Servos - PWM - 555 - how to get *really* low switching Hz and duty cycle

[Nick Mulder]

Hi,

I am trying to make a servo driving unit for time-lapse photography on an old 60's 16mm camera 8) - the servo will simply push a mechanical button but it needs adjustability that other systems wont give (like cam-follower DC motor or solenoid) - I have so far managed to get the mechanics all fine but only about 1/2 of the electronics -

The servo needs to go from 0 degrees to 180 degrees with a regular frequency spanning from the fastest being the time it takes for the servo to make this movement ~ 1-2 sec (you couldn't go any faster would make the servo not reach its full angle) up to about 60 mins - however if this is unfeasible I'll settle for less ...

I can make the pulses that tell the servo to go to these angles with 555/556 timers (haven't done it yet but have found plenty of tutorials online) - I would have two, one outputting a ~ 0.6ms pulse and the other a 2.4ms pulse @ around 50hz . I would have a flip-flop relay switching output to the servo between the two...

However, not only do I need to be able to adjust the timing of the switching between these two 555/556 circuits I also need to be able to adjust the duty-cycle of this timing (so I can open the shutter for longer periods for night photography in case your interested) ...

So heres the issue, how do I drive this timing flip-flop ??

I considered another 556 circuit doing exactly the same thing as the servo drivers but with a much larger time scale e.g.. 0.000278 Hz (60 mins period) - but this would result in very large RC constants ...

is there another way ?

(also, as the system driving the switching between the two PWM generators is actually doing the same thing as the two PWM generators themselves but over different time scales could I also use your potential suggestions to also run the servo's themselves ?)

cheers,
hope this makes sense,
Nick

[email protected]

PS.

Sorry,

should also add i am a bit of a beginner with practical electronics - I only learned about PWM and this stuff yesterday trolling the net ...

:P