Schematic Rise

The prototype of this device will be used in a hospital operating theatre in unijnction with battery operated medical equipment (powered by four pen-light cells).
A moving coil voltmeter was not appropriate as, in the designers’ experience, medical staff 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 ‘l hour); 3) current consumption of the indicator be 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 flash a light emitting diode (L.E.D.) indicator.  The circuit is shown in the figure. The PUT (Q1) is used in a relaxation oscillator circuit. As the voltage being monitored (Vm,,,,) falls, the voltage on the gate (Vg) falls whilst the anode voltage (V,) remains essentially constant. Oscillation commences when V, falls below V, by 0.6 volts. As Vm, falls further, Vg falls and the PUT triggers at lower values of Va. Thus the cycle time shortens and the frequency of flashing increases giving a sense of urgency as the battery  approached exhaustion. Transistor O2 and C2 act as a pulse stretcher and amplifier to drive the L.E.D. display. In the prototype the trigger point can be adjusted from 4.5-5.5 volts and the current drain when V,,,,,,, is 6 volts is 1 mA (controlled primarily by R1). This is considered acceptable as the device being monitored draws 17 mA. All the requirements have been met. The components of this low battery monitor circuit are mounted on the printed circuit board of the main device.