Watch Dog Timer Circuit

Date: Sun, 17 Nov 2002 21:36:14 +0000
From: Roger Barker G4IDE (now SK)
Subject: Re: UI-View and AGWPE

Someone asked
> I certainly would not leave a packet system unattended if it used a PTT
> arrangement that did not include a hardware watchdog timer - it's asking
> for trouble.
>Anyone got a circuit for a watchdog timer which I can build to use with my
>soundcard, please?

G8AMG,- one of the great UK packet "gurus", once sent the circuit below in
a packet bulletin.

Disclaimer - I have never tried it.

*****  Paraphrasing G8AMG *****

Might I suggest that this device be a MOSFET, such as a VN10KM. These can be
driven by a capacitor and, using a high gate resistor to ground, will prevent
the MOSFET from staying switched on and screwing the channel AND your radio.

     Port drive                  ___________ Radio PTT
                            G |/ D
           o------||---+-----||  VN10KM
                 +  -  |      |\ S
                 10uF  |-/\/\/-| 4m7
                       |       |
                       +-|<----| 1N4148

When the Port drive line (TTL) goes high, the uncharged capacitor presents 5V to
the MOSFET, which turns ON. However as the capacitor charges via the
4m7 resistor, the voltage on the gate reduces until the MOSFET turns off.
This is not a fast change, so the PTT circuit needs to be capable of
working with this slow switch-off. A logic gate (most radios today) or a
relay (older radios) both work.

The timing of the above is between 30 & 40 seconds, quite long enough for
ANY packet (even IP).  I use 2m2 and 4.7uF on a 9600 packet link. This is
probably good enough for 1200, too.

Regular operation is allowed for by the diode which allows the capacitor to
discharge quickly once the port drive is removed. This does however mean
that the port drive needs to be TTL- compatible, in order to present a l
ow resistance ZERO state to ground. If you are concerned about damage to
the port drive line, then include a 220 Ohm resister in series with the port line.

Ideas gleaned from the 9600 tips & tricks list published by me in 1994.