Download designing a temperature sensor

DESIGNING A
TEMPERATURE SENSOR
We will be using the following format for
designing this circuit:
INPUT
CONTROL
OUTPUT
INPUT
CONTROL
OUTPUT
CREATING THE INPUT
The input is the part that senses
temperature and converts it to a voltage.
Firstly we need a THERMISTOR.
This alters its RESISTANCE according to temperature.
As it heats up, its resistance lowers. This is type has a
NEGATIVE TEMPERATURE COEFFICIENT, (NTC), -tº
The type used here has a resistance of 5Kohms at 25ºC
Symbol
INPUT
CONTROL
OUTPUT
CREATING THE INPUT
We want a VOLTAGE to represent the
temperature, but the sensor only changes
RESISTANCE. So we add another resistor
to form a POTENTIAL DIVIDER.
INPUT
CONTROL
OUTPUT
CREATING THE INPUT
+9V
Voltage
Output
When this POTENTIAL DIVIDER is
connected across the supply, the output
produces a voltage proportional to the
temperature.
The hotter it gets, the higher the voltage.
0V
INPUT
CONTROL
OUTPUT
CREATING THE CONTROL
The voltage from the input changes only a small amount so we need to
use a device that is very sensitive to changes in input.
An OPERATIONAL AMPLIFIER is an
ideal choice.
One type designed specifically for
this application is known as a
COMPARATOR.
IC number: LM311
INPUT
CONTROL
OUTPUT
CREATING THE CONTROL
The connections for the COMPARATOR are shown below:
A basic rule on how a COMPARATOR
operates is:
+V supply
Vin+
Output
Vin-V supply
If Vin+ > Vin- then the output is ON
So if we connect the signal from the
sensor to Vin+, we can connect a
voltage to Vin- as a comparison.
INPUT
CONTROL
OUTPUT
CREATING THE CONTROL
Remember:
If Vin+ > Vin- then the output is ON
To produce the REFERENCE
voltage to compare against,
we need another
POTENTIAL DIVIDER.
Reference
Voltage
The one shown has had a
POTENTIOMETER (variable
resistor) added to provide an
adjustable output:
The range is about to 0.75v
to 8.25v
INPUT
CONTROL
OUTPUT
CREATING THE CONTROL
Here is what we have so far with the inputs connected to the
COMPARATOR
R5 has been added
to provide some
HYSTERISIS.
This means it has
slightly different
switch-on and
switch-off points – it
prevents
CHATTERING!
INPUT
CONTROL
OUTPUT
CREATING THE OUTPUT
We want to switch on a relay when the circuit activates, but the output
from the COMPARATOR is not powerful enough. We need a DRIVER.
The most basic one is a
TRANSISTOR driver. We
need around 150mA for a
relay. A BC337 can deliver
around 500mA so this will be
suitable.
Collector
Base
Emitter
INPUT
CONTROL
OUTPUT
CREATING THE OUTPUT
The transistor needs a couple of additional components to protect it.
The resistor R6 restricts the
current flow into the transistor
protecting it from damage.
Notice the addition of diode
D1, this is to prevent damage
caused by high EMF voltages
generated in the coil.
INPUT
CONTROL
OUTPUT
PUTTING IT ALL TOGETHER
This is what we have so far.
INPUT
CONTROL
OUTPUT
PUTTING IT ALL TOGETHER
So connecting them all together produces:
INPUT
CONTROL
OUTPUT
PUTTING IT ALL TOGETHER
The voltage supply needs to be DECOUPLED. This means putting a
capacitor across the supply to smooth it and improve circuit performance.
INPUT
CONTROL
OUTPUT
COMPLETED CIRCUIT
The circuit is now complete, showing the switched output connections
from the relay. Try this on Crocodile Clips or LiveWire.