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Op amp uses
Two practical uses of the voltage comparator are as a flame sensor and as a heat sensor.
(i) Flame sensor
The circuit in Figure 1 might be used in a gas boiler to monitor the pilot light. The alarm is
required to sound when the flame of the pilot light goes out.
R2
R1
P
Figure 1
alarm
Q
R3
LDR
A light-dependent resistor (LDR) is the sensor: its resistance is low in light and high in the dark.
When the flame is on the resistance of the LDR is low, the voltage at Q (V 2) is low, (V2 - V1) is
negative and the alarm is off. If the flame goes out, however, V2 rises such that (V2 - V1)
becomes positive, the op amp switches to positive saturation and the alarm is switched on.
(ii) Heat sensor
This sensor (Figure 2) is designed to operate one of two LEDs to show whether the heat sensor,
the thermistor, is hot or cold.
LED 1
P
Q
Figure 2
LED 2
When the thermistor is cold its resistance is large, the voltage at Q (V2) is large, and the op amp
is in a positively saturated condition, that is, the output voltage (VO) is positive. LED 1 therefore
conducts.
When the thermistor warms up a point will be reached when the voltage at Q falls below that at
P; (V2 - V1) is therefore negative, the output voltage is negative and LED 2 conducts.