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I'm trying Project 15 from the Arduino Uno Projects book (optocoupler).

I manage to turn on and off a LED in a separate circuit using my Arduino-driven optocoupler.

Now I tried to also turn on some real device (battery-driven toy from my kid that she doesn't used anymore). Replacing the toy's original switch with jumper wire leaves it running, so I'm confident that it basically works. But putting my optocoupler in between, it just won't fire up.

It looks like this

More detail

Measuring resistance on the 4N35's output bins as advised here, I obtain -15k Ohm (and 1 when measuring the other way round, i.e., as if there were no connection at all). Switching wires also exactly switches the measurement, i.e., I never obtain any positive resistance measurement.

What can go wrong here? What else can I try to find out what's wrong? Thank you!

  • how did you connect the 4N35 between the toy and the Arduino? – jsotola Dec 30 '18 at 22:36
  • @jsotola added pictures above. – bonifaz Dec 31 '18 at 13:16
  • @bonifaz I suggest you to measure the current flowing when you short circuit the wires in the RC car, so that you can guess both orientation and magnitude (if it is more than a couple of milliamps you will need to drive a transistor with the optocoupler) – frarugi87 May 31 at 13:59
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First of all there’s no negative resistance, resistance is a positive only measure, from 0 to infinity.

My best guess is you have your optocoupler the other way around, so you are measuring the input rather than the output: is that possible?

  • I understand that resistance is supposed to be positive; that's why I'm confused that the mediameter displays negative resistance. I believe that my optocoupler is in the right direction, b/c using a LED-circuit instead of the toy works. – bonifaz Dec 31 '18 at 12:28
  • Then maybe the line, that you want to control, has a higher current normally flowing through it. It is totally possible, that the phototransistor site of the optocoupler has a too high resistance for this current to flow, or that in a previous try the current flowing through it was too high for it (so that the dissipating heat killed the site). Try to control a higher current MOSFET with the optocoupler – chrisl Dec 31 '18 at 15:52

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