Posted on Feb 6, 2014

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

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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...

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