I want my mini Christmas light set to be at half brightness. A diode should work right? Does it need to be a particular diode? Would a 1N4001 do it?

 

The 1N4001 has a PIV ov only 50v or so.
Far better to use a 1N4007.

JimB

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Experience is directly proportional to the value of the equipment ruined.

 

1 N 4007 as already mentioned before is a good option.
You can even fit the diode in the mains plug.

Also an other option is to put 2 strings in series on the mains voltage which will run them at exact half voltage and quarter power.

I have both options done this with my Christmas lights for the last 20 or so years with no problems at all.

On off via timeclock so no further work required.

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Electricity, Electric clocks, Meters and Trains are great.