I have an address sign that is supposed to light when it gets dark outside. The issue is the photodiode (assume that's what the part is) is broken and has broken off the board. I have tried connecting it manually for testing purposes, but it appears the component is somehow damaged.

I'm therefore looking for advice as to what component is the correct one to swap in for the broken one. The catch is, there are no markings or information on the packaging. I have marked up the circuit and measured voltages with the legs of the "photodiode" shorted and open. Shorted the LEDs are off and open the LEDs are on.

Any help or advice would be greatly appreciated.

C1 = 25 V, 100 µF

R1 = 114

R2 = 621

R3 = 4R70

BDR = MB105

Q1 = Photodiode ??

Q2 = 1AMI

Q3 = NCE60R540K enter image description here

enter image description here

  • \$\begingroup\$ Welcome! Not an answer to your question but a resistor marked ”114” means 11*10^4 ohm or 110 kohm in engineering notation. \$\endgroup\$
    – winny
    Mar 31 at 18:21

1 Answer 1


Looks to me like it's a phototransistor with 110kΩ R1 establishing the switching brightness level.

Phototransistors in LED packaging are cheap. I suggest you just purchase one in a similar package (probably 3mm or 5mm diameter) and try it. For example, this 5mm one.

Note: Get the kind with clear epoxy housing, not the kind with built-in IR pass filtering which have a black-looking or light-blue package color. Also, pay attention to the polarity.


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