Digital Voltmeter

Construction

Digital Voltmeter


Raj K. Gorkhali

A

voltmeter finds its importance wherever voltage is to be measured. Here we present an easyto-build and accurate digital voltmeter that has been designed as a panel meter and can be used in DC power supplies panels or where it is necessary to have an accurate indication of the voltage.

Circuit description
The circuit uses an analogue-todigital converter ICL7107 made by
INTERSIL. Fig. 1 shows the pin detail of IC L7107. This IC incorporates in a
40-pin case all the circuitry necessary to convert an analogue signal into digital and can drive a series of four seven-segment LED displays directly.
Parts List
Semiconductor:

IC1
- ICL7107 3½ digit analogue-to-digital converter
IC2
- 7805 +5V regulator
IC3
- 7905 -5V regulator
DIS1-DIS4
- LTS542 CA seven segment display
Resistors (all ¼-watt, ±5% carbon):

R1
R2, R7
R3
R4
R5
R6
R8
VR1
VR2

Capacitors:

C1
C2
C3
C4, C6, C7
C5
C8

- 470-kilo-ohm
- 100-kilo-ohm
- 22-kilo-ohm
- 220-ohm
- 10-ohm
- 1-kilo-ohm
- 1-mega-ohm
- 2-kilo-ohm trimpot
- 1-kilo-ohm trimpot
- 0.22µF ceramic disk
- 0.47µF ceramic disk
- 0.01µF ceramic disk
- 0.1µF ceramic disk
- 100pF ceramic disk
- 220µF electrolytic

Miscellaneous:

S1
- Push-to-on switch
S2
- 2-pole, 4-position (each ganged) rotary switch
BATT.1,
BATT.2
- 9V battery

6 2 • J u ly 2 0 0 7 • e l e c t ro n i c s f o r yo u

Fig. 1: Pin details of ICL7107

The circuits built into the IC are an analogue-to-digital converter (ADC), a comparator, a clock, a decoder and a seven-segment LED display driver.
The circuit can display any DC voltage in the range of -1999 volts to +1999 volts. It operates off ±5 volts.
In order to understand the operating principle of the circuit, it is necessary to explain how the ADC IC works.
An analogue-to-digital converter, better known as a dual slope converter or integrating converter, is generally preferred over other types as it offers
accuracy,