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 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 d...