I would like to build simple 555 based fan controller. The requirements are:

  • at start the controller generates one longer pulse (which can be configurable)
  • after one longer pulse it generates PWM signal with configurable duty cycle

I found one question which is related to my problem (Astable 555 Timer creates longer pulse when just turned on) but there the longer pulse at the beginning was undesirable. Maybe I can modify the above circuit to be able to configure how long the first pulse is. Below is the circuit from the related post

Quote from https://electronics.stackexchange.com/questions/208869/astable-555-timer-creates-longer-pulse-when-just-turned-on

  • 1
    \$\begingroup\$ The longer initial pulse comes from the fact that the timing capacitor is charging from zero volts. After that, it's always charging from 1/3 Vcc. Since the same components are involved in both scenarios, you don't get independent control of the initial pulse width and the duty cycle. \$\endgroup\$
    – Dave Tweed
    Commented Aug 7, 2021 at 11:00
  • \$\begingroup\$ How long do you want the initial pulse to be relative to the PWM period? \$\endgroup\$ Commented Aug 8, 2021 at 3:42
  • \$\begingroup\$ I want it to be completely independent because I have different types of fans and everyone oth them has higher treshold to start. \$\endgroup\$
    – eerefresh
    Commented Aug 8, 2021 at 18:52

1 Answer 1


In the above circuit, the initial pulse is longer because the capacitor (call it C1) initially is at 0V, and so the initial pulse is for the time until it charges from 0 to 2/3 VCC. Subsequent pulses are for the time to charge from 1/3 to 2/3 VDD.

By connecting another capacitor (call it C2) from VDD to ~TRIG, you can control the width of this initial pulse. The PWM frequency will be controlled by the sum of these capacitances (C1 + C2). As VCC is applied, the initial voltage at ~TRIG will be VCC times the division ratio of the two capacitors: VCC*C2/(C1+C2).

So, if you make the new capacitor 5 uF, then (since the first is 10 uF), the ~TRIG pin will be charged to 1/3 VDD when power is applied (assuming it is applied faster than the PWM frequency), and the 1st pulse will be the same as the subsequent ones.

Similarly, making it equal (e.g. 10 uF), will make the 1st pulse shorter, and making it 2x (e.g. 20 uF) will tend to make the 1st pulse disappear (in practice there may be a very narrow one as the 555 initializes.

  • \$\begingroup\$ Can I have a different circuit to be able to control this first pulse length independently? \$\endgroup\$
    – eerefresh
    Commented Aug 8, 2021 at 18:59
  • \$\begingroup\$ Depends on what type of control you need. Likely you will need some additional logic components to 'remember' that the 1st pulse has/has not happened, and use that to switch in a different capacitor or resistors. That logic will need initialization. Approximately what parameters for pulse width/frequency do you need ? \$\endgroup\$
    – jp314
    Commented Aug 9, 2021 at 0:02
  • \$\begingroup\$ sorry for late reponse. I have 4 different fans, they differ in size, max current and (which is the main problem I want to solve) they hava a different voltage treshold at which they start. So that is why I need one longer pulse to start them off and then I will control every of them with separate PWM (I do not know what parameters should it have, I will test it). One solution that I came up is one 555 monostable with long enough pulse to make all the fans spinning, an OR gate and second 555 to control the actual speed of the fan. What do you think about this approach? \$\endgroup\$
    – eerefresh
    Commented Aug 13, 2021 at 19:12

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