I made this relay board because sometimes controlling a light isn't enough you need to learn the output state of the relay. Like a wifi light relay with both digital and physical control, if someone opened the light the system is still believes light is close.

To make programming the feedback system I used an LED and a LDR. To make a sensor with LED and LDR I used a heat shrink to hold them together and get rid of the light coming from other light sources.

Assembly Tip:Solder SMD parts before the THT parts.

Video link for DIY LDR optocoupler:

https://drive.google.com/file/d/1SBVrfAwivtBD26G0oaLDu6JXZxFsdvAU/view?usp=sharing

Feedback system works with a basic principle. If you use it with AC, first you rectify 220v AC(swings between +220v and -220v) to 220v DC(swings between +220v and 0v). If you use it with DC there is nothing different but thanks to bridge rectifier we don't need to pay attention to polarity of connection. After rectifying it we connect the LED through a current limiting resistor (470K for 220v and 330R for 12v) to limit the current running through the led. Because we didn't smooth out the rectified voltage brightness of the LED will change according to the swinging voltage. So we can detect the brightness of the LED with a LDR.

Parts:

UR1 MB10S-TP Bridge Rectifier

R1 1/4W Resistor (you need to calculate the resistance for the voltage you want to use the feedback for)

R2 1/4W 1K Resistor

R3 1/4W 10K Resistor

R4 5mm LDR

LED1 5mm LED

D1 M7 SMD Diode

Q1 BC817 SMD NPN Transistor