Great idea, I'll look into it

Mauricio Jancic
Janso Desarrollos
Microchip Consultant Program Member
(54) 11-4542-3519
info@janso.com.ar
www.janso.com.ar


>>-----Original Message-----
>>From: pic microcontroller discussion list
>>[mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Nigel Orr
>>Sent: Friday, July 30, 2004 5:13 AM
>>To: PICLIST@MITVMA.MIT.EDU
>>Subject: Re: [EE]: Discrete challenge
>>
>>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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