This is a simple continuity tester using a green L.E.D and two button cells. It is built into a clear ball point pen body.
Step 1: Description
Electronic circuits are built by connecting components together. This connection might be direct, as when you twist the leads of two components together. Or it might be using a length of wire, or via a trace on a circuit board.
In any case, it is essential that some points are connected together, and equally essential that some other points are NOT connected together. A continuity tester enables you to quickly check whether points intended to be connected are really so, and that points intended to be isolated are also really so. Such a check is usually performed on a circuit board before any components have been loaded into it.
The technician's instrument of choice, the multimeter, is not really suited for a quick check. It gives you a reading, which then has to be interpreted. However, some meters do have a "continuity" position with a beeper for this purpose.
I have here attempted to provide instructions to make a small intrument with visual indication of continuity, suitable for checking continuity of conductors, low value resistors and high value capacitors. It is not suitable for checking semiconductor devices. It might be possible, depending upon your LED, to check diodes with it.
Step 2: Parts
First get the parts together. A green light emitting diode has been chosen because its forward voltage is about three volts, around the same value as that of the two button cells when connected in series. The two button cells have been selected of a size small enough to fit into the ball pen body. The LR41 cells I could find would fit only inside the cap of my ball point pen, but you might be able to find smaller cells - or a larger pen. Sundry other parts are also required, like a spring to press the two cells together to make electrical contact, wires, and a crocodile clip. An alligator clip will do too, or you might prefer to leave the end of the wire stripped a bit. A solder pin or similiar stiff piece of wire is required for the probe tip. If you intend to use a steel pin or needle be warned that it might prove to be impossible to solder.
Step 3: Battery
Check that the two cells and spring fit in the pen. You might have to discard some parts and file down some bits to make them fit. The outer case of the cells are their positive terminals. The negative terminal is that bit of metal showing at the unmarked end. The spring should be placed at the positive side, to avoid the risk of the two terminals being shorted together. The two cells are placed the same way, with the negative ends facing away from the spring. Like this: (- cell +)(- cell +)/\/\/spring\/\/
Step 4: Probe Tip
Fit the probe tip into the other end of the pen. Extract the part that closes the bottom, melt a hole through it with a piece of fine wire (heat it in a flame) and fit your probe tip and fix it in position with a little superglue.
Step 5: L E D
Now fit the LED into a length of empty ink refill tube. Solder wires to its ends, cut a slit in the side of the tube, insert the wires into it and pull until the LED lies next to the tube. Flatten the tube so that the assembly will fit inside the pen. If you can't solder - clean the wires well, and twist them together tightly.
The LED will only light when it is connected the right way around. If you haven't cut its leads yet, its longer lead has to connect to the battery positive (the battery outer, contacted by the spring). If your LED is round, it will have a flat on the body next to the negative lead. This lead has to connect to the probe tip.
Make a trial connection and ensure that the LED lights up before fitting it in position. They go together like this: (- cell +) (- cell +)/\/\/spring\/\/----wire---long lead (LED) flat side ---- probe tip
Connecting the short lead of the LED to the battery negative should make it light up. If it doesn't, get a new LED, fresh batteries, another spring, and good wire and try again until it does.
Step 6: Putting It Together 1
Connect the probe to the LED and fit the assembly into the body of the ball point pen. The LED should point upwards, away from the probe, and should be close to the probe tip. That way, the person using it does not have to shift his eyes too much away from the circuit being probed to look at the LED, and its light shines into his (her?) eyes.
Step 7: Putting It Together 2
At the other end, the wire from the LED should be poking out of the ball pen barrel. Clean its end well and wrap it around the spring. The spring should press the end of the wire against the battery positive. This is the end of the battery that has the maker's name stamped on it, together with other useful information.
Step 8: Putting It Together 3
Melt another hole in the top of the cap of the pen, this time with thicker wire. Take a length of flexible wire and fit an alligator or crocodile clip (depending upon which side of the creek you are in) to the far end. Pass the other end of the wire through the hole in the cap (because the clip is too big to pass through) and take off the insulation for about an inch (around 25.4 millimeters) and wrap it around a short flat faced screw (or nail) for the negative battery contact.
Step 9: The Complete Assembly
Assemble it, inserting the two cells into the cap and pressing it into the barrel. The LED should light when the probe touches the clipodile croc, and that's it, it's finished. It might be a good idea to wrap some tape at the join of the cap so that it doesn't come open accidentally and spill the button cells on the floor so that they roll over to where your toddling pet is going to eat it up.
Step 10: Using It
Use it to check for shorts and opens, obviously.
It can also be used to obtain a rough idea of the resistance of a component. A 470 ohm resistor between clip and probe resulted in a distinct reduction in the brightness of the LED. Higher valued resistors resulted in further reduction in the glow, and, at the other end of the scale, the glow due to a 10K resistor could just be distinguished if I shielded the thing from light. In a dark room, with dark adapted eyes, I might be able to make out the glow due to a 100K resistor.
Connecting a capacitor (about 10 microfarads) resulted in a brief flash of the LED. Reversing the capacitor gave me another flash. Thus the probe can be used to check capacitors too, provided they are larger than about 10 microfarads or so.
Connecting it to a diode resulted in the expected behaviour: ie, the LED lit when it was connected one way only. If your green LED requires more than 2.4 volts to light it, however, it might not be possible to check diodes using this probe.
LEDs require more than a volt to light them and so it is not possible to check them using this probe.
This probe makes a good gift to the electronically oriented guy (or gal) and does not require too much resources to put together. So if your friend is into electronics and even if you are not - pick up a few pens and a couple of odds and ends and gift a continuity tester probe, which you have made yourself, this Christmas. Even if that guy is neck deep in skads of test equipment he will welcome this addition to his arsenal.