Integral timer in temperature-frequency converter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Radio amateur designer
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An integral timer type 555 in self-excited multivibrator mode can be used to generate a square-wave voltage whose frequency exactly matches the measured temperature. In this case, the thermistor with negative TC is used in the charging circuit of the timer.
Ris.1
When the temperature changes from 3 to 46 ° C, the frequency at the output of the circuit changes according to an almost linear law in the range from 38 to 114 Hz. In this entire temperature range, there is not a single point where the frequency deviates from the ideal dependence by more than ±1 Hz. Few parts, low cost, and low power requirements (9,3mA at 10VDC) make this temperature-to-frequency converter suitable for telemetry applications.
Ris.2
Typically, when turning on a 555 timer in a self-excited multivibrator circuit, two fixed resistors are needed. In the converter circuit, instead of one of them, a thermistor and a resistor are connected in series, and instead of the other, transistor Q1, which turns on during the charge of the time-setting capacitance and turns off during its discharge. The resistance of this transistor when it is on is close to zero, and when it is off it is very high.
Author: D. DeKold; Publication: N. Bolshakov, rf.atnn.ru
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