In this instructable, we can see how to make one simple tool to verify the electrical continuiity, using 2 old pens and one old 9V battery.

Step 1: Material

Parts and materials:
2 old pens (optionally equals)
1 old 9V battery
1 LED (of any colour)
2 pieces of a single-line wire of 1.5mm (5cm long)
1 piece of a flexible wire of 1.5mm
solder

Tools:
Drill
Universal Pliers
Soldering iron

Step 2: Prepare 2 Pairs of Cells

After disassembling the old 9V battery, we must keep 4 inner round cells, in order to make a series of 6V DC (4x1.5V), making 2 pairs of cells, soldering each of those pairs of cells to one single-line wire, like shows the picture.

Step 3: Assembling Process

Drill one hole in both caps of the pens. Stick the flexible wire in one of those caps and one pin of the LED in the other cap.

With the 2 pairs of cells prepared, stick them inside the pen's casings, keeping their tops out, in order to allow to solder the flexible wire and the LED, matching the positive top of a pair of cells with the Anode of the LED and the negative top of the other cells with the Cathode of the LED.

Solder it all together, like it's shown in the picture.