Login
Register

ENGLISH简体中文
(Chinese)日本語
(Japanese)РУССКИЙ
(Russian)

Products

Solutions

Design

Buy

Support

About Us

My Maxim

Maxim > Design
support > App notes
> Amplifier
and Comparator Circuits > APP 4443
Maxim > Design support >
App notes
> Automotive
> APP 4443
Maxim > Design support >
App notes
> Measurement
Circuits > APP 4443

Sep 16, 2009

Keywords: op amps, quad op amps, thermometer
scale, LED column, shift registers
Related
parts

Download,
PDF format (250kB)

[?]

APPLICATION NOTE 4443

ICs Form Simple Thermometer-Style Voltage
Indicator

By: Ahmad
Ayar

Abstract: This
application note describes a circuit that produces a
thermometer-style voltage indicator. The circuit illuminates a
portion of 32 LEDs in continuous sequence starting from the
bottom. The MAX4478 op amp is featured in the design.

This
design idea appeared in the October 24, 2006 issue of EE
Times magazine.

Unlike the analog
D'Arsonval meter, a thermometer scale is an analog indicator
with no moving parts. It is useful when you must acquire
information quickly but do not need high accuracy. Typical
applications for such a thermometer-style voltage indicator
are control panels, vehicle dashboards, retrofits for
pneumatic controls, and cosmetic indicators in entertainment
equipment.

Oddly, a thermometer indicator takes an
analog value, converts it to a digital number, and then
displays that number in an analog format. The circuit in
Figure 1 transforms its input voltage into time (a
proportional pulse width). With 32 LEDs in a vertical column,
it then displays an analog of the voltage by illuminating all
the LEDs in sequence, from the lowest to the one that
represents the input-voltage value.

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 (the MAX4478 low-noise
and low distortion
op
amp) is forced to zero by transistor
Q1, and then restarted. All shift-register stages are set to
digital zero as well. The MAX4478's amplifier C compares the
ramp to the input voltage. When the input and ramp voltages
are equal, the circuit generates a pulse.

The cascaded
shift registers, IC2 to IC5, are also reset to zero at the
start of a cycle. After that the shift function is driven by
pulses from a clock oscillator (amplifier A on the MAX4478).
The data input of the first shift
register (IC2) is always connected to a high level (i.e.,
a logic "1"). When amplifier C on the MAX4478 detects that the
input voltage and the ramp are equal, its low-to-high output
edge feeds the ST_CP line. This, in turn, 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; 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 1 LED in the column. The LEDs
associated with digital "1s" are illuminated, thus 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 the MAX4478's amplifier A.
Q1 resets the ramp. Amplifier A inverts and buffers the signal
before feeding it 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 overrange
inputs. Therefore, the 1N4148 diode
feeds the "1" from the top shift-register stage to the ST_CP
line. It lights the entire column by causing a transfer of "1"
to all the output registers. Linearity and stability are
better than 1 LED step for the allowed range of input voltage
(4.5V to 5.5V). You can add more steps (i.e., more LEDs) by
adding extra shift-register ICs (each of which drives 8 LEDs),
and by recalculating the ramp and clock periods. A timing
diagram (Figure 2) gives the relative time
relationships for the circuit's more important
waveforms.

Figure
2. These waveforms illustrate the circuit operation in Figure
1. Trace 1: linear ramp generated by amplifier B of the
MAX4478. Trace 2: output of amplifier C, which compares the
ramp and input voltages. Trace 3: shift-register reset pulse
generated by amplifier A. Trace 4: the last shift-register
output.

Related parts

MAX4478

SOT23, Low-Noise,
Low-Distortion, Wide-Band, Rail-to-Rail Op Amps

Free
samples

Automatic updates
Would you like to be
automatically notified when new application notes are
published in your areas of interest? Sign
up for EE-Mail™.

Download,
PDF format (250kB)
APP 4443: Sep 16, 2009

AN4443, AN 4443, APP4443,
Appnote4443, Appnote 4443

|

|

|     Privacy Policy
|     Legal Notices

Copyright © 2011 by Maxim Integrated
Products