LED Relay circuit problem

Thread Starter

fshgtai

Joined May 12, 2024
3
Hi, I am working on a design that uses a relay to control LED lights. The schematic shows +12VDC LEDs connected to (+) of terminals, the other end (-) of terminal is connected to an opto-coupler and relay, which feeds into the MCU. I only see a high signal at the opto-coupler output whether the relay is on or off. I'm not sure what the issue is with the attached circuit. Can anyone help me identify and rectify the problem? I have already designed a PCB with this circuit and would like to avoid repeating the same mistake in the new build. Thank you

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LowQCab

Joined Nov 6, 2012
4,290
If J-6 & J-7 are actually Jumpers, or maybe External-Switches ??
then the LEDs in the Opto-Couplers have +12-Volts applied to both sides.

Or, is this 2-Connectors for 2-Batteries ?, or 2-Power-Supplies ????

The Schematic is not clearly marked.
It doesn't matter if You know what they are, if nobody else can see it plainly.
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AnalogKid

Joined Aug 1, 2013
11,196
You have the same GND symbol on both sides of the optos. This usually means there is only one GND net, and connecting it across the optos defeats the isolation. Optos usually cannot sink much current, so U5 and U6 probably can be replaced with small signal transistors similar to Q1. Part numbers would help.

ak
 

Thread Starter

fshgtai

Joined May 12, 2024
3
If J-6 & J-7 are actually Jumpers, or maybe External-Switches ??
then the LEDs in the Opto-Couplers have +12-Volts applied to both sides.

Or, is this 2-Connectors for 2-Batteries ?, or 2-Power-Supplies ????

The Schematic is not clearly marked.
It doesn't matter if You know what they are, if nobody else can see it plainly.
.
.
.
The J6 and J7 are terminals where LEDs + and - connects. When relay energized by NPN transistor 2n3904, circuits completes and connect LEDs with ground. Opto-coupler datasheet is here https://www.digikey.ca/en/products/detail/everlight-electronics-co-ltd/ELM453-TA-V/5142599
 
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