This project illustrates the use of a V/F converter in monitoring temperature in degrees Fahrenheit (0F).
The block diagram of the temperature indicator is shown in Figure 1-1.
The indicator is composed of a temperature sensor, amplifier, V/F
converter, three-digit binary-coded-decimal (BCD) counter, time base,
and LED display In addition to the 9400 V/F converter, other ICs needed
for this project include the LM334 temperature sensor, LF353 dual
op-amp, NE555 timers, 74LS00 NAND gate, MC14553 three-digit BCD counter.
MC14543 BCD-to-seven segment decoder/driver/latch and three
seven-segment (common anode or common cathode) LED displays with three
PNP switching transistors.
Working of the system
Figure 1-2 shows the schematic diagram, which is designed to display
temperatures from 0° to 100°F. Operation of the circuit is as follows.
The output of the temperature sensor changes linearly as a function of
temperature (10 mV/ K). This output is an input to the summing
amplifier, which is used to calibrate the output of the temperature
sensor for a desired temperature type (K,
0C, or
0F) and an intended range. That is, to display the temperature in either K, °C, or
0F, potentiometer R
4
is adjusted accordingly so that a suitable voltage appears at the
output of the summing amplifier. Since the output of the temperature
sensor is directly proportional to temperature changes-, R
4
needs to be adjusted at only one temperature. The output of the summing
amplifier then drives the inverting amplifier. The purpose of the
inverting amplifier is twofold: (1) to invert the input so that its
output voltage is positive, which is necessary for the V/F converter,
and (2) to provide a suitable gain, which depends on the
voltage-to-frequency scaling used for the V/F converter.
The output of the inverting amplifier is the input to the V/F
converter; therefore, the output frequency of the converter is directly
proportional the output voltage of the inverting amplifier. For example,
as the temperature goes up the output voltage of the summing amplifier
increases in the negative direction, Whereas that of the inverting
amplifier increases in the positive direction, which in turn causes the
frequency of the V/F to increase in the positive direction.
The output frequency of the converter is then ANDed with the gating
signal to produce the clock signal for the three-digit BCD counter. The
BCD output of the counter drives the three LED displays sequentially via
the BCD-to-seven segment decoder/latch/driver stage, and the
temperature is displayed on the LEDs, depending on the relationship
between the frequency of the V/F converter and the gate signal. The
gate, latch, and reset signals are generated by the time-base circuit,
which consists of a free-running multivibrator and two one-shot
multivibrators.
PARTS LISTS
Resistors (all ¼-watt, ± 5% Carbon)
R1 = 1 kΩ potentiometer at 230 Ω
R2, R7, R12, R13, R21 = 10 kΩ
R3, R5, R6, R11 = 100 kΩ
R4 = 10 kΩ potentiometer
R8= 1 MΩ potentiometer
R9 = 180 kΩ
R10, R15 = 50 kΩ potentiometer
R14 = 510 kΩ
R16 = 3 kΩ potentiometer
R17 = 15 kΩ
R18 = 20 kΩ
R19 = 10 kΩ potentiometer
R20 = 1 kΩ potentiometer
R22 – R28 = 220 Ω
R29 – R31 = 1k Ω
Capacitors
C1 = 1000 pF
C2 = 100 pF
C3, C6, C9 = 1 µF
C4, C5, C7, C8, C10 = 0.01 µF
C11 = 0.001 µF
Semiconductors
IC1 = LM334 temperature sensor
IC2 = LF353 dual op-amp
IC3 = Teledyne 9400 V/F convertor
IC4 = 74LS00 NAND gate
IC5, IC6, IC7 = NE/SE 555 timers
IC8 = MC 14553 three-digit BCD counter
IC9 = MC 14543 BCD-to-seven segment decoder/driver/latch
Q1, Q2, Q3 = 2N1305 switching transistors
D1, D2 = 1N914 signal diodes
Three seven-segment common anode LEDs: MAN72A or equivalent
Circuit Description
