Wiring two LED lights such that when one fails, the other turns off (not series) - Page 1
Forum section: electronics / beginners

author: doublec4
author_rank: Regular Contributor
timestamp: January 16, 2021, 05:11:31 am
content: Hi all,I have a 12V power supply and two LED modules that require 12V. They are LED marker lights that you would find on emergency vehicles (police/ambulance/etc). They have an internal circuitry that allows them to produce flash patterns, etc, and so they only require 12VDC, no external drivers/resistors etc. The internal circuitry PWMs the lights at some frequency.Currently the lights are wired in parallel from a single relay.I have a current sensing circuit that I built to monitor the status of the lights (0-5V analog signal to my microcontroller) to ensure that they are on. Both lights from the single relay are going through the current sensor since they are wired in parallel. Due to the PWM nature of the lights, the current measurements are erratic so it is difficult to determine if both lights are on, or if only one has failed. If both were to fail it would be easy to see that there is no current draw.I would like to know if there is some way to turn both lights off if one fails, making it easy to detect using my current sensor? I cannot wire them in series because they both need 12V and my supply voltage is only 12V... and I cannot wire them through two separate relays with two separate current sensors because I do not have any more relays/analog inputs on my controller. I have maxed them out.So is there any suggestions on how to wire them in parallel such that they still both see 12V, but if one fails it takes the other one down with it?Thank you!

author: ledtester
author_rank: Super Contributor
timestamp: January 16, 2021, 05:27:02 am
content: What's your controller?If you have a couple of digital I/Os you can use an analog mux to add more analog inputs:https://electronics.stackexchange.com/questions/67004/how-do-i-extend-the-number-of-analog-input-pins-available-to-me

author: doublec4
author_rank: Regular Contributor
timestamp: January 16, 2021, 05:48:28 am
content: Controller is X400 from control by web:https://www.controlbyweb.com/x400/I am using their X19s I/O expansion board:https://www.controlbyweb.com/x19s/The same company offers more expansion boards etc. but they aren't exactly cheap. I am hoping there is a less expensive solution than more of their products.I'll take a look at that link you sent ledtester, thank you

author: Terry Bites
author_rank: Super Contributor
timestamp: January 16, 2021, 02:40:36 pm
content: I think current sensing is eminently sensible. Measure the average working current for the pair and use the result to disable driver if its too low. Lots of current sense chips include a comparator. For automotive I like Allegro CS chipshttps://www.allegromicro.com/en/products/sense/current-sensor-ics/zero-to-fifty-amp-integrated-conductor-sensor-ics

author: Siwastaja
author_rank: Super Contributor
timestamp: January 16, 2021, 03:16:08 pm
content: Low-pass filter the current measurement signal. First-order RC filter before the ADC (for example, fc = a few hundred Hz), then more averaging by software (for example, sample at 10kHz, average 1000 samples to get 100ms long average).