A Simple Continuity Checker

This simple continuity checker is a nice experiment and toy for kids. You can easily build it with them and for them and afterwards they can explore their surroundings, finding out which things conduct and which don't. It is also a way for kids to learn a thing or two about simple electrical circuits since the checker is a broken circuit, that can be closed by bringing the tester tips to the same conductor. In this case the LEDs will illuminate.

I installed everything without soldering using crimp bootlace ferrules to make it even easier and more child-friendly. These crimps are commonly used to hold the strands of flex wire together.

Note, that this device is not nearly a professional tester. It is not meant to measure sensitive electronics (for which you still care), circuits under current and in no case to poke it in the electrical outlet. So be careful your kid doesn't any of this things and don't do so yourself.

For the battery holder use the following materials (or just buy battery contacts):

• the 9V battery package
• 2x short M3 screw
• 1x M3 washer
• 2x M3 nut
• some wire
• some cardboard
• some office tape

For the electronics use:

• 2x banana socket (or use a 2 way terminal block, to connect the test leads directly)
• 4x 3mm LED
• 1x resistor (something between 100 and 200 Ohm)
• 6x crimp bootlace ferrule
• some wire

For the tester tips use (or just buy two test leads):

• 2x nail (e.g. 5,5 mm stainless steal)
• 2x 1 way terminal strip
• 2x banana connector
• some insulated flex wire
• some heat shrink tubing

Tool I used:

• scissors
• cutter knife
• marker
• hollow punches (3mm and 7mm)
• wire cutter
• crimping tool
• screwdriver
• metal saw
• a heat gun

Use the battery package to build a battery holder. Mark the position of the contacts an remove the plastic cap carefully. Make holes in the position of the contacts. I used a 3 mm hollow punch, but something pointy like a nail can also do.

Put the screws through the holes. The heads have to be inside, they are the contacts for the battery. For better fit I put a washer under the screw head, which contacts the negative pole. Take some bendable wire and make two loops to fit over the screws. Put it in such a way over the screw, that the loop is pulled more tightly as you tighten the nut.

Put the battery in your holder, stuff some pieces of cardboard around the battery to make a tight fit. Stick the cap back to the cardboard base of the battery package with some office tape.

Make two holes in the cardboard base to thread the wires to the bottom side. I used the 3 mm hollow punch again. Wood drills are also a good tool to make clean holes in cardboard. Just spin them between your fingers carefully. Of course you can also just run something pointy through the cardboard.

Make two holes for the banana sockets. I used the 7 mm hollow punch.

Why do I use 4 LEDs?

• Why not?
• More LEDs are always better.
• We have 9 Volt to waste.

Red, green and yellow LEDs work at a potential difference of circa 2 Volt. At lower voltage they won't let current through. So 4 LEDs add up to 8 Volt and we have to waste 1 Volt using a resistor. This also means, that there will never be more than 1 Volt between the measuring tips, which is nice. I hope you remember to not use this device on sensible electronics (if something bad happens, I am not to blame) but it is better to be safe then sorry.

The 3 mm LEDs can handle up to 10 mA (0.01 A) of current. So we have to limit it with a resistor of the right size. A little less current is not a problem. It will result in lower brightness of the LEDs, but as long as we can see when they are illuminated, everything is good. So we need a resistor of at least:

• R = V / i
• R = 1 V / 0.01 A
• R = 100 Ohm

If you use a LED less, you have a potential difference of 3 V at the resistor and your calculation is as follows:

• R = 3 V / 0.01 A
• R = 300 Ohm

I used a 147 Ohm resistor so we have a current of:

• i = V / R
• i = 1 V / 147 Ohm
• i = 0,0068 A = 6,8 mA

It easily suffices to illuminate the LEDs visibly and a lower current is less probable to damage things. The device is not supposed to come near sensible electronics, but...

Strip the wire, which leads to the positive battery pole and crimp the resistor to it. Crimp the longer leg of the first LED to the resister. The longer leg is the anode and must always be connected to the more positive side. Crimp the shorter leg (cathode) to the longer leg of the next LED and so on. Crimp some wire to the shorter leg of the last LED, make a loop around the more positive banana socket (I used a red one to make it look a little bit more like an actual measuring instrument) and tighten it with the nut. Remember to do it in the direction mentioned above.

Make sure that the wires are insulated where they cross each other and connect the wire leading to the negative pole to the free banana socket. Fix the LEDs with some office tape.

I had some pieces of flex wire with crimp bootlace ferrules at the end from a disassembled project laying around and reused two of them. Screw the banana plug to one end and the 1 way terminal stripe to the other.

Behead the nail so you can fit it in the other side of the terminal and fasten it with the screw.

Put some heat shrink tubing over the test tip and shrink it with a heat gun. (Yes a lighter or candle can also do but be careful, you don't want to burn your test tip or fingers.) Now the tip looks more like the real thing.

Do the same for the second measuring lead and plug the leads to your continuity tester.

You can use the tester to find the connections between a plug and the cable you cut for some project, as shown in the picture. You can also explore what objects and materials are good conductors. Once again: Stay away from high Voltages (9V are okay, really), expensive electronics and the electrical outlet.