PIC Micro Fan Controller - Fail Safe Circuit

[SparkyChris1976]

Hi All,

This is my first visit to the forums, so aplogies if this is in the wrong place!

I have designed a basic PWM fan controller based on a PIC micro-controller and a temperature sensor, which is all working ok:shock:
I would like to add some kind of fail safe circuit that would force the fans to full speed in the event of the PIC crashing and the PWM signal dissapearing , but I am stumped for how I might do this....I would be greatful if anyone has any ideas on how I might do this?

Thanks,
chris

 

[shahbaz.ele]

dear what do you means by crashing, if it means that PIC OFF, then you can add a transistor circuit that is OFF through PIC and when PIC got fail then it automatically turn ON, basic transistor switch circuit will do this.

 

[SparkyChris1976]

dear what do you means by crashing, if it means that PIC OFF, then you can add a transistor circuit that is OFF through PIC and when PIC got fail then it automatically turn ON, basic transistor switch circuit will do this.

Thanks for the reply, what I mean by 'crash' is any event that results in the PIC not providing a PWM signal, so the PIC could still be powered up but not running.
The controller should always maintain a minimum output, therefore a PWM signal should always be present.
It seems to me that I need a way of detecting when the PWM signal stops and applying a fixed, alternative PWM signal (failsafe). Its operation must be independent of the PIC, otherwise its not a fail safe mechanism.

.

 

[KMoffett]

I think you need a "watchdog" timer circuit that monitors the PWM output. It can either reset the PIC or drive the fan full-ON if the PWM disappears, or both.

Ken

 

[nodee]

What about a missing pulse detector? If the PWM pulse train stops the output of the detector will switch, you can then use that signal for the fail safe function. You can do it with a 555 check this link about 3/4 the way down the page.

**broken link removed**

 

[KMoffett]

A "watchdog timer" generally is a "missing pulse detector'.

Ken

 

[SparkyChris1976]

What about a missing pulse detector? If the PWM pulse train stops the output of the detector will switch, you can then use that signal for the fail safe function. You can do it with a 555 check this link about 3/4 the way down the page.

**broken link removed**

Thanks for the link, that looks like it might be anlong the right lines.