This month’s Mini Project may be used to guard any voltage ranging from a minimum of 3 V to a maximum of 25 V. It can, therefore, be used to keep a constant eye on the voltage of a car battery. But it can also be used with the receiver batteries of a radio-controlled model, for example.Even though this is an extremely simple electronic circuit, the voltage guard, equipped with a 10-segment LED display, offers a number of distinct advantages. Firstly, this circuit is very compact, it can therefore easily be built into a suitable case — the circuit board is no bigger than a matchbox and there will generally be no problem to find some space for it. Secondly, the circuit (especially when in ‘dot-mode’) is very thrifty with current.