In this somewhat unusual digital voltmeter, the measurand (voltage to be measured) is digitized in an analogue-to-digital (A-D) converter and then displayed in three decimal digits.

How digital LED voltmeter works?

The display is not the usual seven-segment type, but consists of three groups of ten LEDs. Although this type of display is a little unusual, the measured value can be read without problem after only a short familiarization period: even voltage changes can be readily interpreted. Note that the meter can only be used for measuring direct voltages.

Digital LED voltmeter Circuit Specifications:

The A-D converter is based on a CA3162, which can process direct voltage up to 999 mV (1 V full-scale deflection-f.s.d.).The f.s.d. is extended to 10 V with the aid of potential divider R1-R2-R3. Other ranges are possible by altering the values of the resistors.

How to get output from digital voltmeter?

The measured value is read from three bars of LEDs: the first one of these, D1-D10, shows units: the second, D11 -D20 tens and the third, D21-D30, hundreds. The circuit is nulled with P1 when the input is open circuit. Zero here means that diodes D1, D11 and D21 light. Diodes D10, D20 and D30 represent the figure 9.

Digital LED Voltmeter Circuit Diagram

Next, apply a known voltage to the input and adjust P2 till the LEDs read the correct value. Some people may find it helpful to use a different colour for each of the three groups of LEDs. When the input voltage is too high, the display goes out. When the input is negative, the ‘unit LEDs’ do not light. Note that variations in the supply voltage affect the measurement adversely: it is, therefore, advisable to use a regulated source, for instance, a 9 V PP3 battery and a Type 7805 regulator.

Since only three LEDs can light at any one time, the current drawn by the circuit does not exceed about 30 mA.