Use IC to build a simple thermometer-style voltage indicator

This application note describes a circuit that produces a thermometer-style voltage indication. The circuit can illuminate a portion of 32 LEDs in sequence from the bottom continuously, and is designed using the MAX4478 op amp. < -- ========================================================================== --><-- CONTENT: DB HTML --><-- === ... Typical applications for this thermometer-style voltage indicator include control panels, automotive instrument panels, pneumatic devices, and additional functions for entertainment devices. The process of taking an analog value, converting it to a digital signal, and finally displaying it in an analog format may seem magical on the surface. The circuit shown in Figure 1 converts the input voltage into a corresponding time (proportional pulse width). 32 LEDs are arranged vertically in a row and light up from the bottom to the top, with the LED representing the analog value of the input voltage. Figure 1. A portion of the 32 LEDs are lit up in sequence from the bottom to the top to provide a "thermometer-style scale" indication. At the beginning of each measurement cycle, amplifier B of IC1 (MAX4478 low-noise, low-distortion op amp) generates a linear ramp signal, which is set to zero output by transistor Q1 and then restarted. All shift registers are also set to digital zero. Amplifier C of the MAX4478 compares the linear ramp signal to the input voltage. When the ramp voltage and the input voltage are equal, the circuit generates a pulse. The cascaded shift registers IC2 through IC5 are also reset to zero at the beginning of each cycle. Then, the shifting is controlled by the pulses generated by the clock oscillator (MAX4478 amplifier A). The input data of the first shift register (IC2) is always connected to a high level (i.e., logic "1"). When the MAX4478 amplifier C detects that the input voltage is equal to the ramp signal, it generates a low-to-high jump output trigger ST_CP. The data is transferred between the shift register and its output register in sequence. The shift register chain with a logic "1" input has shifted and outputs a logic high level "1", which refers to the shift register before the output logic "0". According to the data transmission, the output register copies the state of the shift registers at each level. Each register output drives an LED, and the logic "1" output lights up the corresponding LED, thus producing a thermometer-like indication. When the data is transferred from the shift register to the output register, the input "1" will continue to pass through the shift register chain until the first "1" reaches the highest end of the chain (IC5, Q7' output). The Q7' signal acts on the base of Q1 and the input of the MAX4478 amplifier A, and Q1 resets the ramp generator. Amplifier A inverts the signal and buffers it into the MR of the shift register, clearing all shift registers except the output register. When the input is out of range, the comparator cannot detect whether the ramp signal is equal to the input. Therefore, the 1N4148 diode inputs a "1" from the highest-level shift register to ST_CP. Since the logic "1" is transmitted to all output registers, the entire column of LEDs will be illuminated. The linearity and stability are better than 1 level of LEDs within the allowed input voltage range (4.5V to 5.5V). By adding additional shift register ICs (each driving 8 LEDs) and recalculating the slope and clock period, more levels (that is, more LEDs) can be added. The timing diagram (Figure 2) shows the corresponding relationship between some key waveforms in the circuit and time. Figure 2. These waveforms are used to illustrate the operation of the circuit in Figure 1. Waveform 1: The linear ramp signal generated by amplifier B of the MAX4478. Waveform 2: The output of amplifier C, which compares the ramp signal with the input voltage. Waveform 3: The shift register reset pulse generated by amplifier A. Waveform 4: The output of the final shift register.