enter image description here

I'm having an issue with this PWM controller circuit. In this circuit the potentiometer comes with a on/off switch.

The way it should work is: As the potentiometer resistance value varies the output voltage varies (with the help of a 555 timer chip). When the output voltage increases the red LED increases in intensity and the the green LED decreases in intensity.

The problem that is happening when test the PCB is that:

1- The circuit is always "ON" the switch circuit doesn't seem to work

2- The output voltage doesn't vary as I turn the potentiometer, it is always at full intensity

3- The red LED is always at full intensity, the green LED never turn on

This PCB has 12VDC input.

Q1 and Q2 are N-Channel Mosfet (https://octopart.com/buk7y08-40b%2C115-nxp+semiconductors-19677921)

Q3 is a PNP transistor (//octopart.com/bcx5316ta-diodes+inc.-13152586)

U1 is a 555 timer chip.

Is there a problem with the schematic? Or is it that something went wrong during the PCB design?

I'm a hobbyist so forgive any mistake or oversight. Any help would be appreciated

Thanks you,


  • \$\begingroup\$ problem 1: the MOSFET Q1 gate cannot discharge, add 10K between G and S, R1 is probably not needed. problems 2 and 3 are a mystery to me. as drawn it should work. \$\endgroup\$ Apr 13, 2017 at 4:46
  • \$\begingroup\$ @Roni where is your freewheel diode ? \$\endgroup\$
    – Autistic
    Apr 13, 2017 at 5:06
  • \$\begingroup\$ Does the voltage at pin 2, 3 or 7 of the 555 vary as you turn the pot? NOTE: you should wire a Schottky diode across the motor output terminals, with reverse polarity (Cathode to +, Anode to -) to recirculate current through the motor and prevent the FET and LED from being damaged by back-emf spikes. \$\endgroup\$ Apr 13, 2017 at 5:17
  • \$\begingroup\$ @Autistic , do I need a freewheel diode? if so where should it be placed and why? \$\endgroup\$
    – Roni
    Apr 13, 2017 at 6:24
  • \$\begingroup\$ Place the Freewheel diode across the motor terminals such that it appears reverse biased .All motors have inductance which if not dealt with would make voltage spikes which would destroy the main N channel fet .The 555 is featured on www.badbeetles.com .It does have some snags . \$\endgroup\$
    – Autistic
    Apr 13, 2017 at 11:42

1 Answer 1


I took measurements of the 555: Voltage from power supply=12.11v , pin1=0v , pin2=12 , pin3=10.87v , pin4=12.11v , pin5=11.69v , pin6=12v , pin7=12.11v , pin8=12.11v . This voltages remain the same no matter how I turn the potentiometer...

Those voltages are very abnormal. Here's the internal circuit of an NE555:-

enter image description here

The first thing to note is that pin 5 should be 2/3rd Vcc, ie. ~8.1V. Also if pin 6 is higher than pin 5 then pin 3 and pin 7 should both be low.

So either your wiring doesn't match the schematic, or pin 1 is actually open circuit (which would explain why the other voltages are too high), or the 555 is fried.


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