Watchdog with NE555

jmspaggi

Hi there,

I'm trying to build a watchdog with the components.

With 2 NE555 and some BC547 transistor, I'm able to have one output which stay at 1 as long as the input is swtitching from 5 to 0 and then back to 5 in less than a minute.

I built that because I have already 50 NE555 unsused waiting for my on my desk [noparse];)[/noparse]

My concerne is when the output move to 0. I will reset the controller, but the output will stay at 0 until the software start to actiave the pin. But since I'm resetting, the pin stat will never change. Catch 22...

So I'm looking for a solution to have a pulse to 0 when the output move to 0.

Like:

Output
1 ------+                             +-------
        |                             |
0       +----------------------------+

Result
1 ------+  +------------------------------
        |  |
0       +--+

Any clue how I can do that? Maybe with a capacitor? But how? The pulse don't need to be long. But after that, it should not pulse again until Output come back from 1 to 0.

Thanks,

JM

kwinn

Sounds like you need one wired as a missing pulse detector, and have that feeding a second one wired as a monostable. The other option is to have the one wired as a missing pulse detector and feed the output through a capacitor so the edge of the 555 transition produces the reset pulse. You should find both the missing pulse detection and monostable circuits in the data sheet.

jmspaggi

I give up [noparse];)[/noparse]

I tried all day and all evening long and was not able to build it. I think I will simply buy a watchdog component and keep my NE555 for another so good idea [noparse];)[/noparse]

JM

Beau Schwabe

jmspaggi,

Now I'm confused.... wouldn't OUTPUT and RESULT be considered the same thing?

If the RESULT is indeed the desired result you want from the OUTPUT, then there is a simple capacitor solution in mind that might work for you.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

jmspaggi

Hi Beau,

I think a simple capacitor will have been a good idea, but my electronic knowledge is not good enought even to do that. So I bought some MAX6301 instead and everything is now working fine.

Initialy, the goal was to have RESULT pulsing low when and only when OUTPUT was going low.

JM