The good thing is that these kind of rulers are handy because they have a holder which is empty and i decided to fit inside 2 white Leds, a button, 2 cables and 2 button cells. Also you may need some hot glue or any other glue, some tape, a soldering tool and some available time. No skills are required for this very simple but also useful project.
>2 white LEDS 2,2V ~0,20 euro both
>a button switch ~ 0,10 euro
> 2 Button cells of 1,5V each ~ 1 euro the pair
> 2 cables 2x 20cm
> a cable stripper/Cutter
Step 1: Prep the LEDs and the circuit
Try to make clean solders and also use some tape or isotherim plastic tube to prevent any short circuit between the anode and the cathode.
Reply this procedure also for the next LED.
So I have to draw a small schematic so follow this image and no skills needed.
The switcher is a small one like the image's one
With the soldering tool make a small hall to fit the exposed part of the switcher, This is maby the most harder part of the project.
Another thing is the choise of battery cells. I prefer the AG13 1,5V or LR44 size and all you have to do is to fit them in the gap of the ruller and try to solder the cables to to + and - pole of the battery pair. (This isn't to hard enough but you have to apply first some flux in the surfaces and the sordering tool must be very very hot) All you have is to touch for some secs the surface of the battery cells with the soldering tool and some solder 60/40 and is ok. No problem for the batteries if you touch them for some seconds!!!.