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Replacing a Photoresistor with Triac question

Discussion in 'General Electronics Discussion' started by TORCHMD, Jan 28, 2015.

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    Jan 28, 2015

    I'm new to the forum. I'll do my best to ask an intelligent question

    I have a BLDC motor for a sewing machine that I want to fit to my milling machine. It has everything I need, except: the variable speed start circuit is designed around a red led and photoreceptor with a spring activated blade that interrupts the light to the photoreceptor. I want to replace this circuit with a variable resistor like a Triac.

    I have read up on photoreceptors in general, but I can find no markings on the photoreceptor to identified it. I measured resistance across the legs in the dark 35kOhms, and in bright light its 65MOhms.

    Is it safe to replace this with a 100MOhm variable resistor to test and tune the needed resistance?

    Any suggestions?

    Thank you for any help.

  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Hi there and welcome to Electronics Point :)

    Thanks for explaining your whole project. This makes it a lot easier for us to understand what you really need.

    I don't think a triac is what you're after. Triacs are high-power switching devices that can be used to control lamps and motors directly. It sounds like you want a low-power device or circuit to connect into the speed control circuitry of your BLDC motor.

    The photodetector in the sewing machine pedal is probably a photodiode or phototransistor, and you won't get a useful reading if you measure its resistance. The resistance reading will change with the light falling on it, but replacing it with a resistor of that value will probably not cause the controller to run at that speed.

    You can try it though. Set the pedal half way down, measure the photodetector, get a resistor of a similar value and connect it instead of the photodetector. It's very unlikely to damage anything.

    Also you will not find a 100 MΩ potentiometer. 1 MΩ is the highest value that is manufactured by most companies.

    We need to find out more about the photodetector, and the circuit that it connects to inside the BLDC motor controller. Can you upload some photos of the photodetector, and a photo of the motor controller board showing where the photoreceptor connects?
  3. BobK


    Jan 5, 2010
    Hmm. I wonder if the LED / photo-detector is used for isolation? It might not be safe to remove it. How about running the LED from a separate low voltage power supply, and varying the current in it to vary the speed?

  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    I've seen the arrangement he describes before - the LED is driven at constant brightness, and an angled "blade" attached to the pedal is used to vary the amount of light that falls on the photodetector, to vary the speed.

    If the controller is non-isolated, that should be clear from photos of the main circuit board. In that case, yes, some isolation may be needed. I don't think it will be though. I don't think any reputable manufacturer would run live signals through a long cable to the pedal if it could be avoided. (Back in the day of rheostat pedals for speed control, it couldn't be avoided.)
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