This project is not intended to compete with the more elaborate lead-testing equipment which is available to the industrial user; rather, it is designed to provide a quick and reliable method of detecting damaged or wrongly-wired cables.

Construction has deliberately been kept simple in the hope that those readers who up to now have been spectators only will be persuaded to take up a soldering iron and join the revolution.

The only unusual aspect of this design is the bicoloured LED which is to be found in position B on the panel. LEDs are such a convenient source of coloured light that it is easy to forget that they are diodes, and the bi-colour device exploits this fact by incorporating a red and a green LED into a single package. The two devices are connected in opposite directions so that either the red or green part illuminates, depending on the direction of the current flow, but because of the package design, both colours appear to emanate from the same source. By applying an alternating current, both LEDs light alternately, giving the effect of yellow light.

Circuit Operation

In this circuit, when a correctly-wired balanced lead is plugged into the lead checker, all three LEDs light up green. If, however, the two signal wires are crossed over at one end, the current flow through the bi-coloured LED is reversed, resulting in a red light being emitted.

An unbalanced lead will short together the ring and screen connections and so LED C will remain extinguished, only A and B being illuminated.

The operation of the lead tester should be apparent from the schematic diagram, where it can be seen that the lead under test is connected such that its two signal wires and screen are joined by means of the LEDs connected to the jack sockets, so that any break in continuity will prevent any of the LEDs from lighting.

When the lead under test is removed from the sockets, the battery is removed from the circuit and so there is no possibility of the unit being left on and flattening the battery. The 330R resistor limits the current through the LEDs to a safe value.

Assembly

Construction should provide few problems, even for the complete novice. If however, the current circuit does not work first time, the chances are that you have connected one of the LEDs backwards.

LEDs are generally marked by a flat on the body which corresponds to the cathode lead, but in some cases the manufacturer opts for the system of making the cathode lead shorter than the anode lead. This system is fine unless you get a second hand LED where both leads have been cropped to the same length.

If you have any doubt about which way to connect a particular LED, simply connect it to a 9V battery via a 330R (or thereabouts) resistor and make a note of which end has to be made positive in order to coax light out of it. If it won't light either way, then the chances are that it's dead! Don't connect up an LED without a series resistor or it will soon fall into the last category.

In order to avoid damaging the LED by the application of excessive heat (clumsy solderers take note), it is best to leave the legs long and to solder the wires to their ends, not forgetting to fit a price of plastic sleeving for insulation.

All parts including the bicolour are readily available from most of the mail order component suppliers or from a shop such as Tandys, so there is really no excuse for not building one of these. Seriously though, it's no joke when your synth starts cutting out during an important gig attended by the representatives of the only record company that could be bothered to listen to your demo tape.

In Use

So - having bullied you into agreeing to build one, I'd better tell you what it can do for you.

As there are quite a lot of unpleasant things that can happen to a three-conductor cable, I will present the possibilities in the form of a table.

When testing leads, don't forget to bend them and apply a little controlled violence to show up any intermittent fault. The system can be adapted for use with XLR connectors simply by connecting them in parallel with the existing jack sockets.

(Click image for higher resolution version)

[Note: the various circuit images are mixed up between this project and the s-trigger project. We have corrected the images in the article.]