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Subject: Re: How Can I Produce a Triggerless One-Shot?

Date: 08/06/02 at 10:15 AM
Posted by: David Ashby
E-mail: dashby@quin.co.uk
Message Posted:

In Reply to: Re: How Can I Produce a Triggerless One-Shot? posted by Mike on 08/06/02 at 9:02 AM:

Hello Mike,

The formula for the RESET period is

t = -R.C.ln(1-(Vthreshold/Vsupply))

where R is the resistor value (ohms), C is the capacitor value (farads), Vthreshold is the logic threahold voltage for the RESET pin, Vsupply is the supply voltage. 'ln' is the natural logarithm (to base e).

Let's say you aim to make t = 0.5 seconds.
Vthreshold is typically 1.4V for HCT logic.
Assume Vsupply is 5V.
Let's say you use R = 100K

C works out to be 15uF.

To get C down to 4.7uF, just increase R by a factor of 15/4.7, which makes it about 330K.

I wouldn't make R any higher than this because leakage currents could become too significant. There will be some variation anyway because Vthreshold could be a bit higher or lower, and the capacitor value could be +/- 10 or 20 percent. But for most purposes, the reset time will be accurate enough.

Hope this helps,

David Ashby

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