555 monostable chaining - setting initial condition

[smjcuk]

I'm building a simple chained monostable arrangement with three 555's operating in monostable mode with the outs connected to the next stage's trigger. Basically it will produce a 20s high output from the first 555, then trigger the second one which will knock out a 20s high output and then the 3rd one which will then knock out a 4 minute high and then trigger the first one again in an eternal cycle. This is for a simple fan controller arrangement and the first two outputs are connected to relay drivers to turn fans on and off.

Obviously this system is in a non-deterministic state at power on. Does anyone have any hints on how to force an initial condition i.e. trigger the first monostable and ensure the others don't get triggered at power on. At the moment it's random and it settles after a cycle which isn't ideal.

Any help appreciated. Each monostable step is as per the datasheet schematic attached with suitable cap/res values substituted.

I did have an Arduino doing this at the moment but it's a bit overkill and a pain to have to dig out a USB cable and the laptop to frig the timing.

[SL4P]

I suppose without thinking it through too hard, an AC coupled RC time constant to hold all the reset pins at power up...  the C charges up, the pin goes high, then nothing else that isn't part of the 555 schema.

Or if that's a bit flakey, use a 4th 555 (or half a 556) to act as a power-on timer.
Maybe a better solution!

[SeanB]

Pin 4 is a reset input. Use a RC reset circuit to make all the timers reset properly and then simply add another 555 to provide the initial trigger pulse to the first one, and use an or gate ( diodes) so the last one can retrigger it when it cycles around to it again.

[smjcuk]

Thanks for both of your suggestions. I was looking at adding another 555 to the solution for POR and tripped over the fact that my design was really stupid to start with. I have removed the monostable loop and replaced it with an astable that has a cycle time of (approx) 5 minutes then chained the two remaining monostables off that. So when the astable does the high-low transition it triggers the first monostable, then the second one. There is no loop and no non-determinism then.

So, at 3x 555s now and it's working very well.

Now to see if I can find a neat trick to get rid of one of the 555s and I can use a 556 instead!

[SL4P]

or back to the drawing board... an 8-pin uP running the timing loops you need and firing all the needed outputs...

no additional passives, spare pins for control etc!, and one chip

[smjcuk]

or back to the drawing board... an 8-pin uP running the timing loops you need and firing all the needed outputs...

no additional passives, spare pins for control etc!, and one chip

Already there.

That was until I had to go out to the garden with a laptop and USB cable to tweak the timings on it. Decided that standalone analogue was the way to go then. Just a small screwdriver to adjust the trimmers. Plus it's more fun.