While many products can be improved by the incorporation of a digital panel meter, the price of the device often precludes its inclusion. The circuit shown offers a low-cost solution (see the figure). As an added bonus, the circuit is pin-selectable for three different display ranges. The total parts cost is less than $6.00 in quantities of 100.
The low-cost PIC16C711 forms the heart of the panel meter. It is configured with two of its inputs as 8-bit analog-to-digital converters (ADC). While AD0 is the voltage input, AD1 is the range-select input. If AD1 is wired to ground, the unit will display a range of 0.00 to 5.10 V with a 20-mV resolution (the left digit is blank).
With AD1 wired at mid-voltage, the display reads 00.00 to 12.75 V with a 50-mV resolution. With AD1 wired to VCC, the display reads 00.0 to 25.5 V with a 100-mV resolution (the right digit is blank). A total of 256 voltage increments are displayed.
Since the voltage reference is the VCC supply, the accuracy after calibration is as stable as VCC. The 16C711 is rated for ±1/2-LSB over temperature. The micro has an open drain output on RB4, requiring an external pull-up resistor. Also note that external voltage attenuators are required for the upper voltage ranges.
The PIC chip functions as a seven-segment decoder/driver, driving all four seven-segment LEDs at once. Only one of the four seven-segment LEDs is illuminated at a time. It is pulled to ground on the common cathode terminal by Q1 through Q4, respectively. The decimal point is selected through an independent resistor.
The LED pack is MSQC6940C, available from Mouser for $2.06 per 100 units. The PIC16C711-04P is available from Digikey for $2.23 per 100 units. The 4-MHz resonator is ZTT-4.00MG, also available from Digikey at $0.45 per 100 units. The PIC code listing for the PIC16C711 can be viewed or downloaded.
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