Apart from the risk of a (dangerous) electric shock and the near certainty of a very massive short circuit that is the consequence of drilling into a cable, the aftermath of this mishap is no fun either. In any case, the damaged cable has to be replaced – and that is already not nice when these are running neatly through conduit, but in other countries where the wires are often tidily hidden under the plaster work, this is a small catastrophe.
Prevention is better than cure, and that is why the better hobbyist makes use of a cable locator. But what to do on a rainy Sunday afternoon, when it is absolutely essential that a hole is drilled, but the DIY store is closed and you cannot quickly buy a cable locator? Then you build one yourself in a few minutes! Complicated it isn’t, as the schematic of Figure 1 proves.