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A hold/delayed reset switch
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A hold/delayed reset switch

by ThomasBags on Tue Mar 31, 2009 7:28 am

So I've been throwing around an idea to safely power off some of my devices without accidental bump into the reset button. I had the idea of creating something that would act similar to a PC power off switch, in that after you hold it for a certain amount of time it would force shut down.
For this project I would like to shy away from using a microcontroller and just find a way by using regular cheap components as I'd like to incorporate them into a few different project.
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Re: A hold/delayed reset switch

by franklin97355 on Tue Mar 31, 2009 11:58 am

Take a look at the 555 chip. Might be what you want for this.

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Re: A hold/delayed reset switch

by zener on Tue Mar 31, 2009 5:04 pm

To me it sounds like a $20 solution to a $1 problem. If I understand the problem. Just put your reset button where it won't get bumped. However, I look at these things practically, and people here seem to want to have "fun".

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Re: A hold/delayed reset switch

by Amberwolf on Wed Apr 01, 2009 2:40 am

Two-bit solution :) : Use an RC filter on the reset button.

If reset is active low, put a large-value resistor from VCC to reset line of your project.
Large-value capacitor from reset line to ground.
Large-value resistor from reset line to one side of reset button.
Other side of reset button to ground.
Small-value resistor from reset line to actual reset pin of MCU, to prevent any possible large-scale currents from flowing out of MCU into the fully-discharged capacitor.

Calculate values of reset button's resistor and the capacitor such that it will take a few seconds to discharge the capacitor down to a level that will trigger reset.
Ensure that the resistor from VCC to reset line is such that it cannot charge the cap faster than the other one discharges it (should be several times the value of the reset button resistor, must be sufficiently larger that the voltage divider created by it will allow a low enough voltage to cross the reset threshold).

Pressing the reset button will ground the capacitor thru the reset button resistor, discharging it at the rate of RrbC minus whatever current is being fed back into it via the divider created by the VCC-reset line resistor. At some point, the voltage will be below the reset threshold and the project/device/etc will reset.

If reset is active high, reverse all the components' VCC/ground connections.
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Please be positive and constructive with your questions and comments.