555 Timer Question

http://www.mouser.com/ds/2/405/ne555-453433.pdf

Page 17 of that datasheet depicts a circuit that would be useful to me. I am trying to figure out how to make this circuit start on power up (no switch to pull trigger on first 555 low) and then loop the output of the last timer back to the trigger of the first, such that the circuit runs continuously in a loop.

Any ideas?

Well now, as it turns out, that circuit has a bad flaw insofar as the coupling from the first to the second, and the second to the third, is such that the second and third will be triggered upon switch-on if power is applied sufficiently quickly. As such if you loop the third device to the input of the first using the same capacitor, it will in most (but not all) cases start up as a "ring counter" and do just as you describe after the initial erratic sequence.

So the question then is - why would you use NE555s when you could use a 74HC4017 driven by a NE555 if you want the intervals to be equal, or a Pro Mini and generate accurately whatever complex pattern of intervals you could dream up?

Need to drive 5 mosfets sequentially, at about 5% duty cycle with about 100us on time. I want to use 5 timers with 100us on time and then a final 1500us timer before the circuit loops back to the start. Simple as that. I considered the 4017 but I am not sure how to implement the 1500us delay. (Using mosfets to drive groups of 10 parallel LEDs at 1A per LED). I want to do it without a microcontroller just because lately I have been using microcontrollers for just about every project and for such a simple task it is too boring...

Open to any suggestions as to a better way. I considered using AND gates to combine the PWM signal and the outputs of a 4017, but I am not sure how to synchronize the group selection and pwm.

Not tried... but thinking about it.

The timing cap on the 555, how about we have the 4017 control the timing cap?

Eg pin 1 (not actual pin on the ic) could have a 5k pin 2 10k pin 3 20k each pin change would vary the delay on the 555 right!

Could it be used?

Why are you using 555's for this. Surely this is an ideal use for a stand alone arduino board e.g. a Pro Mini?

Or perhaps the clock resultion is not accurate enough.

Or use my new favorite board the 72 Mhz Maple mini (clone for $4) from AliExpress,

I also say "an uC" but I understand your point of wiew.
If you place an 555 freerunning oscillator instead of the switch "S" with a cyckletime of 2000 us you do not need to "loop back"

If it's problem with all outputs will go "on" under power up, connect all pin 4 together with a pullup resistor and a capacitor to ground.

Pelle

Hi,

Need to drive 5 mosfets sequentially, at about 5% duty cycle with about 100us on time. I want to use 5 timers with 100us on time and then a final 1500us timer before the circuit loops back to the start. Simple as that.

You need 5 outputs that do what?
5% duty with about 100uS on time?
Is that a single cycle, 100uS in period with 5% duty cycle?
Or the 5% duty is to be 100uS long.
Please draw a diagram, graph of the 5 outputs and how they relate to each other on a time line.

Tom..... :slight_smile:

I think he means 100 µs on for each output in sequence and a delay for 1500 µs, a total of 2000 µs and then repeat/start it again

100/2000*100% = 5%

Pelle

Thanks for the help everyone. Here is a diagram of what I want to accomplish.

Either is perfectly acceptable.