MK125 Photo Diode

I have bought the MK125,
I have build the kit, but in stead of the LDR they delivered a photodiode.
I can’t get this to work properly. I have tried several things. The voltage over the photodiode responds to the light, but the variation is to small to work properly.

Why do they deliver a photodiode and not an LDR?

LDRs are prohibited by the EU now due to being not RoHS compliant.
We’re sorry you experience the kit in such way.
Is everything solder properly?
Please post a picture of the soldered kit.
Best Regards,


I have tested the kit with an LDR and it works fine.
When i tried with the photodiode the relay switches when i turn the trimmer but it doesn’t responds to the light.
I will sent a picture of the kit asap.
I assume the photodiode has a direction like a real diode. In what direction do I have to place this. Can it be broken when I connected it the wrong way? I have tried both directions.

I don’t think the photodiode will be damaged.
Have you tried putting a shrink tube on it.
This will make the viewing angle narrower which should yield better results

Best Regards,


I have tried again with the photodiode. It seems i haven’t tried both directions previous.
It only works with the photodiode in one direction.
Perhaps Velleman should ad a remark to the instuctions about the mounting of the photodiode?

Thanks for the support.

I experience the same problem with the mk125 kit I bought. I’m not very experienced in electronics.
Could you tell or show me in wich direction (polarity) i have to place (weld) the photodiode.

Helo Fred,

See below the adapted circuit diagram in function of the photodiode (correct polarity)
to replace the LDR, due to EU legislation (RoHS directive).

Hope this will help You further.

Best regards,
Velleman Support

Thank u 4 the fast support.
Will try 2 do so

I have same problem but replace the LDR worked just nice. Meanwhile I needed to check how does a photodiode works to applicate it nice but it was worthy.