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 390129061402 Rev 001 Page 2 of 10 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 390129061402 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. 390129061402 Rev 001 Page 4 of 10 Oct-2007 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 390129061402 Rev 001 Page 5 of 10 Oct-2007 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 390129061402 Rev 001 Page 6 of 10 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 390129061402 Rev 001 Page 7 of 10 Oct-2007 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 390129061402 Rev 001 Page 8 of 10 Oct-2007 Design Example Thermostat with MLX90614 Assembled: Top view Assembled: Bottom view 390129061402 Rev 001 Page 9 of 10 Oct-2007 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 390129061402 Rev 001 Page 10 of 10 Oct-2007