LED voltmeter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology
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If you decide to assemble a voltmeter, and you do not have a pointer device, use an LED indicator instead. The electronic device is a three-stage (VT1-VT3) DC amplifier with a VD2 LED at the output.
Before measurement, the engine of the variable resistor R4 is set to the upper position according to the scheme, while the transistor VT1 is securely closed. By lowering the voltage at its emitter, they reach a value at which the measured voltage Ux will be slightly higher than the reference voltage, then all three transistors will open and the LED will light up. Since the voltmeter has hysteresis, calibration is performed only by the ignition threshold of the LED, moving the R4 slider down. The measurement limit is 2 ... 10 V with an accuracy of 0,1 V, the input resistance of the device is about 70 kOhm.
Possible replacement: VT1 KT3102 (h21e > 200); VT2 KT3107, KT361; VT3 KT315, KT312 - with any letter indices. VD2 - LED suitable for current.
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