Talkback for article: 239, July2002

Hardware watchdog and shutdown button

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From: Greg Pratt <gpratt3151(at)> [ date: 2002-07-04 ]
Once again, thanks for a great article! Interfacing external hardware to
Linux systems is an important topic and you've done a great job starting
from the basics and building up from there.
From: Alain Gauthier <AlGauRythme(at)> [ date: 2002-07-08 ]
We need good articles like yours, especially in hardware. You do a great
job. Congradulations and keep on the good work.
From: William Hathaway <william(at)> [ date: 2002-07-08 ]
Very good article.

However you noted only in passing a realworld application for this circuit. That is rebooting a hung computer that controls hardware is in an inaccessible place such as on the top of a mountain.

From: colyli <colyli(at)> [ date: 2002-08-03 ]
This watchdog maybe can resolve Linux SMP auto powerdoen problem. When we try to type "halt -p" to shutdown SMP Linux system, the machine often failure for powerdown. If this watchdog can set a timer after we shutdown sytem, and act hardware to powerdown when timer expired, we can implement system auto powerdown!

great designing!
From: Freak [ date: 2002-09-18 ]
Great article...(on of the best i've read in the last month)

one question left: do you think it is right, that the RTS Voltage goes to the Reset Pin(4) of the NE555 insted of the Control-Votlage Pin(5), like in your drawing??
May be, i am fault, but i don't understand, why the NE555 schould be under reset all the time...
From: guido [ date: 2002-09-18 ]
HI "Freak",
no the NE555 schould not be under reset. That's why it has pin 4 connected
to supply voltage. Reset connected to logical "1" means reset off for the

From: Freak [ date: 2002-09-18 ]
ok, thank you very much for your reply, i will try to build this circuit soon...
From: guido [ date: 2002-12-29 ]
There is a new and updated design available (version 0.4):
From: atrak <watchdog(at)> [ date: 2003-02-10 ]
А это наш watchdog

From: Bhavesh Gudhka <gudhkabhavesh(at)> [ date: 2004-03-04 ]
I am looking for a circuit that can power off computer after halt. Is it posible with your circuit?

please suggest if any customization is required.

From: Guido Socher [ date: 2004-07-29 ]
Kurt Schlager has some variants of this watchdog on
his homepage:
You can find there also a driver for Windows.
The site is in German.
From: flavio vasconi <flaviova(at)> [ date: 2004-12-31 ]
hi Guido, thanks for the great job you did.
By the way there is a small problem: if the application sending the pulses to DTR dies and the DTR remain permanently on, the wachdog does not detect it. A solution is to drive with this signal the reset 4. If you are interested i can send the diagram to you.
Thanks flavio
From: james last <sdf(at)aol.spam> [ date: 2005-02-09 ]
great watchdog !!!!!!!!
Exactly what I was looking for .. Thanks you very very much !!

From: hygy <hygy(at)> [ date: 2005-10-06 ]
So is the DTR remain permanently bug resolved?
From: hygy <hygy(at)> [ date: 2005-10-08 ]

So the solution of the DTR stay on high level when crashed is:
DTR --------||-----------R3 (3k3)-----
| |
LED 2k6 ohm
| |
| |
1k ohm |
| |


I put a led to see the signals comming from serial port. (I try this on a test board with 12V)
Now I connect it to serial and check it there too.

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