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[Andy M]

Googled it but the kit never showed in the results? Dont have a number for it do you?

[RST]

MK120 from a google search. On phone so links are hopeless am afraid. May not be any good but I can't download the pdf manual at the moment to check myself. Before anyone else says, no doubt you can import a Chinese copy for pennies, but I've used velleman etc kits before and they're ok for the money.

[Andy M]

Hi , had a look, it says range of 1-4metres. I need 0-35 cm. I dont know if that means it wont work on my set up. Doesnt mention whether its a momentary action or if it stays on.

[malcolmfrary]

The kits pictured will very nearly do the job.  Their task is to switch on when they see light, off when they don't.  Using a relay, and a non-battery supply, the relay can be permanently operated, releasing to the dark.  Or the sense pot and sense device can be transposed.
The drawing (found on the internet) shows the modification to change it to operate the relay when it loses sight of the light source.

[Andy M]

Hi, sorry not replied sooner, wondered what rl was and also I assume vr is variable resistor? Excuse my ignorance.

[RST]

Hi , had a look, it says range of 1-4metres. I need 0-35 cm. I dont know if that means it wont work on my set up. Doesnt mention whether its a momentary action or if it stays on.

Hi Andy, sorry red herring I think. The info available for it is pretty pap also.




Rich

[malcolmfrary]

Hi, sorry not replied sooner, wondered what rl was and also I assume vr is variable resistor? Excuse my ignorance.

Yes, VR has been the standard shorthand on diarams for variable resistance for a while, and while relays get a lot of different names, especially in circuits with a lot of relays, this one at least says " relay" alongside it.
Any of the kits should come with a diagram, but most switch the relay on when light falls on the sense element.  The application here requires that, unless the relay is permanently operated until the light beam is interrupted, the logic will need to be inverted somewhere along the line.  Having the sensor (which goes to a low resistance state when lit up) switch the 1st transistor off by shorting the base to emitter rather than applying a reducing resitance to the base to switch the transistor on, is the easiest way.  This does mean that the variable resistor needs a bit of padding to ensure that current via the base is limted whatever the setting of VR.