Msg # 3251  Type:B  Stat:$  To: ALL   @RTON    From: AD8I    Date: 16-Jul/0710
Subject: Timer for unattended TNCs  ...
Bulletin ID: AD8I0716    
Path: AD8I
 
 
With the recent cases of Kantronics lockup,  It may be time to
remind those with stock KPC-x (other than KPC-4 or KAM) of the
danger in unattended operation of any TNC which lacks a watch-
dog timer  (just ask W9ZRX about the damage  to his HF station
when his KPC-2 locked up).
 
Here is the schematic of a timer designed for the KPC-2.  How-
ever, it will work with with any TNC lacking a safety timer.
 
 
         (R39)
   _____/\/\/\/\____________ Q3 (base)
  |       4.7K
  |                   ___________ +5V
  |                  |        |
  |       ___________|___     >
  |      |               |    >
  |      |           8   |    > 100 K
  |______| 3             |    >
         |   (NE555)     |    >
         |               |____|
  U14 ___| 2,4      6,7  |    |
Pin 12   |               |    |
         |   5        1  |   ---
         |_______________|   --- 470 uF
             |        |       |
            --- .01   |       |
            ---       |       |
             |        |       |
            -+-      -+-     -+-
 
All of the parts are available from your local Radio Shack.
 
NE555     (RS 276-1723)  $1.19
470 uF    (RS 272- 957)  $ .89
.01 uF    (RS 272- 131)  $ .49
100 K     (RS 271-1347)  $ .39
pc-board  (RS 276- 159)  $1.49     (optional)
 
The constants shown will give about a 51 second timeout which
is suitable for HF and VHF.  Reducing the capacitor to 220 uF
(RS 272-956) would reduce the time out to 24 seconds  for VHF
only work.
 
The original KPC part numbers refer to the KPC-2, they may be
different in the KPC-1 (Q3 is the PTT transistor, R39 is the base
current limiting resistor for Q3).
 
I suggest that all KPC owners install a hardware timer in their
unit. It may save a rig, power supply or worse if the TNC ever
locks up.
 
73,
 ...Joe, AD8I