While back I stuck a CDS light sensor on the front porch light. Wired it up myself based on one of these “Trinity Lighting Post Sensors” that I picked up from Lowes. Well it finally died and had to be replaced. So opportunity to disassemble something!
Strange what I found. This light sensor really is a temperature sensor! The CDS part is pretty obvious since it’s sticking out of the front, but I had (wrongly) assumed there was some type of TRIAC circuit inside that turned the light on and off based on the CDS reading. Not so!
It’s a mechanical switch. The metal vanes flex and turn the circuit on and off. What flexes the circuit? look at the picture. Looks like a resistor fixed on the back of one of the metal vanes. When the CDS is in the dark it’s high resistance keeps all current out of the resistor on the metal vane. But… when it’s light outside, the CDS sensor conducts, causing the metal vane to heat, bend and open the light circuit.
I always wondered why it took several minutes after turning the power on in the daylight for one of these guys to actually sense the daylight and turn off. Appears it’s because it takes awhile for the metal to heat enough to open the light circuit!