thrifty LED indicator

It is often necessary that the current consumption of an essential status indicator is minimal. In the circuit shown, dependent on the level of the supply voltage, a number of LEDs drawing a current of only 10...15 mA may be switched on or off as desired. Moreover, the entire indicator may be switched off if none of the LEDs lights. The circuit is based on switched cur- rent source T1. The base current of this transistor is set at c. 15 mA with Rx. The value of this resistor is calculated from R„ = [4x1061(Ub-0.7)] Q where libis the supply voltage in volts. Transistor T2 conducts when the input to. inverter N1is logic 0: when this becomes a logic 1, the current source and, consequently, the indi- cator are switched off. If the input to one of the buffers N2. . . N4 is a logic 1, the associated LED is switched on. More LED-FET combinations may be added to the circuit as long as the supply voltage permits this. Also, the dissipation of Ti has to be kept within certain limits. A BC...