# Astable 555 Timer creates longer pulse when just turned on

I have a question regarding the behavior of a 555 timer circuit that I have made. It is depicted below.

Now, here is some interesting behavior, I wrote a program on arduino that measures the lengths of the pulses from this 555 timer. Here is the output that I had.

  Was on for: 472
Was on for: 279
Was on for: 279
Was on for: 277
Was on for: 278
Was on for: 278
Was on for: 278
Was on for: 279
Was on for: 277
Was on for: 278
Was on for: 278
Was on for: 277


Now, as you can see, when the 555 timer is first turned on, the pulse is much longer (measured in milliseconds). My question is, how would I avoid this behavior? In other words, how can I turn an astable 555 timer on and off and have the first pulse be the same length as all of the other ones?

• Does anyone else feel that on calculation last pulse deviates just too much from actual answer that is around 303ms. And value of control capacitor is too high. – manav.tix Jan 1 '16 at 8:09
• @manav.tix, that last pulse is an anomaly because I turned off the circuit, I will remove from the question since it is confusing. – Anthony Dito Jan 1 '16 at 8:10

## 1 Answer

Add a 4.7uF capacitor from pin2 (top end of 10uF) to vcc reduce the timing resistors by one third.

this way the capacitor pair will power up as about 1/3 VCC and the pulses will go from there all coming out at about the same size,

• Thanks for the help! I just threw in another 10uF capacitor into the circuit where you said (I don't have a 4.7uF capacitor around) without changing the resistors and I am getting pulses of the same length. Great answer. – Anthony Dito Jan 1 '16 at 8:40