I am trying to make an alarm system (audible and visual) using a siren and 3 strobe lights. I am using 12v battery like the one used in cars. Now I would like to detect when any of the alarms have failed. I think a strobe-light/siren would fail when it is open circuited or short circuited. So what came to my mind is that the current will increase dramatically when the strobe/siren had just failed. I need a comparator circuit (maybe Schmitt trigger? or an op-amp circuitry?) that will detect over-current in those alarms. Once that alarm failure is detected, I would like the user of the alarm system to be alarmed maybe by a piezo or LEDs.

  • \$\begingroup\$ How do you power your strobes and the siren? A system may fail in many unpredictable ways, odds are that periodic inspection will be needed too. \$\endgroup\$ Jul 14, 2014 at 8:05
  • \$\begingroup\$ The strobes and the siren are powered by the 12v battery. They are triggered by a relay board which is controlled by a controller. \ \$\endgroup\$
    – aloano
    Jul 14, 2014 at 9:21
  • \$\begingroup\$ I am only concerned about the siren and strobes failing, not the system. \$\endgroup\$
    – aloano
    Jul 14, 2014 at 9:22
  • \$\begingroup\$ The siren is a system, as well as the strobe. My point is that unless they are designed to detect and report fault conditions making something that handles all the fault statuses is quite impossible. I'd say measuring the current is a good idea, I was also thinking of a microphone and light detectors... \$\endgroup\$ Jul 14, 2014 at 9:24
  • \$\begingroup\$ I thought about the microphone and light detectors, but that would not function properly for the strobes as the system is already exposed to sunlight, or would the light detector (photo-resistor) be sufficient regardless of sunlight? do you recommend an over-current detection circuitry? \$\endgroup\$
    – aloano
    Jul 14, 2014 at 9:29

1 Answer 1


For the lights I suggest to use synchronous light detection.

Because your system will be exposed to the ambient light, using a simple threshold would not works (or at least not very well).

What I suggest instead is to take a measurement when the flashing light is supposed to be off and another when it is supposed to be on. Then you subtract the two. If the difference is above a defined threshold, the flashing lights are working. If they are not working, the two measurements would be close to each other and the diff would be close to zero.

If you repeat this several you may reject thing such as ambient light switched-on, sun, flicked of the nearby lamp, etc.

You may use whatever light sensitive device as soon as it is never saturated (when exposed to the sun for instance). Otherwise the synchronous detection won't work and will report a broken light when it is working fine.

Other thing such as current measurement are not completely closing the loop. What you care is that light is emitted, not that the electrical behavior of the light is correct. Think about your light covered with paint. This should be detected as a light failure (we are considering an Alarm system here, right?) Light measurement would detect this but electrical one would say that the system is fine.

The same thing apply to the sound. If you measure the sound using a mic and ensure that is it running loud. Then it's fine. This is what matter at the end.

  • \$\begingroup\$ I do not think that would work because what if all 3 strobes turn on simultaneously. \$\endgroup\$
    – aloano
    Jul 14, 2014 at 11:18
  • \$\begingroup\$ You have to choose to turn them on at the right time. You are not controlling the strobe? \$\endgroup\$
    – Blup1980
    Jul 14, 2014 at 11:21
  • \$\begingroup\$ the alarms are controlled based on given event. when event A happens, strobe #1 triggers. I will try to fit a photo-resistor inside the enclosure of the strobe if that is what you mean. But then how do i go about the circuitry of detecting whether a failure is present? do you have a circuit in mind? \$\endgroup\$
    – aloano
    Jul 14, 2014 at 11:24
  • \$\begingroup\$ Please let me know if something is unclear. My English is not so great. \$\endgroup\$
    – aloano
    Jul 14, 2014 at 11:31

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