It sounds
rather mysterious: a switch that is controlled by its ambient
temperature. All without the touch of a human hand, except for when
you’re building this sort of electronic thermostat. There are a lot of
handy uses for a thermally controlled switch. If the temperature inside
your PC gets too high sometimes, the circuit can switch on an extra
fan. You can also use to switch on an electric heater automatically if
the room temperature is too low. There are innumerable potential
applications for the thermostat described here.
Circuit diagram:
There
are lots of ways to measure the temperature of an object. One very
simple way is to use a semiconductor sensor, such as the National
Semiconductor LM35 IC. This sensor is accurate to within 0.5 °C at 25
ºC, and few other sensors can do better or even come close to this level
of accuracy. In the circuit described here, the sensor (IC2) generates
an output voltage of 10 mV/°C, so the minimum temperature that can be
measured is 0 °C. At 25 °C, the output voltage of the sensor is (25 °C ×
10 mV/°C) = 0.25 V.
The circuit uses a TLC271 opamp as a
comparator. It compares the voltage from the temperature sensor, which
is connected to its non-inverting input (pin 3), with the voltage on its
inverting input (pin 2). The latter voltage can be set with
potentiometer P1. If the voltage from the sensor rises above the
reference value set by P1 (which represents the desired temperature),
the output of the comparator toggles to the full supply voltage level.
The output is fed to transistor T1, which acts as a switch so the output
can handle more current.
This makes it possible to energize a
relay in order to switch a heavy load or a higher voltage. The
transistor also supplies current to LED D1, which indicates whether the
temperature is above the reference value. The reference value can be
adjusted by P1 over the range of 18–30 °C with the indicated component
values. Of course, you can adjust the range to suit your needs by
modifying the value of R1 and/or R2. To prevent instability in the
vicinity of the reference value, a small amount of hysteresis is
provided by resistor R4 so the temperature will have to continue rising
or falling by a small amount (approximately 0.5 °C) before the output
state changes.
The LM35 is available in several different
versions. All versions have a rated temperature range of at least 0–100
°C. One thing you may have to take into account is that the sensor has a
relatively long response time. According to the datasheet, the sensor
takes 3 minutes to reach nearly 100% of its final value in still air.
The opamp has very low drift relative to its input voltages, and in the
low-power mode used here it draws very little current. The sensor also
draws very little current, so the total current consumption is less
than 80 µA when LED D1 is off.
The advantage of low current
consumption is that the circuit can be powered by a battery if necessary
(6 V, 9 V or 12 V). The sensor has a rated operating voltage range of
4–30 V, and the TLC271 is rated for a supply voltage of 3–16 V. The
circuit can thus work very well with a 12-V supply voltage, which means
you can also use it for car applications (at 14.4 V). In that case, you
must give additional attention to filtering out interference on the
supply voltage.
Source: Elektor Electronics 12-2010
rather mysterious: a switch that is controlled by its ambient
temperature. All without the touch of a human hand, except for when
you’re building this sort of electronic thermostat. There are a lot of
handy uses for a thermally controlled switch. If the temperature inside
your PC gets too high sometimes, the circuit can switch on an extra
fan. You can also use to switch on an electric heater automatically if
the room temperature is too low. There are innumerable potential
applications for the thermostat described here.
Circuit diagram:
Temperature-Controlled Switch Circuit Diagram
There
are lots of ways to measure the temperature of an object. One very
simple way is to use a semiconductor sensor, such as the National
Semiconductor LM35 IC. This sensor is accurate to within 0.5 °C at 25
ºC, and few other sensors can do better or even come close to this level
of accuracy. In the circuit described here, the sensor (IC2) generates
an output voltage of 10 mV/°C, so the minimum temperature that can be
measured is 0 °C. At 25 °C, the output voltage of the sensor is (25 °C ×
10 mV/°C) = 0.25 V.
The circuit uses a TLC271 opamp as a
comparator. It compares the voltage from the temperature sensor, which
is connected to its non-inverting input (pin 3), with the voltage on its
inverting input (pin 2). The latter voltage can be set with
potentiometer P1. If the voltage from the sensor rises above the
reference value set by P1 (which represents the desired temperature),
the output of the comparator toggles to the full supply voltage level.
The output is fed to transistor T1, which acts as a switch so the output
can handle more current.
This makes it possible to energize a
relay in order to switch a heavy load or a higher voltage. The
transistor also supplies current to LED D1, which indicates whether the
temperature is above the reference value. The reference value can be
adjusted by P1 over the range of 18–30 °C with the indicated component
values. Of course, you can adjust the range to suit your needs by
modifying the value of R1 and/or R2. To prevent instability in the
vicinity of the reference value, a small amount of hysteresis is
provided by resistor R4 so the temperature will have to continue rising
or falling by a small amount (approximately 0.5 °C) before the output
state changes.
The LM35 is available in several different
versions. All versions have a rated temperature range of at least 0–100
°C. One thing you may have to take into account is that the sensor has a
relatively long response time. According to the datasheet, the sensor
takes 3 minutes to reach nearly 100% of its final value in still air.
The opamp has very low drift relative to its input voltages, and in the
low-power mode used here it draws very little current. The sensor also
draws very little current, so the total current consumption is less
than 80 µA when LED D1 is off.
The advantage of low current
consumption is that the circuit can be powered by a battery if necessary
(6 V, 9 V or 12 V). The sensor has a rated operating voltage range of
4–30 V, and the TLC271 is rated for a supply voltage of 3–16 V. The
circuit can thus work very well with a 12-V supply voltage, which means
you can also use it for car applications (at 14.4 V). In that case, you
must give additional attention to filtering out interference on the
supply voltage.
Source: Elektor Electronics 12-2010
