This is an easy to build, but nevertheless very accurate and useful digital voltmeter. It has been designed as a panel meter and can be used in DC power supplies or anywhere else it is necessary to have an accurate indication of the voltage present. The circuit employs the ADC (Analogue to Digital Converter) I.C. CL7107 made by INTERSIL. This IC incorporates in a 40 pin case all the circuitry necessary to convert an analogue signal to digital and can drive a series of four seven segment LED displays directly. The circuits built into the IC are an analogue to digital converter, a comparator, a clock, a decoder and a seven segment LED display driver. The circuit as it is described here can display any DC voltage in the range of 0-1999 Volts.

Characteristics:

Supply Voltage: ............. +8...30V

Power requirements: ..... 200mA

Measuring range: .......... +/- 0-1,999 VDC in four ranges

Accuracy: ....................... 0.1 %

The value of R3 controls in fact the range of measurement of the voltmeter and if you provide for some means to switch different resistors in its place you can use the instrument over a range of voltages.

For the replacement resistors follow the table below:

0 - 2 V ............ R3 = 0 ohm 1%

0 - 20 V ........... R3 = 120 Kohm 1%

0 - 200 V .......... R3 = 12 Kohm 1%

0 - 2000 V ......... R3 = 1.2 Kohm 1%

Connect the circuit to a suitable power supply and turn the supply on. The displays should light immediately and should form a number. Short circuit the input (0 V) and adjust the trimmer P1 until the display indicates exactly ?0?.