sensor and mounting magnets in the wheel of my RC car, but I thought it would be pretty hard to do it this way. Instead, I want to use a photocollector that I already have and mount it to my RC Car. (here is the datasheet btw )

    Basically I found a metal shaft that is attached to the gear of my motor. I want to put a metal ring and mount it to this shaft. I then am planning on cutting a slit out of the ring and mounting the photosensor around the ring so that the photosensor can "count" each time the ring makes a revolution.

    My circuit that I am using consists of one side of a 10k resistor tied to 5v and the other side tied to the collector of the phototransistor. The emitter is tied to ground. I then take the collector voltage and send it to a 74ls14 to get a digital output.

    I have just tested this circuit with a piece of paper and everything seems to work well, I get 5V when the paper is present and 0 V when the paper is gone, but I am concerned that it will not be able to switch fast enough when the motor is at high RPMs. Does anybody know if this circuit is feasible at a high RPM setting? I didn't see anything in the datasheet about anything involving any sort of response time, so I'm a bit concerned about this....

    Any help would be appreciated,


  2. debjit625

    Well-Known Member

    Apr 17, 2010
    It have i.e.. Rise and Fall time typ 15 micro sec for HQA1887-011,-012 and typ 75 micro sec for HQA1887-013 .

    For example the device HQA1887-011 will take typ 15 micro seconds to change form low to high or high to low...

    Good Luck
  3. marshallf3

    Well-Known Member

    Jul 26, 2010
    If it's a modern photocoupler it should be plenty fast enough.

    Do a little math to figure out what RPM that shaft is turning at.
  4. Wendy


    Mar 24, 2008
    You could also paint a series of strips on the road, or use a piece of paper with them on there, and put an optosensor for them underneath. It would be similar to the bar code readers I've been hear about, but in this case the frequency would be directly proportional to speed (and very accurate).