c520
#thermostat
#temperature control
#AD converter
#digital display
#home automation
#ambient temperature
#sensor
#C520
#temperature range
#household
Description: This schematic illustrates a digital thermostat designed to control the ambient temperature within a house. The thermometer displays temperatures in the range of 0°C to 99.9°C, while the thermostat can set a temperature within the range of 0°C to 99°C. The actual temperature range for indoor use is between 12°C and 25°C. The C520 is an older model of an analog-to-digital (AD) converter, developed over 10 years ago in the former German Democratic Republic. The original purpose of the C520 was as a digital voltmeter, capable of measuring from -9.9 mV to +999 mV, displayed in three digits using an external BCD to 7-segment decoder. Two trimmers for zero adjustment and full-scale adjustment, along with some external components, were also necessary. In this application, the C520 functions as a relative digital voltmeter, with all trimming settings made in the thermometer section.
The schematic for the voltmeter section indicates that the zero scale adjustment resistor has been replaced with resistors R1 and R2, while the full-scale adjustment resistor, typically connected between pin 13 and ground, is absent. Capacitor C1 sets the number of readings per second to a reasonable value (3 to 6) for good visibility. The demultiplexing circuit, comprising transistors Q1 to Q3, supplies the common anode digits. U2 is a standard TTL BCD to 7-segment common anode decoder, which has the limitation of not being able to decode a negative sign, appearing instead as a 'c' on the display. However, since only positive temperatures are measured, this limitation is not a concern.
The analog signal is applied between the HI and LOW pins and originates from the thermometer section, which utilizes a Romanian temperature sensor, the BM135. This sensor operates similarly to a diode but features improved characteristics, providing a sensitivity of +10 mV/°C compared to -2.2 mV/°C. For optimal replaceability of the temperature sensor, an optional adjustment resistor should be included. In this application, the sensor is biased under constant current.
Calibration of the thermometer requires repetitive operations at two fixed temperature points (0°C and 100°C). At 0°C, a ZERO ADJ is performed, and at 100°C, a FULL SCALE ADJ is completed. Due to the interdependence of these adjustments, they must be repeated 3 to 4 times until U6 and U7 latch the tens and units digits of the temperature measurement simultaneously with the multiplexing signals M (Most Significant Digit) and I (Intermediate Significant Digit). To prevent false spikes during the small pauses between these multiplexing signals, integration is achieved using R11, C8, and R10, C7.
The tens digit is compared in U10 ALU with preset tens from a BCD inverted programmer connected at JP2, and the unit digit is compared in U11 ALU with preset units from a BCD inverted programmer connected at JP1. When the preset temperature equals the measured temperature, relay K1 is released, and the last digit displayed at U5 becomes blank. A more efficient approach would be to replace the 181 ALU with two 485, 4-bit magnitude comparators or, ideally, to implement the thermostat using a PIC16F84 microcontroller. When this project was initially developed, these were considered the best available devices.
It is important to note that C9 should be a high-quality polyester capacitor, while U10 and U11 are CMOS ALUs and U3, U4, and U5 are low-current display digits. Additional 100nF capacitors (not shown in the schematics) should be soldered on the power pins near the C520 converter if a cost-effective power supply solution without a transformer is utilized.This schematics show a digital thermostat designed for controlling ambient temperature inside a house. Thermometer display temperature in 0C. 99. 9C range and the thermostat can set a temperature in 0. 99C range. The temperature real range is between 12C and 25C inside a house. C520 is an oldest but goldest AD converter designed more than 10 years ag o in Deutchland Democratic Republic. The first purpose for C520 was digital voltmeter -9. 9mV to +999mV range in three digits display using external BCD to 7 segments dedicated decoder. Two trimmers ( for zero adj. and full scale adj. ) and some external components were also necessarily. In this application C520 acts as a relative digital voltmeter, all trimming settings were made in thermometer section. Here is the schematics for voltmeter section: The zero scale adjust resistor was replaced with R1 and R2 and full scale adjust resistor (normally connected between pin 13 and ground ) is missing.
C1 set the numbers of readings/second at a reasonable value (3 to 6 for a good visibility). Demultiplexing circuit Q1 to Q3 supplies the common anodes digits. U2 is a normal TTL BCD to 7 segments, common anodes decoder, it has only the disadvantage that can`t decode - sign, this appears like a c sign on display. Because only positive temperature are measured this situation will never be visible. Analogic signal is applied between HI and LOW pins. This signal comes from the thermometer section: The thermometer use a romanian temperature sensor BM135, it`s like a diode with better characteristics: +10mV/C instead of -2.
2mV/C. For a good replaceability of temperature sensor an optional adjustment resistor must be added. In this application the sensor is biasing under constant current. Repetitive and successive operation must be done for calibrating the thermometer: two fixed temperature points ( at 0C and 100C ) are necessarily, with sensor at 0C a ZERO ADJ. is done, with sensor at 100C a FULL SCALE ADJ. is done. Because these two adjustments are interdependently, must be done for 3. 4 times until U6 and U7 latch the tens and units digits of temperature measurement simultaneously with multiplexing signals M ( Most Semnificative Digit ) and I (Intermediate Semnificative Digit ).
Because multiplexing signals comes with a small pause between them, these signals must be integrated with R11, C8 and R10, C7 to prevent false spikes. Tens digit is compared in U10 ALU with preset tens from BCD inverted programmer connected at JP2. Unit digit is compared in U11 ALU with preset units from BCD inverted programmer connected at JP1. When the preset temperature is equal with the measured temperature, K1 relay is released and the point of last digit display ( U5 ) become blank.
A better way is to use instead of 181 ALU two 485, 4bit magnitude comparator or much better is to built a thermostat using a PIC16F84 microcontroller. A few years ago when I made this project these were the best devices I`ve had. One more phrase: C9 must be a very good polyester capacitor, U10 and U11 CMOS ALU and U3, U4 and U5 low current display digits.
Supplementary 100nF capacitors ( not shown in schematics) must be soldered on power pins near the C520 converter if the cheaper solution of power supplies ( show above ) without transformer is used.
The schematic for the voltmeter section indicates that the zero scale adjustment resistor has been replaced with resistors R1 and R2, while the full-scale adjustment resistor, typically connected between pin 13 and ground, is absent. Capacitor C1 sets the number of readings per second to a reasonable value (3 to 6) for good visibility. The demultiplexing circuit, comprising transistors Q1 to Q3, supplies the common anode digits. U2 is a standard TTL BCD to 7-segment common anode decoder, which has the limitation of not being able to decode a negative sign, appearing instead as a 'c' on the display. However, since only positive temperatures are measured, this limitation is not a concern.
The analog signal is applied between the HI and LOW pins and originates from the thermometer section, which utilizes a Romanian temperature sensor, the BM135. This sensor operates similarly to a diode but features improved characteristics, providing a sensitivity of +10 mV/°C compared to -2.2 mV/°C. For optimal replaceability of the temperature sensor, an optional adjustment resistor should be included. In this application, the sensor is biased under constant current.
Calibration of the thermometer requires repetitive operations at two fixed temperature points (0°C and 100°C). At 0°C, a ZERO ADJ is performed, and at 100°C, a FULL SCALE ADJ is completed. Due to the interdependence of these adjustments, they must be repeated 3 to 4 times until U6 and U7 latch the tens and units digits of the temperature measurement simultaneously with the multiplexing signals M (Most Significant Digit) and I (Intermediate Significant Digit). To prevent false spikes during the small pauses between these multiplexing signals, integration is achieved using R11, C8, and R10, C7.
The tens digit is compared in U10 ALU with preset tens from a BCD inverted programmer connected at JP2, and the unit digit is compared in U11 ALU with preset units from a BCD inverted programmer connected at JP1. When the preset temperature equals the measured temperature, relay K1 is released, and the last digit displayed at U5 becomes blank. A more efficient approach would be to replace the 181 ALU with two 485, 4-bit magnitude comparators or, ideally, to implement the thermostat using a PIC16F84 microcontroller. When this project was initially developed, these were considered the best available devices.
It is important to note that C9 should be a high-quality polyester capacitor, while U10 and U11 are CMOS ALUs and U3, U4, and U5 are low-current display digits. Additional 100nF capacitors (not shown in the schematics) should be soldered on the power pins near the C520 converter if a cost-effective power supply solution without a transformer is utilized.This schematics show a digital thermostat designed for controlling ambient temperature inside a house. Thermometer display temperature in 0C. 99. 9C range and the thermostat can set a temperature in 0. 99C range. The temperature real range is between 12C and 25C inside a house. C520 is an oldest but goldest AD converter designed more than 10 years ag o in Deutchland Democratic Republic. The first purpose for C520 was digital voltmeter -9. 9mV to +999mV range in three digits display using external BCD to 7 segments dedicated decoder. Two trimmers ( for zero adj. and full scale adj. ) and some external components were also necessarily. In this application C520 acts as a relative digital voltmeter, all trimming settings were made in thermometer section. Here is the schematics for voltmeter section: The zero scale adjust resistor was replaced with R1 and R2 and full scale adjust resistor (normally connected between pin 13 and ground ) is missing.
C1 set the numbers of readings/second at a reasonable value (3 to 6 for a good visibility). Demultiplexing circuit Q1 to Q3 supplies the common anodes digits. U2 is a normal TTL BCD to 7 segments, common anodes decoder, it has only the disadvantage that can`t decode - sign, this appears like a c sign on display. Because only positive temperature are measured this situation will never be visible. Analogic signal is applied between HI and LOW pins. This signal comes from the thermometer section: The thermometer use a romanian temperature sensor BM135, it`s like a diode with better characteristics: +10mV/C instead of -2.
2mV/C. For a good replaceability of temperature sensor an optional adjustment resistor must be added. In this application the sensor is biasing under constant current. Repetitive and successive operation must be done for calibrating the thermometer: two fixed temperature points ( at 0C and 100C ) are necessarily, with sensor at 0C a ZERO ADJ. is done, with sensor at 100C a FULL SCALE ADJ. is done. Because these two adjustments are interdependently, must be done for 3. 4 times until U6 and U7 latch the tens and units digits of temperature measurement simultaneously with multiplexing signals M ( Most Semnificative Digit ) and I (Intermediate Semnificative Digit ).
Because multiplexing signals comes with a small pause between them, these signals must be integrated with R11, C8 and R10, C7 to prevent false spikes. Tens digit is compared in U10 ALU with preset tens from BCD inverted programmer connected at JP2. Unit digit is compared in U11 ALU with preset units from BCD inverted programmer connected at JP1. When the preset temperature is equal with the measured temperature, K1 relay is released and the point of last digit display ( U5 ) become blank.
A better way is to use instead of 181 ALU two 485, 4bit magnitude comparator or much better is to built a thermostat using a PIC16F84 microcontroller. A few years ago when I made this project these were the best devices I`ve had. One more phrase: C9 must be a very good polyester capacitor, U10 and U11 CMOS ALU and U3, U4 and U5 low current display digits.
Supplementary 100nF capacitors ( not shown in schematics) must be soldered on power pins near the C520 converter if the cheaper solution of power supplies ( show above ) without transformer is used.
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