The prototype of this device will be used in a hospital operating theatre in conjunction with battery operated medical equipment (powered by four ‘pen··light' cells). A moving coil voltmeter was not appropriate as, in the designers experience; medical staffs have difficulty in interpreting a voltmeter — and sometimes find themselves half way through an` operation with exhausted batteries. Therefore, the requirements for the indicator were that:
1) The display be eye catching, easily understandable and provide a sense of urgency as · the battery approaches exhaustion;
2) Provide adequate warning of battery failure (at least 1 hour);
3) Current consumption of the indicatorbe low in relation to the main equipment;
4) Preferably, be more rugged and cheaper than a moving coil meter.
The design was based on a programmable unijunction transistor (PUT), because its threshold characteristics can be well defined, arranged to flesh alight emitting diode (L.E.D.) indicator. ‘
In the illustrated circuit diagram of a low battery warning indicator, the PUT (Q1) is used in a relaxation oscillator circuit. As me voltage