Thermostat with MLX90614 DownloadLink 5327

Design Example
Thermostat with MLX90614
Scope
This design example describes an example of a thermostat with a MLX90614 without
using a microcontroller. Temperature control is non-contact thanks to the factory calibrated IR
thermometer MLX90614.
Applications
■ Cooling system
■ Heating system
■ Alarm system
Related Melexis Products
EVB90614 is the evaluation board which supports the MLX90614 devices
Other Components Needed
C1
C2
C4
C3
C5
C6
C7
C8
C9
R1
R2
R3
R4
R5
R6
R7
R8
R9
R10
R11
R12
R13
R14
R15
100n
100n
1u
1u
10n
10n
10n
470/16
100/10Ts
360/0.6
360/0.6
620
3.3k
100k
330/0.6
10k
1.8k
43k
620
330/0.6
50k
10M
330k
330k
390129061402
Rev 001
R16
R17
R18
R19
R20
R21
D1
D2
D3
D4
F1
F2
J1
J2
J3
T1
U1
U3
U2
U4
U5
U6
U7
Page 1 of 10
5.1M
15k
27k
16k
0Ω
3.3k
LED R 3MM
BAS21LT1
B8S
LED G 3MM
FUSE
FUSE
CON5
JUMPER
CON2
EE20/4-0.08VA
PVN012/MOC3083
PVN012/MOC3083
74LVC1G86
MLX90614AAA
LTC1041
BC847
LP2980
Oct-2007
Design Example
Thermostat with MLX90614
Typical Circuit
Fig. 1: Block Diagram
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Oct-2007
Page 3 of 10
1
2
3
4
TP2
TP
TP3
TP
~220V
J3
1
2
POWER SUPPLY
MLX90614
SCL
SDA
Vdd
Vss
U4
TP1
TP
MLX90614
R14
330k
LPF
R8
1.8k
F2
FUSE
C1 C2
100n 100n
R7
10k
C3
1u
R15
330k
+5V
7TP29
6
TP30
1
2
3
4
TP
TP
TP28 TP
TP27 TP
R17
15k
EE20/4-0.08VA
1
4 T1
C4
1u
POT
R12
R9
43k
1
LTC1041
-
ON/OF
V
Vin
Vpp
SPoint Osc
GND Delta
U5
8
7
6
5
B8S
D3
+
C5
10n
R13
10M
COMPARATOR
Jumper OFF: HEATER
Jumper ON: COOLER
4
3
2
2
1
C7
10n
C6
10n
C8
+
470/16
J2
JUMPER
R5
100k
R18
27k
3
2
1
D1
LED R 3MM
R20
0
R19
16k
N/C
Out
4
5
TP25 TP
+ C9
100/10Ts
U6
BC847
R10
620
TP
R3
620
TP
TP
TP24
TP22
TP26
R4
3.3k
TP23 TP
D2
BAS21LT1
4
5
On/OFF
GND
In
U7
LP 2980
74LVC1G86
B
VCC
A
GND Y
R16
5.1M
1
2
3
U2
XOR
TP21 TP
A
C
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Rev 001
C
+5V
A
U1
PVN012
MOC3083
U3
PVN012
MOC3083
6
5
4
6
5
4
R1
360/0.6
LED G 3MM
D4
R21
3.3k
+5V
1
2
3
1
2
3
Photo Relays
R2
360/0.6
R11
330/0.6
R6
330/0.6
F1
FUSE
1
2
3
4
5
CON5
J1
Design Example
Thermostat with MLX90614
Fig. 2: Schematic diagram
Oct-2007
Design Example
Thermostat with MLX90614
Explanation
MLX90614 has thermostat mode with programmable threshold and hysteresis. Once set in the
on-chip EEPROM this mode will operate without the need of microcontroller. However, versatile
change of setpoint might be an issue (because of the need of supporting keyboard, display and
SMBus Master to change the EEPROM value). An easy way to implement that function is
described herein – a simple potentiometer is used to set the threshold.
Fig.1 shows the basic idea of such a thermostat with MLX90614. The MLX90614 IR
thermometer works in PWM mode. The PWM output passes Low Pass Filter (LPF). The output
voltage of the LPF is proportional to the temperature measured by the MLX90614. Further this
voltage Vin is compared with a reference voltage Vref. The BANG-BANG controller LTC1041 is
used in this design, with a transfer function shown on Fig. 3.
Vout
Vdelta
Vdelta
GND
Vref
Vin
Fig. 3
When Vin < (Vref-Vdelta) the output of the LTC1041 is HIGH and when Vin > (Vref+Vdelta) the
output is LOW. For versatility the output of the LTC1041 passes an Exclusive OR (XOR) gate.
The truth table of this element is given in Table 1. The output of the logical element depends on
the JUMPER. When the JUMPER is ON the XOR’s output follows the comparator’s output and
when JUMPER is OFF the XOR’s output is opposite of the comparator’s output. XOR’s output
controls photo relays intended to drive loads or trigger higher power switches like triacs.
Table 1
A B
0 0
0 1
1 0
1 1
Y
0
1
1
0
With jumper J2 left open high object temperature will turn the U3 on. Thus a thermostat can be
built with a heater controlled by the U1 (and jumper opened).
Schematic diagram of the thermostat is given on Fig. 2. The Power Supply is also designed to
be versatile. The module can use different power sources as shown on the following schematic
diagrams.
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Design Example
Thermostat with MLX90614
220V (110V) AC:
~220V
4 T1
6
U7
LP 2980
D3
1
2
1
+
-
2
1
B8S
1
7
2
EE20/4-0.08VA
+
3
470/16
In
Out
5
+5V
GND
On/OFF
N/C
+ C9
100/10Ts
4
R21
3.3k
D4
3
C7
10n
C8
A
F2
FUSE
J3
4
POWER SUPPLY
C
LED G 3MM
External 9V AC/DC:
The external LDO regulator is used:
POWER SUPPLY
TP
4
TP27
1
-
U7
LP 2980
+
2
1
B8S
2
+
C8
3
470/16
Out
5
+5V
GND
On/OFF
N/C
4
+ C9
100/10Ts
R21
3.3k
D4
3
C7
10n
In
A
D3
~9V
LED G 3MM
C
TP30 TP
or
The zener diode integrated in the MLX90614 (5V version, SCL pin to Vss pin) can also be used:
D2
BAS21LT1
U6
BC847
POWER SUPPLY
To SCL pin of a
MLX90614
R19
16k
TP
4
TP27
D3
1
~9V
-
+
2
+5V
+
C8
470/16
+ C9
100/10Ts
LED G 3MM
TP
C
TP30
R21
3.3k
D4
3
C7
10n
A
B8S
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Design Example
Thermostat with MLX90614
External 5V DC:
POWER SUPPLY
R20
0
+5V
5V
C7
10n
+
C8
A
+
+ C9
100/10Ts
470/16
R21
3.3k
D4
LED G 3MM
C
-
The MLX90614 can be configured in PWM Open Drain or Push Pull. In the latter case the
resistor R8 is not needed.
The LPF filter consists of R14, R15, C3 and C4. The output signal is fed to pin 2 (Vin) of the
LTC1041. On pin 3 (SPoint) the reference voltage Vref (from the divider R9-R12-R17) sets the
threshold. Vref can be adjusted with the potentiometer R12. R9 and R17 set the adjustment
range. The divider consisting of R16 and R18 defines the hysteresis (see Vdelta on Fig. 3). The
output of the LTC1041 through the logical element Exclusive OR ( 74LVC1G86) drives two
photo relays (the presence of the both relays simultaneously is not mandatory). Different types
photo relays can be used.
Different output configurations are shown below. The pinout of the output solid state relays /
opto-couplers is valid for large number of relays.
Using MOC3083: The MOC3081, MOC3082 and MOC3083 devices are designed for use with a
triac in the interface of logic systems to equipment powered from 240 Vac lines.
With the jumper J2 left open thermostat application is possible with upper opto-coupler (U1)
controlling a heater and/or the lower one (U3) controlling a cooler.
Photo Relays
F1
FUSE
R1
360/0.6
TP22
R2
360/0.6
U1
TP
R3
620
1
2
3
6
5
4
MOC3083
HOT
R6
330/0.6
J1
Q1
~240Vac
1
2
3
4
5
TP
TP24
39
0.01
LOAD
NEUTRAL
Q2
CON5
U3
R10
620
TP
TP26
1
2
3
39
0.01
6
5
4
LOAD
MOC3083
R11
330/0.6
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Oct-2007
Design Example
Thermostat with MLX90614
Using PVN012: It is ideally suited for switching high currents or low level signals without
distortion or injection of electrical noise. The shown connection allows maximum continuous
load current 2.5A. With a single opto-coupler U1 the module will operate as a thermostat with a
heater and jumper J2 open or a cooler and shorted J2.
Photo Relays
TP22 TP
F1
FUSE
U1
R3
620
1
2
3
6
5
4
PVN012
J1
1
2
3
4
5
TP24 TP
LOAD
~
AC/DC
CON5
Photo Relays
F1
FUSE
JUMPER ON: U1 is COOLER
JUMPER OFF: U1 is HEATER
U1
TP
TP22
R3
620
1
2
3
6
5
4
PVN012
J1
AC/DC
1
2
3
4
5
TP
TP24
~
LOAD
LOAD
CON5
U3
R10
620
TP
1
2
3
TP26
6
5
4
PVN012
JUMPER ON: U3 is HEATER
JUMPER OFF: U3 is COOLER
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Design Example
Thermostat with MLX90614
PCBs overview
The described design is implemented on the PCB shown below. It contains two units,
power supply and the thermostat itself. Voltage regulator is present on the thermostat part thus
allowing a local power source to be used instead of power line.
Unassembled: Top view
Unassembled: Bottom view
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Design Example
Thermostat with MLX90614
Assembled: Top view
Assembled: Bottom view
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Design Example
Thermostat with MLX90614
Conclusion
The shown schematic is just an example of simple thermostat with adjustable set point
with MLX90614. It is designed to be versatile and easy to adapt to different application
requirements.
References
1. IR thermometer MLX90614 family datasheet
2. LTC1041 datasheet
3. 74LVC1G86 datasheet
4. Series PVN012 datasheet
5. MOC3081/82/83 datasheet
6. LP2980 datasheet
7. Laminated Transformers Type EE20/4-0.08VA datasheet
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Oct-2007