# Triggering a 555 timer IC

I need to trigger a 555 timer IC in astable mode as soon as it has power. It CAN NOT be done manually through a negative pulse like it usually is. The solution must be compact as space in my project is certainly at a premium. I either need a method of converting a constant high to a negative pulse, however this would require other components, or another method of triggering it. If neither are possible, is there an alternate astable IC I can use which would enable me to provide a 60s mark time followed by 1s space? Thanks in advance for your time, Regards, Eerf

• 1) How much space $L \times W \times H$ do you have available? 2) How accurate do you need the output timing to be? 3)How much current do you need to drive your load? Oct 4, 2015 at 18:58
• My load is 150ma +-5% and I can spare no more than 2.5x2.5 for LxW but height isn't an issue. The timing really has to have no accuracy. The shorter the interval between pulses and the shorter the length of the pulses, the better, but 10s between pulses is plenty.
– Eerf
Oct 4, 2015 at 19:20
• You don't usually have an external trigger in astable mode. Oct 4, 2015 at 19:30
• 2.5 x 2.5 centimeters? millimeters? Oct 4, 2015 at 19:38
• Re: "The timing really has to have no accuracy." then 60s $\approx$ 0s. Problem solved. Oct 4, 2015 at 21:40

60 s is too long a time for a old analog timer like the 555. Use a microcontroller.

This can be done by the cheap and tiny PIC 10F200. It has only 6 pins and comes in the same package individual transistors do. All it needs is power and a bypass cap. The oscillator is built in, which is good to a few percent. This solution will be smaller and take less power than a 555 timer, even if a 555 could reasonably do 60 s.

While what you need can be easily accomplished using a 555, in this instance - because of your space constraints - it seems a cheap 8 pin microcontroller might be a better fit.

In order to get the astable duty cycle you want, the 555 circuit will need two resistors and two capacitors and, if you need to use the bipolar for the highish source and sink currents its output can provide, the timing capacitor will be in the tens of microfarads for for an output high 60 seconds long.

A microcontroller will require, at the most, a power supply bypass capacitor and a ceramic resonator or crystal, or at the least, just the bypass cap if you choose one with an internal oscillator.

• Thanks for the quick response,however surely another circuit in an 8 pin package with 2 resistors and caps will take up the same space as a 555?
– Eerf
Oct 4, 2015 at 19:08
• Edit: Dupe. Ignore this.
– Eerf
Oct 4, 2015 at 19:21
• @Eerf: You missed the point, I think. A 555 will require three supporting discretes, while an MCU will require only one. And, possibly, even that one can be gotten rid of. Oct 4, 2015 at 19:41
• Ah I didn't realize a microcontroller would not require external components. Thanks!
– Eerf
Oct 4, 2015 at 19:57