July 3, 1997

Battery monitor emulates auto dashboard

TG Barnett and J Millar, University of London, UK

When you use battery-powered scientific-recording equipment, you must know the state of the battery, because an unnoticed battery run-down can lead to loss of data. Several types of warning indicators are available, but all have their drawbacks. Moving-coil indicators, test terminals, and digital flags, for example, all are wanting in some respect--they can be inaccurate or unreliable, have an unacceptable battery drain, or have some other operational problem. Ideally, you need a passive and fail-safe system requiring no action on your part. However, if anything goes wrong, you should receive an automatic alert. The automobile industry inspired the simple circuit
in Figure 1.

When you start the ignition in a modern vehicle, the warning lights on the dashboard briefly come on and then go out if all is well. The same thing happens with the configuration in Figure 1: Upon equipment switch-on, the super-bright LED momentarily lights up, thereby confirming an adequate power-supply voltage. The LED then goes out, but if the battery voltage falls below a set level during operation, it permanently switches on. The circuit uses a MAX951 ultralow-power, single-supply op amp/comparator/reference, configured as a unity-gain comparator.

The 2 Megaohm potentiometer sets the voltage at the positive input of the op amp; in this example, at 1.44V. When the battery voltage falls to 5V, the comparator output goes low, thus illuminating the LED. At switch-on, with a 0.01-µF value for C₁, the LED gives an approximately 30-msec flash. Note that the MAX951 can operate with a supply voltage as low as 2.4V. (DI #2052)

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