pic microcontroller discussion list <> wrote:
> operator. I need a discrete circuit that operate a RELAY which enable
> the motor ONLY when the PWM is present and (before, in time, of the
> PWM) the FET was not conducting.

I have a design, untested, to do this (it is designed as a watchdog for
some home automation kit, to return to normal manual operation if a
'watchdog' output from the processor is not received).  More reliably than
my untested circuit(!), you could just use a standard watchdog chip.  If it
doesn't get a pulse in Xms, it fires an output to turn off the relay.

If you want my design, I can send it as an LTspice/SwitcherCAD schematic,
you can get LTspice as a free download.  It's basically a series capacitor
then a diode charging another capacitor with a resistor across it.  As long
as the signal is AC it will charge the cap and hold a FET on.  If the
signal 'sticks', the cap discharges and the FET turns off.  It operates
over a period of a couple of seconds, a watchdog chip would be better for
fast response.

Nigel
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