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I want to be able to use a MOSFET switching circuit to be able to turn a motor on when a CDS Cell detects light and is off when the CDS cell is covered (for example in a dark room the motor is off). I have made a quick schematic in Multisim using an LED as a motor and a switch as the CDS. Whenever I breadboard the schematic everything works fine using the motor and a push button switch, but when I replace the switch w my CDS cell the motor will not function. I am a beginner and was under the impression a CDS cell would essentially act as a switch for my circuit and vary the resistance as it detects light or not. I replaced the switch with a variable resistor in my schematic as well and it still worked fine.

Multisim Schematic

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That's because the photocell can't get a low enough resistance to turn on your MOSFET. Depending on your photocell, the illuminated resistance could be 10k-100k ohms. I'd use a comparator and probably implement some hysteresis in there while you're at it.

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  • \$\begingroup\$ Hmm okay, I tried a simple circuit with just an LED and the CDS and it worked fine on my breadboard as far as turning on/off the LED. If the CDS cell were that high in its illuminated resistance wouldn't it not work using a 5V supply? I should probably just measure the resistance of it covered/illuminated. \$\endgroup\$
    – acc13241
    Commented Nov 1, 2021 at 22:21
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    \$\begingroup\$ You're probably juuuuust hitting the threshold voltage to get the handful of mA you need to get the LED to turn on. Using a comparator gives you a nice 5V to really turn the transistor on (although it really wants 10V) and the ability to adjust the trip point with a potentiometer on the inverting terminal. Hysteresis will prevent the motor from turning on and off repeatedly at the transition point. Don't forget your flyback diode! \$\endgroup\$
    – vir
    Commented Nov 1, 2021 at 22:30
  • \$\begingroup\$ Okay cool, thanks for the advice! \$\endgroup\$
    – acc13241
    Commented Nov 1, 2021 at 22:34
  • \$\begingroup\$ Use any 74HC CMOS logic gate and measure the resistance at the light threshold U want then match it and use 220 ohm to drive LED. This is ideal. Simple then ground all unused inputs. \$\endgroup\$
    – D.A.S.
    Commented Nov 2, 2021 at 2:29

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