Heartbeat monitor in rack
The heartbeat monitor in the rack box project, connected and working.

As I started putting more and more modules into my rack box project, the previous monitoring module quickly ran out of inputs. I needed to build something bigger and better. I realized that using LEDs to show module status just wasn’t practical as the number of inputs increased. So this unit shows the module statuses on an LCD display instead, and has 15 inputs.

Table of content


If you’ve read about my first module heartbeat monitor you already know why I built it, if not; here is a recap: I needed a way to know that the modules inside the rack box project were not only powered, but running. Monitoring their heartbeat output was the solution.

If more than 10 seconds passed, without receiving a heartbeat this monitor unit would go into alarm state. A blue LED would flash, and the LCD display would show which module failed.

10 FAIL!

All alarms had to be manually reset, even if the heartbeat came back. The reason for this was so that I would see it if a module had been dead for some time, but came back by itself. This would still indicate a problem with that module. If a heartbeat was not detected at all after a reset, that input was marked as inactive and #n/a was shown on the LCD. Inactive inputs did not produce alarms.

4  #n/a

The remaining time was also shown on the LCD display, it was split into half seconds; so 20 meant that there was 10 seconds left.

1  Ok>20



  1. PA2 Module 1
  2. PA3 Module 2
  3. PC2 Module 3
  4. PC3 Module 4
  5. PC4 Module 5
  6. PC5 Module 6
  7. PC6 Module 7
  8. PC7 Module 8
  9. PD0 Module 9
  10. PD1 Module 10
  11. PD2 Module 11
  12. PD3 Module 12
  13. PD4 Module 13
  14. PD5 Module 14
  15. PD6 Module 15
  16. PD7 Reset


  1. PB0 LCD Db7
  2. PB1 LCD Db6
  3. PB2 LCD Db5
  4. PB3 LCD Db4
  5. PB6 LCD E
  6. PB7 LCD Rs
  7. PA0 Module error signal
  8. PA1 Run LED


D-Sub 9-pin

  1. 5v
  2. 0v
  3. Reset
  4. Module error LED

D-Sub 15-pin

  • Module 1 15

LCD display

I used a broken LCD display on this module, of the 16 characters on the display only the first 8 worked. Below are the different messages and statuses shown on the display.

1  Ok>21
4  #n/a
10 FAIL!

Please leave a comment or tweet if you have any questions or feedback on this project.

Schematic drawing

Module heartbeat monitor with LCD circuit
Schematic drawing for module heartbeat monitor with LCD; with AVR AT90S8515.

Source code


Detects and alerts if any of the monitored modules goes silent; meaning their heartbeat stops. Written in BASCOM-AVR.

Master Releases

0 0 0 v1.0 Visual Basic 2 months ago

Image gallery

Parts list

Project: Module heartbeat monitor with 15 inputs and LCD by Thomas Jensen is licensed under CC BY-SA 4.0 with attribution required.

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  •   Created Dec 28, 2006
  •   Last modified 2 months ago
  •   Project status: Completed