Many battery-operated appliances have an LED indicator that shows the state of the battery. If the battery is almost flat, the LED flickers. The circuit described enables this facility to be built into apparatus that has no indicator. The drop across D1 (LED) depends on the type of LED and is normally around 2 V. This voltage is applied to the input of a discrete differential amplifier, T1-T2 When the base voltage of T1 is smaller than ULED• it switches on, whereupon C1 gets charged via R1. The rising potential on C1 will cause -T3 to switch off, whereupon T4 begins to conduct. Because of that, D1 goes off and remains so until Ci is discharged. Assuming that the base voltage of T1 is still smaller than LED, the cycle starts afresh. In this way, the LED continues to flicker until the base voltage of T1 becomes larger than LED. Only Tr) and T3 are then on and T4 is off so that the LED lights continuously.
The setting of preset P1 determines the point at which the LED begins to flicker. The frequency at which it flickers depends on the supply voltage and the value of C1. This means that the frequency may be changed by altering the value of this capacitor.
The value of Rv determines the current through D1 and thus the brightness at which it lights. In practice, the LED and Rv are often already present in the appliance.