Eight LEDs Make a 100-Division Voltmeter

The circuit in this Design Idea makes a voltmeter that reads to 0.99V. The idea uses a counter IC to drive two sets of four LEDs (Figure 1). Each of these two sets represents a BCD (binary-coded-decimal) value. With all of the LEDs off, the voltmeter reads 0V. With all of the LEDs on, the reading is 0.99V. Op amp IC1A generates a predictable voltage ramp.

Figure 1

Voltmeter Circuit Diagram

You use op amp IC1B as a comparator to compare the ramp to an input signal. The higher the input voltage, the longer the output pulse from IC1B is. You use this pulse to gate free-running oscillator IC2B. A potentiometer on this multivibrator circuit allows you to adjust the full-range count. The voltmeter has a maximum input of 1V and uses three dual-part packages. You make output counter IC3 work as a two-digit counter by strapping the enable pin of the IC3B part to the MSB (most-significant-bit) output of the IC3A part.

A dual op amp is used to create the comparator function and the ramp generator. The design also uses a dual 555-type timer chip. You use IC2A to create the ramp and to reset it and the output counter, and you use IC2B as a free-running oscillator that drives the counter chip. To blank the output LEDs when the chip is counting, Q3 disables drive current to the LEDs when IC3 is incrementing. You use IC4 to derive a reference of 2.5V.

Figure 2
Eight LEDs Make a 100-Division Voltmeter

Tests of the design use TL084 op amps, but you can also use an LM358. A top view of the 15×15-hole prototype board shows component placement (Figure 2a). Figure 2b shows a bottom view of the board, with the connection and three resistors. You might use flat-green LEDs with the sides painted black or covered with black-plastic sleeves for good visibility. link

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