simulate this circuit – Schematic created using CircuitLab

The circuit reverse-engineered above is a LED dimmer which is driven by 555 timer in astable mode which then drives an op-amp to provide current for the gate of the MOSFET. The circuit dims quite well but it has an issue, when you power it up for a brief moment the LED strip receives non-PWM voltage what results in full brightness and dimming according to the 10K pot position after a while. I was thinking in order to fix this I could remove the 7805 regulator and just replace it with buck converter which powers the PWM circuitry constantly, but I'm asking if there's maybe more elegant solution here.

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    \$\begingroup\$ Where are the 7805 input and output decoupling capacitors? Schematic elegance (and therefore readability and quite likely accuracy) usually leads to an elegant solution. \$\endgroup\$
    – Andy aka
    Oct 17, 2020 at 15:50
  • \$\begingroup\$ @Andyaka, this is what is on the circuit board I've bought, I guess those caps aren't necessary in China! Also I've updated the schematics to something others can work with. \$\endgroup\$ Oct 17, 2020 at 17:06
  • \$\begingroup\$ Are you sure your schematic is correct and you are SHORTING out the LED yet don’t power them from anywhere. My guess is no and that you have a boy standard low side switching dimmer. \$\endgroup\$
    – winny
    Oct 17, 2020 at 17:23
  • \$\begingroup\$ @winny I put the 12V source wrong before also I've added line that ties ground and 5V through a capacitor. \$\endgroup\$ Oct 17, 2020 at 18:06
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    \$\begingroup\$ Vcc and Gnd are shorted on your schematic. And the opamp is comparing the pot voltage with your shorted Vcc/Gnd line. \$\endgroup\$ Oct 17, 2020 at 18:18

1 Answer 1


Presumably the 555's threshold pin is connected to the inverting input of the op amp.

Swap the inputs over on the op amp. 555's threshold pin to the op amp's non-inverting input and the pot to the op amp's inverting input.

And do put a current limiting resistor in series with the LED.

  • \$\begingroup\$ I've checked once again and the trigger and threshold pin of 555 is connected to the inverting input of the op-amp. \$\endgroup\$ Oct 17, 2020 at 17:56
  • \$\begingroup\$ @Dreaded Singleton But you've drawn the op amp's inverting input connected to ground. Also 555's reset input should be connected to +5V. Swap the op amp's inputs over with each other. \$\endgroup\$
    – James
    Oct 17, 2020 at 18:20
  • \$\begingroup\$ @Dreaded Singleton The GND pin of the 555 should be at ground potential and connected to C1 and C2. It should not be connected to the top of C3 nor the top of the op amp. Remove that connection with the 90 degree bend. If you swap the op amp's inputs over with each other it may well cure your original problem. \$\endgroup\$
    – James
    Oct 17, 2020 at 19:11
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    \$\begingroup\$ @Dreaded Singleton Your mosfet/led connections are incorrect as drawn. I would expect the source (arrowed terminal) to be connected to ground and the LED with a series resistor to be connected between the drain and +12V. \$\endgroup\$
    – James
    Oct 17, 2020 at 22:49
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    \$\begingroup\$ Thanks! I've updated schematic according by request of other posters to represent signal flow and connected MOSFET properly. Also the fix worked great and the LEDs go from 0V to desired brightness instead of 12V to lower level. \$\endgroup\$ Oct 18, 2020 at 20:52

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