LED bar-graph display represents two digits

This circuit uses two National Semiconductor LM3914 dot/bar-display-driver ICs to implement a two-digit, 0-5V LED voltmeter that mimics a subranging flash ADC.

An LED bar graph comprising five LEDs, each representing 1V of input signal, stands for the most-significant digit (MSD). Nine LEDs in dot mode, in which only one LED lights up, represent the least-significant digit (LSD). The circuit senses the operation of the MSD LEDs and uses them to change the input reference ladder of the chip that drives the LSD. The input signal ranges from 0-5V, and accuracy is better than ±50mV. The circuit operates over a supply voltage range of 5-8V.


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R1 and R2 divide the input voltage in half, such that a 5V maximum input is 2.5V at the LM3914s, IC1 and IC2 (Figure 1). Strap the mode pin of IC1 high, so it operates as a bar graph, and use VR1 to adjust the REFOUT pin of IC1 to 2.5V. Thus, each of the IC1 output pins lights successively in 0.5V increments. Because this IC makes the MSD, wire in only five LEDs on every other output, starting at output D2, meaning that the five LEDs will light at 1V intervals from 1-5V. The LM3914’s data sheet explains how to use R3 to set a constant-current output on the LED pins. The current in each LED is approximately 10 times the current that can be drawn from the REFOUT output pin. The part maintains 1.25V between the REFADJ and REFOUT pins. The VR2/R10/R13 voltage divider causes a load, which, along with the 1.5kΩ value of R3, sets a fixed output current in LEDs D1 through D5. Select these LEDs from the same batch so that their forward voltage drops match.


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Wire a resistor and a transistor around each of the four LEDs. The voltage across the LED also presses across the resistors, so these LEDs form four constant-current sources that operate in conjunction with the LEDs. Adjust VR3, such that each LED when on adds 500mV to their summed output. Send this signal to RLO, the bottom of the internal resistor string in the second LM3914 (Figure 2), and then the 50 percent-divided input signal to the SIG Pin of IC2. Use an op amp, IC3, to add a fixed 500-mV offset plus the summed-current signal from the outputs of IC1. R1 and R2 reduce the input signal to the circuit by 50 percent, so a 500-mV excursion at IC2’s SIG Pin input represents 1V of the input excursion.

As the input to the circuit goes from 0-1V, the SIG inputs to both bar graph ICs go from 0-0.5V. No LEDs light on IC1, meaning that IC2 has RLO at 0V and RHI at the 500mV offset that was adjusted with VR2. The LED outputs of IC2 now light in sequence as the input to the chip goes from 0-0.45V, corresponding to a 0-0.9V input at the Signal-in Port. When the input signal is high enough to light LED D1, the value at IC2’s RLO jumps to 500mV, and the input at RHI jumps to just 500mV higher than RLO, or 1V. Because IC2’s internal resistor ladder is now biased between 0.5V and 1V, IC2 indicates 0.1V step between 1V and 2V at the Signal-in Port. Leave the Mode Pin on IC2 floating so that the part operates in dot mode instead of bar-graph mode.

At a 4.9V input to the Signal-in Port, LEDs D1 through D4 illuminate, resulting in 2V at the RLO input of IC2. The op amp adds 500mV to that value and presents it to the RHI input of IC2 for a total of 2.5V. The input to IC2 is 2.45V, so the D9 output of IC2 lights D14, correctly indicating the least-significant bit (LSB) of the measurement as nine-tenths.