measure temperatures up to
150°C with an accuracy of ±1°C.
The temperature is read on a 1V full
scale-deflection (FSD) moving-coil
voltmeter or digital voltmeter.
Operational amplifier IC 741 (IC3)
provides a constant flow of current
through the base-emitter junction of
npn transistor BC108 (T1). The voltage
across the base-emitter junction
of the transistor is proportional to its
temperature. The transistor used this
way makes a low-cost sensor. You can
use silicon diode instead of transistor.
The small variation in voltage across
the base-emitter junction is amplified
by second operational amplifier (IC4),
before the temperature is displayed
on the meter. Preset VR1 is used to
set the zero-reading on the meter and
preset VR2 is used to set the range of
temperature measurement.
Operational amplifiers IC3 and IC4
operate off regulated ±5V power supply,
which is derived from 3-terminal
positive voltage regulator
IC 7805 (IC1) and
negative low-dropout
regulator IC 7660 (IC2).
The entire circuit works
off a 9V battery.
Assemble the circuit
on a general-purpose
PCB and enclose
in a small plastic box.
Calibrate the thermometer
using presets VR1
and VR2. After calibration,
keep the box in
the vicinity of the object
whose temperature is
to be measured.
No comments:
Post a Comment