Variable 555 circuit with equal LOW&HIGH frequency

555

I've started to play with the 555 chip. There is a lot of documentation out there. So I want to create a strobe light project, with an adjustable frequency.

I've played with a lot of capacitors (1nf, 1uf, 10uf, 100uf, 1000uf) and resistances (330ohm, 1kohm, 10kohm, 100kohm, 1mohm). If I can change the frequency, I run into the problem where the light does not stay off for an equal amount of time than it stays on.

For example, right now I have this kind of output when I play with my potentiometer (numbers are not accurate):

  • 100ms on, 100ms off
  • 500ms on, 100ms off
  • 1000ms on, 100ms off

But I'm trying to achieve this kind of scale:

  • 100ms on, 100ms off
  • 500ms on, 500ms off
  • 1000ms on, 1000ms off

I have multiple 555 chips, but I can't find a way to have the on and off signal to have an equal duration.

Best Answer

Dave Tweed has indicated a simple solution (+1 from me). The circuit for which is...

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By charging and discharging the capacitor (C) through the resistance connected to the output (pin 3) the charge and discharge times are the same. By using a fixed resistor (R1) you can set the minimum time for the astable. By adding a variable resistor (VR1) you can easily alter the time without altering the mark/space ratio. The voltage across the capacitor (C) charges and discharges between 1/3 and 2/3rds of the supply voltage set by the internal comparators of the 555. The ON time = OFF time = T = 0.7CR (where R = R1 + VR1) - C in Farads, R in Ohms, T in Seconds.

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