Monostable that recharges quicker than the time it is on

[m1tch37]

Hi,

I want to build a monostable which stays in its unstable state (for examples sake) for 100ms, but can be triggered as frequently as every 120ms (that is to say, if it was consistently triggered every 120ms, the output would be periodic and high for 100ms and low for 20ms.) The reason I want to use a monostable is because I require the high time to be known and fixed, however the frequency of the triggering (and hence the off time) is unpredictable.

The issue I am having is all the Monostable circuits I have found must recharge a capacitor before the output can be retriggered, and this recharging actually takes longer than the time the output remains unstable. I have not found any Monostables that could achieve an output duty cycle of 50% or ideally even greater.

Are monostables with a fast retrigger possible? Does anyone know of any example circuits?

Cheers,
Mitch

 

[ericgibbs]

hi,
The attached image shows a 555 timer which gives a very close to 50% duty cycle.

I dont follow your other requirement.?
If you had a repeating 120ms trigger into a 100mS Monostable you would get close to 100mS high 20mSec low, providing it was a fast/narrow trigger pulse.

EDIT:
The 2nd image is what I 'think' you are asking.

 

[alec_t]

How about this? The diode allows rapid re-charge of the cap. The monstable period is ~0.7*R1*C1.

 

[k7elp60]

You might take a look at the 74LS123 retriggerable muiltivibrator