Unlike the analog D'Arsonval meter, a “thermometer scale” is an analog indicator with no moving parts. (If you don't know about this ubiquitous and historical analog meter movement, click here.) It is useful when you must acquire information quickly but without high accuracy. Typical applications are level and volume indicators, control panels, dashboards, retrofits for pneumatic controls, and cosmetic indicators in entertainment equipment.
Ironically, a thermometer indicator takes an analog value, converts it to a digital number, and then displays that number in an analog format, from the viewer's perspective. The circuit of Figure 1 transforms its input voltage into time (a proportional pulse width), and then displays an analog representation of the voltage by illuminating, via a vertical column of LEDs, all the LEDs in a sequence from the lowest LED to the LED representing the input-voltage value.
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Figure 1: This circuit produces a “thermometer scale” by illuminating a portion of the 32 LEDs in continuous sequence from the bottom up.
At the beginning of each measurement cycle, a linear ramp generated by amplifier B in IC1 is forced to zero by transistor Q1 , and restarted. All shift-register stages are set to digital zero as well. Amplifier C in IC1 compares the ramp to the input voltage, causing the circuit to generate a pulse when the input and ramp voltages are equal.
The cascaded shift registers IC2 through IC5 are also reset to zero at the cycle start, and after that the shift function is driven by pulses from a clock oscillator (IC1 amplifier A). The data input of the first shift register (IC2 ) is always connected to a high level (logic “1”). When the comparator (IC1 amplifier C) detects equality between the input voltage and the ramp, its low-to-high output edge feeds the ST_CP line and causes a transfer of data between the shift-register stages and their output registers.
All the shift-register stages to which the digital “1” at the input of the shift-register chain has been already shifted contain “1” as an output, and all stages above that level contain “0.” Upon transfer of data, the output registers copy the state of the shift-register stages. Each output register drives one LED in the column, so the LEDs associated with digital “1s” are illuminated, producing a display similar to that of a thermometer.
After data from the shift register is transferred to the output registers, shifting of the input “1” continues through the shift-register chain until the first “1” reaches the top stage of the chain (IC5 , Q7 ' output). This Q7 ' signal is applied to the base of Q1 and to the input of IC1 amplifier A. Q1 resets the ramp and amplifier A inverts and buffers the signal before feeding to the shift registers' MR line, which zeroes all the shift-register stages (but not the output registers).
The comparator can never detect equality between ramp and input for over-range inputs, so the 1N4148 diode feeds the “1” from the top shift-register stage to the ST_CP line in that case, where it lights the entire column by causing a transfer of “1” to all the output registers. Linearity and stability are better than one LED step for the allowed range of input voltage (4.5 to 5.5 V). You can add more steps (more LEDs) by adding extra shift-register ICs (each of which drives eight LEDs), and recalculating the ramp and clock periods. The timing diagram of Figure 2 gives the relative time relationships for the circuit's more important waveforms.
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Figure 2: These waveforms illustrate the circuit operation in Figure 1. Trace 1 is the linear ramp generated by amplifier B of IC1 . Trace 2 is the output of amplifier C, which compares the ramp and input voltages. Trace 3 is the shift-register reset pulse generated by amplifier A. Trace 4 is the last shift-register output.
About the authors
Alfredo H. Saab and Ahmad Ayar are with Maxim Integrated Products, Sunnyvale, CA, where Alfredo is Applications Manager and Ahmad is an Applications Engineer.