[nos-bbs] PTT protection circuit (finally found it) !

[M Langelaar]

Happy Friday !

I found a very interesting circuit recently, see further below, an 
article from
the year 2002, by Roger Barker (SK), talking about circuit from G8AMG.

NOTE : Use at your own risk. I'm not to blame if you blow up your
             Rig or Transmitter or Antenna System or whatever ...

I have successfully used the circuit below (using junk box parts), so be
warned, my values are not the same as those shown below. Also, I use
the circuit to DRIVE MY EXISTING optocoupler PTT interface. I'm a big
fan of completely isolating computer from RIG, so optocoupler stays.

<<< article starts now >>>

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 low 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.

*************************