As well as having to provide a number of different DC voltages, your PC's power supply has to deliver an appreciable amount of power - hundreds of watts. This is the main reason why switchmode power supplies are used, because they're much more efficient than the older "linear" type of power supply. However, they're also more complex and this tends to make them slightly less reliable.

The completed PC Power Rail Monitor simply clips in the front of the PC's case, in place of an existing drive blanking plate.

Also, some PC power supplies really do have trouble supplying all that current and sometimes they fail to deliver just the right voltage at critical times - like when you are in middle of a big download off the Internet. If you build this unit, it will give you a visual and audible warning of the problem so that you can have it fixed.

Of course, apart from data loss, if a PC's power supply does happen to develop a fault, this can have quite disastrous (and costly) consequences. Replacing a blown CPU chip can involve many hundreds of dollars, while replacing blown DIMM modules can be almost as costly.

Fortunately, many of the latest PC power supplies incorporate special circuitry to detect when any of the main power rail voltages fail or go high and shut down the supply if such a fault occurs. However, such protection circuitry does not always do the job, so this monitoring circuit can still be a worthwhile addition. It's good to know that if a fault develops, you'll be warned straight away so you can "pull the plug" before much damage is done.

So that's the idea of this project. It's a low-cost, easy-to-build circuit which can continuously monitor the main power rails in a PC and display their status via columns of LEDs. At the same time, whenever it senses that any of the rail voltages has moved out of the safe operating range (too high or too low), it sounds a small piezo buzzer to draw your attention to a possible problem.