I have been requested to look at a project. The first half of the project is complete, but I am struggling with the second half of the requested specification.

For the second half of the project, I require a six month timer. Accuracy is not an issue as long as the output action is within 4-5 days of timer reset. At the six month point, an LED is illuminated and an internal sounder activates. The sounder can be muted, via a push button, but resound after a 24 hour period and repeats until the service counter is reset, which also extinguishes the LED and the timer process re-starts.

Looking into it, I believe I require a microprocessor to handle the timer event, but not to sure how to handle the buzzer/mute function. Could this be via a secondary processor (i.e. primary processor outputs to a secondary processor input and PIC is programmed to handle the LED and buzzer functions)? If so, is there anything I should be aware of under this set-up?

Appreciate any advice and help with this.

  • \$\begingroup\$ (1) Does your system already have the LED, buzzer, mute button? If so, where are they connected to? (2) How do you plan to keep track of time over a six month period? How are you going to create a real time clock (RTC)? How are you planning to power the RTC? \$\endgroup\$ Nov 17, 2018 at 19:26
  • 3
    \$\begingroup\$ What is controlling the existing project? If it has a controller then why not build the functionality into that? We're missing a lot of context here. \$\endgroup\$
    – Transistor
    Nov 17, 2018 at 19:28
  • \$\begingroup\$ The first part of the project is an interface isolation unit for a fire suppression system. The PCB is powered by an on-board 230VAC - 12VAC transformer. The service module is not physically part of the isolation unit, but more of a service reminder system. The 'service reminder' will be housed within the same enclosure as the interface isolator and is the only association to this. \$\endgroup\$
    – Titch2K6
    Nov 17, 2018 at 19:33
  • \$\begingroup\$ I do not need to track 'time' as such. More of a 180 day countdown timer.... \$\endgroup\$
    – Titch2K6
    Nov 17, 2018 at 20:06
  • \$\begingroup\$ So maybe a RTCC processor? \$\endgroup\$
    – Titch2K6
    Nov 17, 2018 at 20:28

1 Answer 1


as it's a fire alarm I'll assume it has a near continuous power supply and that outages are both brief and infrequent.

have a microcontroller count time and 5 minutes into every hour update the non-volatile memory with elapsed time (so after 5 minutes it writes 1 hour elapsed, and then 60 minutes later writes 2 hours elapsed etc) this could be done in trhe form of tallies where each hour represernted by a bit until you get to 100 hours and then you write an integer value or elapsed hours and clear all the tallies - this reduces the number of erase cycles needed prolonging the life of the EEPROM

for the led and button just connect them to GPIO pins (like normal), for the sounder you can also use a GPIO PIN and use that to control an sounder, or amplify it an connect it to a loud-speaker.

it may be possible to use the timing loop to also conditionally drive the speaker, or have two different timing loops, one noisy, and one silent.


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