# variable 555 circuit with equal LOW&HIGH frequency

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.

• I think it is not possible to get a 50% duty with a 555, At least I didn't find to do it when I was in the University, and related books didn't talk about it. It always was >50% or <50%. Nov 28, 2013 at 22:41
• There are many ways to get 50% duty cycle with a 555. A simple Google search will turn up many examples. The simplest is to connect the timing resistor to the output (pin 3) instead of the usual connection to Vcc via pin 7. Nov 28, 2013 at 23:01
• @DaveTweed Maybe I just don't use the good keywords, but I've searched all day prior to writing up this question... could you point me to some links on the subject ? Thanks ! :) Nov 29, 2013 at 10:34
• @Chirry: For my application, I don't need a very high precision, could be 45%+55% and it would do just fine. But right now my results are not anywhere close to 50%+50%, more like 95%/5% when at a low frequency. Nov 29, 2013 at 10:37
• @FMazoo8 What I used to do in those cases, is to find a combination of capaticor-resistor that could work in the frequency i need, then with a potenciomenter as resistor I find the resistance it fix better. As you know, it's easier to find resistors than capacitors at a custom value. Nov 29, 2013 at 14:46