Monitoring module in rack box
The monitoring module in the rack box project, surrounded by wires.

This was the first AVR microcontroller module I ever built, it was made for and installed in the rack box project. As the name suggests it monitored that the essentials were OK, that means power supplies, fuses and communication. It used optocouplers to detect if the 5 or 12 V power supply failed, but depended on other modules for the fuse, emergency supply and communication failure.

Table of content


When a situation were detected the corresponding LED would flash, and there was a sound signal. These LED outputs were later also used to notify other modules, like the controller for the stack lights and the sound alarm controller. So it was a daisy chain of connected inputs and outputs.

One problem with monitoring the 12 V power supply was that this module needed 12 V to work… So it bypassed the 12 V main fuses, but if that supply failed completely the monitoring module would also fail. And thus no alarms, it wasn’t really a bullet-proof solution :p

Alarm situations

Server timeout

To make sure the server was present, responding and the I/O module was OK; a heartbeat signal was sent every 60 seconds. If this signal went silent for over 310 seconds, meaning 5 minutes and a bit, the server was regarded as timed out and the alarm sounded.

Emergency supply failure

Without the emergency supply voltage the emergency stop would not work, so that was important. A N.C relay was connected to this supply, and if the supply dropped the relay would activate the emergency failure input.

Power supply failure

The 5 and 12 V supply were monitored with internal optocouplers, so if any voltage or polarity was dropped; it was detected by the module. When a 24 V power supply was added later an additional optocoupler was installed between 5 V ground and the monitoring module input.

Fuse error

The detecting of broken fuses happened in the fuse boxes (this and this), they sent a signal when a fuse broke which caused this module to sound the alarm.

Try and try again

This being the my very first AVR microcontroller project, it took a bit of time to get it to work and I made some schematic mistakes. That’s why you can see a few extra stripboards stuffed into the box in the image gallery, I used the wrong resistor values for all the inputs and had to redo those. That was also when I added the optocouplers.


In retrospect I don’t think the module was very useful, but it was a fun learning experience back then. If a main power supply fails, it is going to have a big impact. Like if the 5 V supply failed, all microcontrollers would stop. Meaning no controller would be alive to receive the error signal. If the 12 or 24 V supply dies there would be no horn, sirens or lights to show the error. So it seems that the best option for a monitoring module like this would have been to have been completely self sufficient, own warning lights, horns etc. And a battery failover. Maybe next time :)



  1. Server heartbeat
  2. Fuse error (N.O)
  3. Emergency supply failure (N.O)
  4. 5V supply OK, from optocoupler (N.C)
  5. 12V supply OK, from optocoupler (N.C)


  1. Buzzer
  2. Alarm signal (to other modules)
  3. Fuse error LED
  4. Supply failure LED
  5. Run LED
  6. Heartbeat output
  7. Server timeout LED

Schematic drawing

Supply and fuse monitoring module circuit
Schematic drawing for supply and fuse monitoring module; with AVR AT90S2313.

Source code


Simple monitoring module for the essentials in the rack box project; power supplies, fuses and communication. Written in BASCOM-AVR.

Master Releases

0 0 0 v1.0 Visual Basic 3 months ago

Image gallery

Parts list

Project: Power supply and fuse monitoring module by Thomas Jensen is licensed under CC BY-SA 4.0 with attribution required.

Similar resources

  •   Created Jul 29, 2005
  •   Last modified 5 days ago
  • Project status: Completed