Battery guard

This protective circuit is readily in- corporated in battery-powered equipment which is typically in- tended to operate for less than about a minute; possible applications that come to mind include IR remote con- trol units, calculators, etc. Forgetting to switch off such devices irrevo- cably causes the built-in batteries to be exhausted after a while, however "low" the standby current. The proposed battery guard auto- matically switches off the supply cur- rent to the circuit, either after about one minute has lapsed after power- on, or when the battery voltage has fallen below the acceptable level for normal operation. Series regulator FET T1 can pass a maximum current of 150 mA in the circuit as shown, and it is advisable to use a more powerful type than the BS250 in case more than about 100 mA is expected to be consumed by the equipment connected to the output terminals. The Type BS250 FET drops about 0.5 V at a drain cur- rent of 100 mA, and 0.8 V at 150 mA, whence the foreg...