I just built a fan speed controller, using the following schematic:


Now if you connect 12V from 555 and connect pin 7 to the blue CPU fan wire, it controls the speed, but whenever the PWM signal is approx 20% duty cycle, the fan runs approx 80% of full speed.

I liked the simplistic design and want to someday turn this into an LED dimmer. But I am not sure how to drive a N-channel FET or NPN transistor with the switched ground as opposed to the normal switched Vcc. How can I accomplish this?

Thanks for reading!


Pin 7 of the 555 is an open-collector output, while pin 3 is a normal totem-pole output that can be used to drive a transistor (BJT or MOSFET).

  • \$\begingroup\$ don't forget a resistor between pin 3 and the NPN base, the MOSFET OTOH can be driven directly \$\endgroup\$ – Jasen Aug 2 '15 at 4:53
  • \$\begingroup\$ So how could I modify the schematic to use pin 3 instead of 7 with the same amount of parts? Or is that not possible? \$\endgroup\$ – Matt Aug 3 '15 at 2:05
  • \$\begingroup\$ Just connect your transistor to pin 3. If it's a BJT, use a current-limiting resistor between pin 3 and the base. Connect the emitter/source to ground. Connect your load between the positive supply and the collector/drain. No other modification is needed, but note that the duty-cycle control will now work "backwards" -- when the pin 7 output is on, your transistor will be off, and vice-versa. \$\endgroup\$ – Dave Tweed Aug 3 '15 at 3:32
  • \$\begingroup\$ Works great! Thanks! Care to explain why this works? I've never seen a potentiometer directly on pin 3, thought it would interfere. Apparently not tho. \$\endgroup\$ – Matt Aug 4 '15 at 21:34
  • 1
    \$\begingroup\$ You can think of pin 3 as being connected alternately to Vcc and Gnd. When it is high, the bottom half of R1 and D1 conduct current to charge C1. When C1 reaches 2/3 Vcc, pin 3 goes low, and the top half of R1 and D2 conduct current to discharge C1. When C1 reaches 1/3 Vcc, the output goes high and the cycle starts over. Since the two halves of R1 -- with the split being determined by the wiper position -- are used independently, you get a wide range of control over the output duty cycle. \$\endgroup\$ – Dave Tweed Aug 5 '15 at 0:54

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