Heat Sensitive Switch

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At the heart of this heat-sensitive switch
is IC LM35 (IC1), which is a linear temperature sensor and linear
temperature-to-voltage converter circuit. The converter provides
accurately linear and directly proportional output signal in millivolts
over the temperature range of 0°C to 155°C. It develops an output
voltage of 10 mV per degree centigrade change in the ambient
temperature. Therefore the output voltage varies from 0mV at 0°C to 1V
at 100°C and any voltage measurement circuit connected across the output
pins can read the temperature directly. The input and ground pins of
this heat-to-voltage converter IC are connected across the regulated
power supply rails and decoupled by R1 and C1. Its temperature-tracking
output is applied to the non-inverting input (pin 3) of the comparator
built around IC2. The inverting input (pin 2) of IC2 is connected across
the positive supply rails via a voltage divider network formed by
potentiometer VR1.

Since the wiper of potentiometer VR1 is connected to the inverting
input of IC2, the voltage presented to this pin is linearly variable.
This voltage is used as the reference level for the comparator against
the output supplied by IC1. So if the non-inverting input of IC2
receives a voltage lower than the set level, its output goes low
(approximately 650 mV). This low level is applied to the input of the
load-relay driver comprising npn transistors T1 and T2. The low level
presented at the base of transistor T1 keeps it nonconductive. Since T2
receives the forward bias voltage via the emitter of T1, it is also kept
non-conductive. Hence, relay RL1 is in de-energised state, keeping
mains supply to the load ‘off’ as long as the temperature at the sensor
is low. Conversely, if the non-inverting input receives a voltage higher
than the set level, its output goes high (approximately 2200mV) and the
load is turned ‘on.’ This happens when IC1 is at a higher temperature
and its output voltage is also higher than the set level at the
inverting input of IC2.

So the load is turned on as soon as the ambient temperature rises
above the set level. Capacitor C3 at this pin helps iron out any ripple
that passes through the positive supply rail to avoid errors in the
circuit operation. By adjusting potentiometer VR1 and thereby varying
the reference voltage level at the inverting input pin of IC1, the
temperature threshold at which energisation of the relay is required can
be set. As this setting is linear, the knob of potentiometer VR1 can be
provided with a linear dial calibrated in degrees centigrade. Therefore
any temperature level can be selected and constantly monitored for
external actions like turning on a room heater in winter or a room
cooler in summer. The circuit can also be used to activate emergency
fire extinguishers, if positioned at the probable fire accident site.
The circuit can be modified to operate any electrical appliance. In that
case, relay RL1 must be a heavy-duty type with appropriately rated
contacts to match the power demands of the load to be operated.