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[SOLVED] how to build a one shot where trigger period > RC time constant?

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berres

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Hi all
I know this component might be considered as digital but "embedded systems and real time OS" didn't feel like the right category to create the thread in either... sry if I placed it incorrectly..

Why I need a multivibrator:
I'm working on a project where two sensors is going to trigger another circuit or microcontroller and the output pulse has to be hold active (high) for like 2sec at least.
The sensors will sense direction and I've built a circuit using 2 retriggerable monostable multivibrators where only one multivibrator can be triggered at a time. So if sensor 1 triggers multivibrator 1 then multivibrator 2 cannot be triggered while multivibrator 1 is active and it could only be triggered when multivibrator 1 has turned to its stable state (low) again (and if it is first of the two to trigger of course).


However all multivibrators I can find consumes at least 4uA when not active and 160uA when active (74VHC123A for instance). For my application this is by far too much and I've tried to build a one shot my self. I've tested the circuits in the following links:
http://www.electronics-tutorials.ws/sequential/seq_3.html
http://www.electronics-tutorials.ws/waveforms/monostable.html

The problem with these is that the period of the trigger pulses has to be greater than the time RC time constant of the multivibrator. The sensors will for sure be triggered with a period lower than the RC time constant (of 2sec) so these solutions will not work.


Do anyone have schematics for such a multivibrator (have to be a retriggerable also)?


Thanks for taking time reading this!
 

crutschow

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Re: how to build a one shot where trigger period > RC time constant?

You might also look at a CD4047. At 5V it draws 1µA maximum quiescent current.

What is your power source that 4µA is too much current? :-?

- - - Updated - - -

You might also look at a CD4047. At 5V it draws 1µA maximum quiescent current.

What is your power source that 4µA is too much current? :-?
 
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berres

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Re: how to build a one shot where trigger period > RC time constant?

You might also look at a CD4047. At 5V it draws 1µA maximum quiescent current.

What is your power source that 4µA is too much current? :-?
Thanks for your answer, I've missed this part.
4uA in standby is not that bad, worse is 160uA when active because in this application it will be in that state very often.

In the datasheet for the 4047B they did not specify the power consumption when active. Is the power consumption when active equal to the sum of the quiscent current of the device and to the dynamic power needed to charge the capacitor?
In the datasheet there is an equation given for that:
P=(2.9C(V^2))*(duty cycle) / T

Another thing that is not that obvious is if both the duty cycle and T refers to the output pulse of the one shot or if any of them refers to the trigger pulse.
 

crutschow

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For a one-shot I would expect the power to be the static power plus P=2.9C*V2*f where f is the frequency of the pulses.
 
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If this is correct then the active power consumption will be 5uA, which will cope the demands well. Thank you for helping.
 
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