Alarm disabling unit on production equiptment
Alarm disabling unit #1 installed and connected to production equipment. Alarm LED is lit.

This module was built for a production equipment at my previous workplace, it made it possible for an operator to silence an alarm without resolving it. Fixing an error could take some time and having the alarm horn sound during that period was very annoying, so many operators removed the alarm relay. This was unfortunate because it was not always put back, so the next alarm situation could go unnoticed. This module would silence the alarm, but still reactive it once the situation had been resolved. Eliminating the need to remove the alarm relay. This production equipment was a temperature tester, keeping it running was vital for the temperature to be stable. Powered by: 9-24V.

Table of content


Alarm reset

When an alarm situation occurs the red LED flashes and the production alarm sounds, by pressing “reset alarm” the voltage to the sound alarm is cut. The red LED lights continuously when the alarm is reset. When the alarm situation is resolved the red LED turns off and the voltage to the sound alarm is restored. When a new fault occurs the procedure is repeated.

Warning long stop

If an alarm situation persists more then 30 or 60 minutes (jumper setting) the module will sound the alarm again. That means if the previous alarm was reset the voltage to the sound alarm will now be restored, blue LED starts flashing. This alarm is reset the same way, by pressing “reset alarm” the voltage to the sound alarm relay is once more cut. The blue LED will now be on continuously. The purpose of this alarm is to make the operators aware that they should consider defrosting the cold temperature chamber if the equipment is to be left stopped.

Service mode

During maintenance or other work where multiple alarms are expected; the module can be put in service mode. This will disable the alarm horn for 1.5 hours, after which the module will return to normal operation. Service mode is aborted if the “reset alarm” button is pressed.

Enter service mode

  • No alarms must be present or silenced.
  • Press and hold the “reset alarm” button for 20 seconds.
  • Alarm LED will flash, let go of the “reset alarm” button within one second.
  • The power LED will start flashing quickly to indicate that service mode is active.


Input pins

  1. PD.0 Reset button
  2. PD.1 Stack lights signal
  3. PD.2 Jumper for setting long stop timer

Output pins

  1. PB.0 Red LED
  2. PB.1 Blue LED
  3. PB.2 Stack lights relay N.O
  4. PB.3 Power LED

D-Sub 9-pin connector

  1. 24V
  2. 0V
  3. Stack lights in + (to optoisolator)
  4. Stack lights in - (to optoisolator)
  5. Stack lights relay +
  6. Stack lights relay - N.O

Internal connectors

Alarm disabling unit #1 internal connections
Where to locate the internal connections on the circuit board.
  1. GND to internal reset button (Without resistor)
  2. Signal from internal reset button (With resistor)
  3. GND to stack lights relay (D-sub 6)
  4. Signal from stack lights
    • Red : + (D-sub 3)
    • Blue : - (D-sub 4)
  5. 24V supply voltage
    • Red : + (D-sub 1)
    • Blue : - (D-sub 2)
  6. 24V to stack lights relay (D-sub 5)
  7. Jumper for setting long stop timer
    • With jumper : 30 min
    • Without jumper : 60 min

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

Schematic drawings

Alarm disabling unit #1 circuit
Schematic drawing for alarm disabling unit #1; with AVR ATtiny2313.
Alarm disabling unit #1 connections
How alarm disabling unit #1 interacts with the production equipment.

Source code


Simple unit to silence the audible alarm on a production equipment, written with BASCOM-AVR.

Master Releases

0 0 0 v1.1 Visual Basic 4 months ago

Image gallery

Parts list

Project: Alarm disabling unit for production equipment with long stop warning by Thomas Jensen is licensed under CC BY-SA 4.0 with attribution required.

Similar projects

  •   Created Feb 27, 2007
  •   Last modified 5 days ago
  •   Project status: Given away