555 water temperature alarm circuit

Published:2011/6/13 20:26:00 Author:TaoXi | Keyword: 555, water temperature, alarm circuit | From:SeekIC

As the figure 15-4 shows, this circuit uses the 556 as the core. The monostable trigger is composed of the left part (1/2 556) of IC, the R1, R3, RP1 and the thermistor probe. The thermistor uses the resistance which has the RT=3kΩ and the negative temperature coefficient. When the water temperature is 100 degrees celsius, the resistance value is about 240Ω; when the water temperature is 50 degrees celsius, the resistance value is about 950Ω. If you pre-set the temperature and adjust RP1 to 50Ω, so the VA=RT/(RP1+R1+RT)<1/3, when the trigger port pin-6's electric potential is lower than 1/3VDD, the trigger is set, the high electrical level which is output by the pin-5 adds to the right part of the 1/2 556's reset port, f=1.44/(R4+2R5)C4, the oscillation frequency is about 1000Hz.



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