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By John Clarke & Julian Edgar
Using the temperature switch, it’s easy to
rig warning lights or alarms for over-high
engine or gearbox oil temperatures. In fact,
anything’s that hot in the car (with the
exception of the exhaust gas and cylinder
head) can be monitored. [Ford photo]
Temperature Switch
A cheap general-purpose adjustable design that can work all the
way up to 245°C!
T
HERE ARE MANY automotive performance applications where you
want to turn something on or off on
the basis of measured temperature. Radiator cooling fans, over-temperature
warning lights or alarms, intercooler
or amplifier fans – they all need a
cheap and easily-adjusted temperature
switch.
Temperature switches are available
commercially but this build-it-yourself design has some major advantages
over normal thermostats and temperature switches. First, it can be adjusted
very finely – you can literally set (to
Main Features
• Adjustable temperature switching from 0°C to 245°C
• Double-pole changeover 5A
relay contacts
• Selectable rising or falling
temperature switching
• Adjustable hysteresis
• Easy to build
22 Silicon Chip
the degree) the temperature at which
the switch triggers.
Second, the hysteresis (ie, the difference between on and off temperatures)
is adjustable. That lets you set the
system up so that the device you’re
switching isn’t constantly cycling at
the trigger point. You can set a wide
hysteresis to switch something on and
off at two widely spaced temperatures,
or a low hysteresis to keep tighter
control – the choice is yours!
Third, the sensor used in this design
is good for temperatures up to 245°C.
This means you can monitor engine
oil or auto transmission oil temperature, or site the sensor near the brakes
to trigger cooling sprays. Basically,
apart from exhaust gas and cylinder
head temperature, you can trigger the
switch with anything on the car that’s
hot or cold!
Finally, you can configure the sensor so that it reacts very quickly to
temperature changes.
Construction
The Temperature Switch is a simple
kit to build but you should make one
decision before starting construction. Will you be using it to detect a
temperature that is rising to the trip
point or falling to the trip point? The
Temperature Switch can be configured
to work either way but if you know
which way you’re going, you won’t
have to make changes later on.
The detection of a rising temperature will be the more common
application – for example, turning
on a warning light or fans when the
temperature gets too high. But if
you want something switched on as
the temperature falls – for example,
activating a warning light when the
outside temperature drops below 3°C
to warn of the possibility of black ice
on the road – then the Temperature
Switch needs to be configured for a
falling temperature.
So what are the changes made for the
differing configurations? They’re simple: for rising temperature detection,
link LK1 is placed in its “L/H” position
(ie, to the left when the board is orientated as shown in Fig.2) and diode D3
is orientated so that its band is closest
to the bottom of the board. Conversely,
siliconchip.com.au
Fig.1: the temperature is monitored using a thermistor, while either op amp IC1a or IC1b drives transistor Q1 and
the relay. Trimpot VR1 sets the temperature trigger point.
How It Works
The temperature is monitored using
a thermistor which exhibits a variable
resistance with temperature. At high
temperatures, the resistance of the
thermistor is low, while at lower temperatures its resistance increases.
A 1kΩ resistor from the 8V supply
feeds current through the thermistor
which then produces a voltage which is
inversely proportional to temperature.
This voltage is filtered using a 100nF
capacitor and fed via a 1kΩ resistor to
the inverting input (pin 2) of op amp IC1a
which is connected as a comparator.
The voltage on IC1a’s non-inverting input (pin 3) is by set-point trimpot VR1 via
a 10kΩ resistor. When the thermistor
voltage at pin 2 is above the voltage
set by VR1 at pin 3, IC1a’s output is
low. Conversely, when the thermistor
voltage is below the voltage on pin 3,
IC1a’s output is high (around +8V).
Hysteresis has been added to prevent the output of IC1a from oscillating
when the inverting input is close to the
switching threshold. This hysteresis is
provided by trimpot VR2 and diode D3
in series between pins 1 & 3.
Trimpot VR2 enables the amount of
hysteresis (actually positive feedback)
siliconchip.com.au
to be adjusted. With low hysteresis,
the temperature only has to drop by
a small amount for IC1a’s output to
switch low again after it has switched
high. If VR2 is set for high hysteresis,
the temperature must fall by a much
larger amount before IC1a’s output
switches low again.
Diode D3 sets the direction of the
hysteresis action. As shown, it provides
hysteresis when pin 1 of IC1a goes
high. Alternatively, if mounted in the opposite direction, it provides hysteresis
when IC1a’s output goes low.
Where the circuit is intended to
provide a switched output when the
temperature goes above a certain
value, the diode is installed as shown
on the circuit and parts overlay (Fig.2).
If you want the switching to occur when
the temperature falls below a certain
value, diode D3 is reversed.
Op amp IC1b is an inverter which
provides a signal opposite in polarity
to IC1a’s output. When IC1a’s output
goes high, IC1b’s output goes low and
vice versa.
Link LK1 provides the option for driving the relay with a rising temperature
(L/H) or a falling temperature (H/L). It
selects the output of IC1a or IC1b to
drive transistor Q1 which, in turn, drives
the relay. Diode D2 is there to quench
the reverse voltage that is generated
by the collapsing magnetic field of the
relay coil each time it is switched off.
Power is obtained from the car’s
+12V ignition supply via D1 which gives
reverse polarity protection. The 10Ω
resistor, 100µF capacitor and zener
diode ZD1 provide transient protection at the input of regulator REG1. All
the circuitry is powered via the 7808
regulator with the exception of the relay,
Q1 and LED1 which are driven from the
11.4V supply following D1.
Resistor Colour Codes
Value
4-Band Code (1%)
5-Band Code (1%)
22kΩ
10kΩ
1.8kΩ
1kΩ
10Ω
red red orange brown
brown black orange brown
brown grey red brown
brown black red brown
brown black black brown
red red black red brown
brown black black red brown
brown grey black brown brown
brown black black brown brown
brown black black gold brown
January 2007 23
The device turns other devices
on or off on the basis of sensed
temperature. Its sensor can work
over the range of 0°C - 245°C,
making it useful for monitoring
engine oil, engine coolant and
transmission oil temperatures, as
well as intercooler and inlet air
temperatures. Note that link LK1 (to
the left of the relay) must be moved
to the H/L position and diode D3
(circled) reversed in orientation if
the switch is to trigger on a falling
(rather than rising) temperature.
Use It To Do This . . .
•
•
•
•
•
•
transistor, voltage regulator and
electrolytic capacitors. During
construction, follow Fig.2 closely to
avoid making mistakes.
The thermistor is of the “bare”
design – ie, it’s not potted in epoxy
or mounted inside a brass fitting. If
you want temperature detection to
occur very quickly (ie, if you want
the thermistor to react quickly, even
to small temperature variations), the
100 µF
10 µF
H/L
CT N
L/H
10k
C ON
CN
1.8k
22k
10k
100 µF
ON
NO
+
NC
CN
H/L
LK1L/ H K
10k
1k
100nF
COM
C
COM
+
10 µF
1k
NO
NC
IC1
LM358
1
K
+
K
1M
A
1N
4148
ZD1
+
TO
THERMISTOR
*D3
10k
A
K
LED1
VR2
REG1
7808
V21+
+12V
DNG
GND
A
VR1 1k
10Ω
D2
Q1
RELAY 1
1 0 1ra c 5 0
A D1 K
H CTI WS ERUTAREP MET
to detect a falling temperature, link
LK1 is moved to its alternative “H/L”
position and diode D3’s orientation is
reversed. Easy, huh?
When assembling the PC board, be
sure to insert the polarised components the correct way around. These
parts include the diodes, IC, LED,
A
*REVERSE D3 IF LINK LK1
IS IN 'H/L' POSITION
Fig.2: this layout diagram shows where each of the parts is placed on the
PC board. Don’t forget to reverse D3 if link LK1 is in the H/L position.
24 Silicon Chip
Operate electric radiator fans
Over-temperature warning light or alarm
Operate amplifier cooling fans
Operate an intercooler water spray or fan
Operate a brake cooling water spray
Reduce turbo boost when intake air
temperature is high
thermistor should be left exposed.
However, if the reaction speed isn’t
so important but durability is, you can
pot the thermistor in high-temperature
epoxy and mount it in the end of a
threaded brass fitting.
Either way, the thermistor will need
to be connected to a length of shielded
single core cable, with the shield (the
braid) connecting to the 0V terminal
on the PC board. The thermistor isn’t
polarised – it can be connected either
way around. Insulate the leads of the
thermistor using heatshrink tubing so
that they cannot short out to each other
or to ground. In many cases, the whole
thermistor itself can then be covered
in heatshrink without slowing its reaction time too much.
Testing
Once the assembly is complete, it’s
a good idea to bench-test the module
to make sure it works correctly. To do
this, you’ll need to connect the thermistor to the input terminals (remember, braided side of the shielded cable
to 0V) and supply power and earth.
First, turn VR2 (just above IC1) fully
anti-clockwise. Then turn set-point
pot VR1 anti-clockwise until the relay
siliconchip.com.au
The thermistor’s
leads should be
insulated and then
covered in heatshrink tubing so
that short circuits
can’t occur. If
durability in
extreme conditions
is required (and the sensor doesn’t
need to react quickly), it can be
potted in high-temperature epoxy
and mounted in the end of a threaded
brass fitting.
clicks and the LED comes on. Because
VR1 is a multi-turn pot, you may need
to rotate it a number of times before
the LED lights.
Once the switch has tripped, you
can then turn the set-point pot back
clockwise just enough to turn off the
LED and disengage the relay. Now
when you heat the thermistor, the LED
should immediately come on and the
relay click over; cooling the thermistor
should cause the LED and relay to turn
off again fairly quickly.
Finally, turn VR2 (hysteresis) clockwise a little and you should find that
the switch takes longer to turn back
off when it is being cooled down after
being tripped.
Fitting
Fitting the Temperature Switch to
a car is easy. You need only provide
an ignition-switched power supply
Fig.3: here is a typical connection set-up, where the Temperature Switch
might be monitoring the temperature of an audio amplifier. The relay’s
Normally Open (NO) connection is made to ignition-switched +12V, while
the adjacent Common terminal is connected to a fan. The other side of
the fan is earthed. When the temperature rises to the set-point, the fan
is triggered. The setting of the hysteresis pot determines how low the
temperature then has to fall before the fan switches off.
and earth, and then install the thermistor where you want to sense the
temperature.
For example, if you are controlling
a radiator cooling fan, you could place
an electrically-insulated temperature
sensor on the top tank of the radiator. Or if you want the Temperature
Switch to illuminate a warning light
when engine or transmission oil gets
excessively hot, you could attach the
sensor to the appropriate sump.
If you want just the warning light
function, you can remotely mount a
high-intensity LED within your line
of sight – just extend the wires that
connect the LED to the PC board,
making sure that you keep the LED
polarity correct.
If you want to turn a device on and
off with the temperature switch, you
can use the relay’s Normally Open
(NO) and Common contacts. Fig.3
shows these connections. The relay’s
contacts are rated to 5A – for currents
higher than this, use the on-board
Table 1: Setting The Trip Point Temperature
°C
Rt
Vt
°C
Rt
Vt
°C
Rt
Vt
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
33,944.034
26,120.042
20,286.407
15,894.535
12,557.604
10,000.00
8023.382
6483.660
5275.206
4319.920
3559.575
2950.420
2459.334
2061.059
1736.202
1469.774
1250.116
7.771
7.705
7.624
7.526
7.410
7.273
7.113
6.931
6.725
6.496
6.245
5.975
5.687
5.387
5.076
4.761
4.445
85
90
95
100
105
110
115
120
125
130
135
140
145
150
155
160
165
1068.105
916.558
789.791
683.278
593.399
517.244
452.462
397.143
349.731
308.953
273.760
243.287
216.818
193.755
173.601
155.938
140.416
4.132
3.826
3.530
3.247
2.979
2.727
2.492
2.274
2.073
1.888
1.719
1.565
1.425
1.298
1.183
1.079
0.985
170
175
180
185
190
195
200
205
210
215
220
225
230
235
240
245
126.739
114.656
103.954
94.454
85.999
78.458
71.718
65.679
60.259
55.384
50.991
47.023
43.435
40.183
37.231
34.547
0.900
0.823
0.753
0.690
0.634
0.582
0.535
0.493
0.455
0.420
0.388
0.359
0.333
0.309
0.287
0.267
You can use this table to set the trip point for the temperature switch, where the temperature is shown in the lefthand
column and the voltage required to be on the middle pin of the set-point pot (VR1) is shown on the righthand side. For
example, if you want the switch to trip at 145°C, the pot will need to be turned until the measured voltage is 1.425V.
siliconchip.com.au
January 2007 25
Parts List
The temperature switch can be used to operate electric radiator fans and you
can adjust the difference between the switch-on and switch-off temperatures.
With high loads like these fans, you should run an extra heavy-duty automotive
relay. [Bosch]
relay to switch another heavy duty
automotive relay.
Note that because a double-pole,
double-throw (DPDT) relay has been
used, another completely independent
circuit can also be switched simultaneously. This other circuit can even
turn off the second device as the first
is switched on.
Setting-Up
There are two ways of setting the
action of the Temperature Switch.
First, if you have another means of
monitoring the temperature (eg, via
an engine-coolant temperature gauge
or a temporary temperature probe
and display), wait until the measured
temperature reaches the desired trigger level, then turn set-point pot VR1
until the Temperature Switch just
turns on.
The turn-off value will be set by
the hysteresis pot (VR2). If you want
the turn-off value to be close to the
Where To Buy A Kit
This design was originally
published in SILICON CHIP’s “Performance Electronics For Cars”. A
kit of parts is available exclusively
from Jaycar Electronics.
“Performance Electronics For
Cars” is available from Silicon Chip
Publications and from Jaycar.
26 Silicon Chip
turn-on temperature, set VR2 fully
anti-clockwise. If you want the turn-off
temperature to be much lower, adjust
VR2 further clockwise.
The other way of setting the trip
point is to make some measurements
on the bench. Table 1 shows typical
NTC thermistor resistance values for a
range of temperatures. The thermistor
is 10kΩ at 25°C and falls to 34.5Ω at
245°C.
Table 1 also shows the expected
voltage across the thermistor at each
temperature value, assuming the regulator output is at 8V. If the regulator
voltage is slightly different to this,
the value will need to be scaled accordingly.
For example, if the regulator output
is 7.8V, then the output voltage will be
the value shown multiplied by 7.8V,
all divided by 8V. You can measure
the regulator’s output voltage by using
a multimeter to probe the righthand
terminal of the regulator with the PC
board orientated as in the overlay and
photos. Connect the other probe of the
multimeter to ground. Make sure that
you don’t slip with the multimeter
probe and short-circuit the regulator!
When VR1 is adjusted so that a particular voltage shown in Table 1 can
be measured on its wiper terminal, the
switch will trip at the corresponding
temperature. For example, if you want
the relay to close at 120°C, set VR1 so
that its wiper voltage is 2.274V. The
accuracy will be within about 2%.
1 PC board coded 05car101,
105 x 60mm
1 NTC thermistor, SOD27
leaded package, -40°C to
300°C (BC components 2322
633 83103)
5 PC-mount 2-way screw
terminals with 5mm pin
spacing
1 12V PC mount DPDT 5A relay
(Relay1)
1 3-way header with 2.54mm
spacing
1 jumper shunt with 2.54mm
spacing
1 3m length of single core
shielded cable
1 50mm length of 4mm diameter
heatshrink tubing
1 1kΩ multi-turn top adjust
trimpot (VR1)
1 1MΩ horizontal trimpot (VR2)
Semiconductors
1 LM358 dual op amp (IC1)
1 7808 3-terminal regulator
(REG1)
1 BC337 NPN transistor (Q1)
1 5mm red LED (LED1)
1 16V 1W zener diode (ZD1)
2 1N4004 1A diodes (D1,D2)
1 1N4148 diode (D3)
Capacitors
2 100µF 16V PC electrolytic
2 10µF 16V PC electrolytic
1 100nF MKT polyester (code
104 or 100n)
Resistors (0.25W, 1%)
1 22kΩ
2 1kΩ
4 10kΩ
1 10Ω
1 1.8kΩ
Remember, if you wish the relay to
close when the temperature goes above
a particular value, install link LK1 in
position “L/H” and install diode D3
as shown on the overlay. For the relay
to close when the temperature goes
below a certain value, install link LK1
in position “H/L” and install D3 the
other way around.
In most applications, once the Temperature Switch is set, it won’t need
to be altered. The PC board fits into a
130 x 68 x 42mm jiffy box, so when
the system is working correctly, it can
be inserted into the box and tucked
SC
out of sight.
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