It is late at night, you are working on a project and you find that your electrical multimeter has a dead battery.
This happened to me, I looked everywhere but there was nowhere I could get a 9V battery until the morning. Not ready to give up yet, I decided to make a quick and easy solution to test continuity in circuits.
Step 1: Materials
This instructable is based around a battery powered LED light. This particular light came with a bit of double sided adhesive tape to stick in a cupboard or under the stairs for extra light but I am sure you could adapt a torch in the same way. You will also need an old cable such as an old phone charger or USB cable pictured here. A USB cable may be better as the cables inside are individually insulated. The last thing you need is something to accurately place on the terminals you wish to test. All i could find was 2 small pins that worked quite well but larger nails would work also.
The tools you will need include:
Small flat head screwdriver
Long nose pliers
Step 2: Dismantling
With the flat head screwdriver you can lever the battery cover off and remove the batteries.
Once the batteries are removed you can squeeze the flat head screwdriver in between the battery housing and the outside casing. With this light the casing popped off quite easily to expose the circuit board with LEDs as well as the wires connecting the battery housing.
These wires connecting the battery housing to the circuit board are what we will be looking at in the next step.
Step 3: Preperation
You can now plug the soldering iron in and allow it to heat up while you prepare the wires.
Taking the cutting pliers cut the end of your phone charger or USB cable etc This will expose a number of cores. In in a USB cable you will find 4 insulated wires with various colours of insulation covered with a metallic shield. You will only need to use 2 of the wires,in this case I chose to use the red and green ones. The wires you use will need to be stripped with the cutting pliers to expose the copper wire. The remaining wires can be snipped or simply bend back out of the way.
Once the cable is prepared the soldering iron should be up to temperature. You now need to hold the soldering iron on one of the wires soldered on to one of the battery terminals until the solder melts and you can remove the wire.
Step 4: Soldering the Cable
We can now begin soldering the stripped wires into the circuit. This light had holes in the back of it so first of all I threaded the prepared end of the cable through. You may have to drill hole in yours if you wish to close it back up neatly.
The first wire I soldered onto the battery terminal that we took the original wire off of. At this stage you might find the need for an extra hand. I found it useful to put the pliers on the cable to weigh it down and hold it in place while I soldered.
Connecting the other wire would be much easier if you had a cable connector. I didn't so used the soldering iron to connect the second stripped cable and the cable that was first removed from the battery terminal. I used a scrap bit of wood to solder on.
When the solder has cooled you can wrap a bit of insulating tape on the exposed cable and assemble the light housing again.
Step 5: Attaching Electrodes
Snip the other end of the cable and strip the wires again. This end requires a lot more of the outer sheath to be removed. I used the long nose pliers to pull the insulation off after I'd cut it. As I was using the red and green wires I could cut the other 2 off.
Connecting the pins to the cable ends will be harder than soldering to the battery terminal because for the solder to take the 2 materials being connected must be the same temperature. To keep the wire in place while the pin heats up I wrapped the wire around the pin head a couple of times. This process was repeated for the other wire.
Step 6: Finish
The last thing I done was wrap some electrical tape around the end of the insulating to tidy it up a bit.
This may be simple but it was a very useful in a time of need and will be kept on the shelf as I am sure it will come in handy again.