The range of this ac-voltage monitor is 100 to 140 Vac, with a resolution of 1 V. The speech processor interprets an 8-bit binary input code from an analog-to-digital converter. The processor's pulse-code-modulated output then passes through a filter and an amplifier before driving the circuit's speaker to vocalize the corresponding number. Each time switch S1 is pressed, the speech-processor program enunciates the monitored voltage readings from 100 to 140 V, depending on the code at the input of a 27C64 EPROM.
The voltage-monitoring circuit consists of a bridge rectifier, filter capacitors, and a 10-K ohms load resistor. A divider, RA and RB, limits the input voltage to a maximum 2.55 V. The a/d converter, IC4, then sends the voltage reading to the 27C64 EPROM, IC5. Pressing S1 sends a negative transient pulse to the write, WR, input of the a/d converter, IC4, which initiates a 100-ms conversion process.