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urgent help needed !!!

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gut_but

New Member
hi,
any body can help me in designing a time delay circuit with only RC components which introduces a time delay of 10ms beacuse i can't use 555 timer. can we use simple low pass filter to introduce a time delay? if , so how to calculate the delay?
electronic newbie
 

audioguru

Well-Known Member
Most Helpful Member
Hi Gut,
A capacitor charges through a resistor to 63% of the supply voltage in a time period that equals R times C, where R is in ohms and C is in Farads. So a 1uF capacitor will charge through a 10K resistor to 63% of the supply voltage in 10ms. You could also use a 10uF cap and a 1k resistor, a 100uF cap and a 100 ohm resistor etc.

It might be easier to detect when a charging capacitor's voltage reaches exactly one-half of the supply voltage. It occurs in a time period that equals 0.71 times R times C.
So a 1uF cap will charge through a 14.1k resistor to half the supply voltage in 10ms.

Don't ask me for the formula. I never use it since it is worn off in my old book. I just use the graph in the book that is still clearly visible. :lol:
 

ljcox

Well-Known Member
I assume you want a digital delay, not an analogue one.

To create a delay, a simple RC low pass filter will suffice, but it needs a Schmitt Trigger such as the CMOS 40106 to detect when the voltage across the capacitor has reached a preset level.

See https://www.electro-tech-online.com/threads/pic-c-complier-free-version.6386/

These posts are for a bounce suppressor, but can be adapted for a delay by removing the switch and connecting the signal to the resistor, ie. remove the Vcc from the resistor and connect the signal to this point.

Len
 
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