LED bar display showing two digits

This circuit uses two LM3914 dot/bar display driver ICs from National Semiconductor to implement a 0 to 5V two-digit LED voltmeter that simulates a flashing ADC. This LED bar graph consists of five LEDs, each representing a 1V input signal, indicating the MSD (most significant digit). The dot mode has nine LEDs, when only one LED is lit, representing the LSD (least significant digit). The circuit detects the operation of the MSD LED and uses it to change the input reference step of the chip driving the LSD. The input signal range is from 0 to 5V with an accuracy better than ±50mV. The circuit can operate with a supply voltage of 5 to 8V.

Figure 1. This voltmeter displays IC1's 1V to 5V voltage as a bar graph. IC2's dot graph shows the least significant digit, and the LEDs show 0.1V to 0.9V.

R1 and R2 divide the input voltage into two, such as a maximum 5V input is 2.5V on the LM3914 (IC1 and IC2) (Figure 1). IC1's mode terminal is tied high, so it works as a bar graph. VR1 is used to adjust IC1's REFOUT pin to 2.5V. As a result, each output pin of IC1 is lit up one by one in 0.5V steps. Since this IC is an MSD, starting from the output of D2, each other output terminal is connected to only five LEDs, which means that the five LEDs will be lit up one by one from 1V to 5V at 1V intervals. The LM3914 data sheet explains how to use R3 to set the LED pin to a constant current output (Reference 1). The current on each LED is about 10 times the current drawn from the REFOUT output pin. The REFADJ and REFOUT pins are maintained at 1.25V. The VR2/R10/R13 voltage divider creates a load that, along with 1.5kΩ R3, sets the LEDs D1 to D5 to a fixed output current. These LEDs should be selected from the same batch so that they have matched forward voltage drops.

Next, connect a resistor and a transistor to each of the four LEDs. The voltage across the LED is also added to the resistors, so the LEDs form four constant-current sources that operate relative to the LEDs. Adjust VR3 so that each LED adds 500mV to its summed output when it turns on. This signal is sent to RLO, the bottom resistor in the second LM3914's internal resistor string (Figure 2). Next, send the 50%-divided input signal to IC2's SIG pin. Use an op amp, IC3, to add a fixed 500mV bias to the summed current signal at IC1's output. R1 and R2 reduce the circuit's input signal by 50%, so a 500mV shift at IC2's SIG input represents a 1V input shift.

Figure 2. The LM3914 IC has an internal reference and a resistor ladder that can be configured for use as a bar or dot LED display (originally from National Semiconductor).

When the circuit's input voltage changes from 0 to 1V, the SIG inputs of both bar graph ICs are 0 to 0.5V. None of the LEDs on IC1 illuminate, meaning IC2's RLO is 0V and RHI is biased to 500mV using VR2. When the chip's input changes from 0 to 0.45V (corresponding to 0 to 0.9V at the signal input port), IC2's LED outputs now light up in sequence. When the input signal is high enough to illuminate LED D1, the voltage on IC2's RLO jumps to 500mV, while the RHI input is only 500mV higher than RLO, to 1V. Since IC2's internal ladder resistors are now biased at 0.5V to 1V, IC2 indicates 0.1V steps between 1V and 2V at the signal input port. Floating IC2's Mode pin causes the device to operate in dot mode instead of bar graph mode.

When a 4.9V input is given to the signal input port, LEDs D1 to D4 illuminate, causing IC2's RLO input to be 2V. The op amp adds 500mV to this value, delivering a total of 2.5V to IC2's RHI input. IC2's input is 2.45V, so IC2's D9 output illuminates, correctly indicating that the measured LSB (least significant bit) is 1/9.