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I am trying to add a sensor to my 8 underfloor heating actuators (24V) so I can monitor when they are 'On'. I thought this would do the job, although I'm only sensing 24VDC. Input goes through MB6M bridge rectifier, then PC817 optocoupler. From the output TTL pin I'm always getting high reading though. I understand DC into bridge rectifier also outputs DC.

Assume this be due the the input voltage being too low so its not suitable for my project? When I bump the input up to 30V and the TTL output went low. Any suggestions on which components I should switch out to convert board to a 24V sensor?!

Existing:

220V input -> Resistor (150k ohm) -> Bridge Rectifier (MB6M) -> Diode -> Capacitor (100uF 25V) -> LED -> Optocoupler (PC817) -> Output pins

Specs from AliExpress:

Feature: Functuon: Test whether AC 220V is existed Ouput TTL Level: can use MCU to test,voltage is 3-5V Output can connect PLC 24V

Wiring: When connecting the PLC input, the PLC's COM terminal is connected to the 24V power supply, the input is connected to signal outputs S1-S8 of the board, optocoupler GND is connected to the power supply negative. Blue 5.0 terminal is connected to 220V voltage, yellow pin signal output. When the 220V voltage is normal, it output low level; when 220V is powered off, it output high level.

AC 220V MCU TTL Level 8 Ch Optocoupler Isolation Test Board

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Reduce the resistor to 15K, or just solder 15K parallel with the existing resistor, 0.05W or larger size will be fine.

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