Project: dim bulb tester

Warning - this project requires you to work with mains connections. Be careful!

This device is useful for testing mains-powered devices that you're not sure will work, or that could be at risk of shorting out and breaking even further - for example, tube radios and old televisions. The idea behind it is quite simple: it works just by placing an old incandescent lightbulb in series with what you're testing. The lightbulb will limit the current going to the device and, in case there is a short, it will simply light up at full brightness.

Building the tester

Building one of these devices is rather simple, and can be done in a number of ways depending on if you want extra features or not. The simplest way to build one is to just take an extension cord, cut one of the wires (or cut all of it and reconnect the others like before) and put a light socket in between.

There are a few extra features that could be added, however, depending on your specific needs, and doing so could be useful for a device set up on a workbench. For example, the simplest thing to add would be a power switch - that'd let you leave the tester always plugged in, and ready to be used.
Another thing that could be added is a second socket that's not going through the lamp - this might be useful if you test things outside,to plug in extra equipment. Additionally, a switch to bypass the bulb (once the device has proven to be working) could be helpful - this would also allow you to connect more than one lightbulb, and select the right one (or use more than one) depending on the device being used.

What I built

I built my tester out of cheap electrical components, and that's probably the easiest way to make one of these devices, since it gives you a lot of flexibility. In particular, in Italy sockets and lightswitches are made with modular systems which let you add different modules depending on your needs, something extremely useful for this project.

The components I got were:

• a 4-module box
• a double-pole on/off switch
• two 2-way switches (I only ended up using one)
• a 16A socket
• a lamp socket

Operation is very simple: the main switch turns the power on or off, and the bypass switch simply bypasses the lighbulb.

The downsides

Unfortunately, due to the extremely cheap components I used, I couldn't actually fit both 2-pole switches in the box like I intended to, so I had to only use one and cover the leftover hole with some tape.
This is obviously not ideal, and if I decide to rebuild this device I'll definitely fix it, although for now it's not a big deal, since I haven't had the chance to use it all that much anyways.