In radio Amateur practice there is often a situation when it is necessary to track the readings of each option. Propose a scheme indicator led rulers. Depending on the input voltage glows more or a fewer number of LEDs arranged in a line (one after another).
Voltage - 4...12V, i.e., when the input voltage will be 4 In burn only one (the first) led, and at 12 In all lines.
The scheme can be easily extended. To track AC stress, quite to the resistor R1 to set the diode bridge from low-power diodes. The supply voltage can be varied from 5 to 15 In, picking up accordingly, the resistors R2...R8. From the supply voltage of the circuit depends mainly the brightness of the LEDs, the input of the same circuit characteristics with virtually no change. To the brightness of the LEDs was the same, you should choose resistors as follows:
where I max is the collector current of VT1, mA;
R3=2R2; R4=3R2; R5=4R2; R6=5R2; R7=6R2; R8=7R2.
Thus, in applying the transistor KTA (lK max=30 mA) R2=33 ohms. Resistor R1 is included in the voltage divider and regulates the mode of operation of the transistor VT1. Diodes VD1 ...VD7 can be replaced by CDA, CD, D, LEDs HL1...HL8 - AL.
Resistor R9 limits the base current of the transistor VT1 and prevents damage when hit last on the circuit's input great voltage.
Author: A. Kashkarov, G. S. Petersburg