Wiring Schematic Diagram

This is simple 3-digits digital volt meter. The range is -99mV to +999mV. Two integrated circuits are used, the CA3161 and the CA3162.

IC1 converts the analogous signal to a digital signal and also produces the multiplex signal to drive T1-T3. The BCD-code from IC1 then goes to IC2, a BCD to 7-segment decoder/driver. From there the signal goes to the displays.

The timing for the CA3162 is supplied by a 786Hz ring oscillator, and the input at pin 6 determines the sampling rate. A 5V input provides a high speed sampling rate (96Hz), and grounding or floating pin 6 provides a low speed (4Hz) sampling rate. When pin 6 is fixed at +1.2V (by placing a 12K resistor between pin 6 and the +5V supply) a 'hold' feature is available. While the CA3162E is in the hold mode, sampling continues at 4Hz but the display data are latched to the last reading prior to the application of the 1.2V. Removal of the 1.2V restores continuous display changes. Note, however, that the sampling rate remains at 4Hz.

The 'EEE' or '---' displays indicate that the range of the system has been exceeded in the positive or negative direction, respectively. Negative voltages to -99mV are displayed with the minus sign in the MSD. The BCD code is 1010 for a negative overrange (---) and 1011 for a positive overrange (EEE).

To callibrate the circuit, connect HIGH and LOW to mass and adjust P1 to read '000'. Then connect a tension from 0 to 1V to HI and LOW with LOW connected to mass. Adjust P2 to read the tension in mV.

The measuring range can be raised, using tension dividers. To use the decimal points, connect DP1, DP2 and/or DP3 to mass.