The monitor, based on Philips Components' Type TEA1041T, enables a check to be kept on the power supply lines of a battery-operated appliance. Switch S1 controls both the appliance and the monitor. The supply voltage is applied to the input (pin 1) of IC1 via potential divider R1-R2. The switching threshold at the input is 1.25 V. When the supply voltage drops below 3.1 V, the potential at pin 1 drops below the switching threshold level. This causes a timer in the chip to be actuated within about 2 s. If in that time the voltage rises above the switching threshold again, the timer is reset. If not, D1 lights for about 2 s to indicate that the battery voltage is low. This means that the appliance should be switched off with S1. Switching off the appliance also prevents the complete discharge of the battery via D1. With S1 open, IC1 continues to operate in the standby mode. The LEDs light alternately for about 4 s, which limits the average current drawn to about 10 μA. During the standby mode, there is no current through the potential divider. The monitor is suitable for use with supply voltages of 1.8–4 V; the LEDs may operate from slightly higher potentials (max. 5.5 V.

   Schematic