Introduction: How to Create a Watchdog Timer for Computers!
Ever wanted a means to reboot a locked up PC? Especially one that is supposed to run 24-7 and does a fairly important job? This instructable is part one of how I achieved this using a neat little programmable timer from Amazon.com. It is powered from 5VDC and has 18 different modes of operation. As a watchdog it needs just one!
I attached the pdf of the timer's manual.
Read on if I have wetted your technical appetite.
Step 1: The Timer's Buttons and Terminals.
The first pic shows 4 buttons. From left to right they are:
SET: This enters/exits/selects the operating mode.
SM1: This cycles through the digits on each menu.
NUM+: Increases a digit from 0 to 9.
NUM-: Decreases a digit from 9 to 0.
Next to the 4 digit LED display are 4 smd LEDs. From Top to Bottom there are:
MD: The menu for selecting the operation mode is active.
T1: The wait period before action occurs of the relay.
T2: The period during which the relay is active (coil energized).
NX: The number of repeats of a particular operation (depending on the mode selected).
The power terminal block is from left to right: 5VDC, 0VDC, Trigger.
The other terminal block are the relay contacts NC, COM, NO.
Step 2: Powering It Up!
I used my portable power supply set at around 5VDC. The digital wattmeter at its output conveniently displays the electrical parameters. I used 2 alligator clips and a single wire to complete the wiring to the timer.
Step 3: Programming!
When powered, hold the MD button a few seconds. The smd LED for MD goes on. For watchdog use, mode 5 is needed and can be selected by pressing the NUM+ button.
Single pressing MD makes the T1 LED on to give the delay time in seconds. For demonstration I set it to 9 seconds but for the final installation I will use 900 seconds or 15 minutes.
Single pressing MD makes the T2 LED on to give the active or on time for the relay coil. To reset a PC a 1 second pulse is just fine.
Single pressing MD makes the NX LED on to give the number of repeats based on T1 and T2. I left this on its default since changing it made the relay behave weird.
Press and hold MD to exit programming mode and the relay starts counting down from T1 to 0. Upon reaching 0, the relay smd LED goes on and the relay pulls in for the seconds in T2 (in my case only 1 second).
Step 4: Resetting the Watchdog!
A watchdog is only useful if it detects the PC is crashed. My home automation PC can send a high (3.3VDC) pulse to the CH1 (or trigger) terminal once every 10 seconds. I have a Labjack U3-HV and a custom flowstone software suite to manage my home (solar energy, dog food, dog water etc). The next instructable will show me installing my new watchdog timer into the embedded control system which is my Home Automation System.
Now to reset the decrementing timer back to T1 is simple. A high (>3.3VDC) pulse to the CH1 terminal does the trick! Mind you it must be a repeating pulse generated from the host PC. If that pulse no longer appears the timer will send a 1 second relay contact closure to the reset button's terminal on the PC's motherboard. Most reset buttons short two terminals together, one being the reset input and the other 0VDC or chassis.
For those of you without a Home Automation setup like mine, a convenient reset signal could be the HDD/SSD activity LED. The HDD- pin from the motherboard actually does the switching and this is perfect for resetting the watchdog. Mind you, the PC LED must be connected to the motherboard for this to work.
I hope I have opened a few minds with this instructable. I actually already have a watchdog timer but I wanted an upgraded version to show timer activity and the inclusion of a manual reset button. Part two of my watchdog timer instructable is here: