Watchdog relay driver chip or circuit

Thread Starter

jdupre

Joined Sep 6, 2018
3
I need a watchdog timer to control some "failsafe" relays.

There are a lot of watchdog chips, but the outputs are typically designed to create a momentary pulse to toggle a reset line. What I need is the output to hold some relay coils energized until the timeout, at which point the relay coils are de-energized. The coils should remain de-energized until the timer is reset with a tickle on the input. If the input is "tickled" before the timeout, then the timer should be reset and the output should continue to hold the coils energized.

Does anyone know of a chip that can do this?

Ideally, the output should keep the relays de-energized at power-on until the first "tickle" of the watchdog, but that is not a hard requirement and it would be OK if the timer starts and the output drives the relay coils at power on.

Thanks!
 

danadak

Joined Mar 10, 2018
4,057
I would do this either with an ATTINY and Bascom Basic or a PSOC 4.

Can you code either in C or Basic ?

What is the timeout length required ?


Regards, Dana.
 

crutschow

Joined Mar 14, 2008
27,472
Sounds like you need a retriggerable one-shot.
How long do you want the relays to stay on?
What is the supply voltage?
 

Thread Starter

jdupre

Joined Sep 6, 2018
3
Sounds like you need a retriggerable one-shot.
How long do you want the relays to stay on?
What is the supply voltage?
Under normal operating conditions, the relays will be energized. If firmware fails to tickle the watchdog, the relays should relax and stay relaxed. If the firmware can manage to wake up to tickle the watchdog after it timed out, then the relays should re-energize until watchdog times out again.

I have 5V and 3V3 on the board. Relays on 5, cpu on 3V3
 

danadak

Joined Mar 10, 2018
4,057
This is one possibility, PSOC.

Might be possible to fit into $2 part, using state machine wizard, I got lazy. This
is a PSOC 5LP

upload_2018-9-6_18-12-32.png

Regards, Dana.
 
Last edited:

AnalogKid

Joined Aug 1, 2013
9,296
Is there only one timer circuit? If so, how many relays is it driving?

Separate from that, how long is the timeout/watchdog period?

ak
 
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