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Alarms can be used for a variety of applications, such as frost monitoring, ambient temperature monitoring, and more. In the idle state, the circuit draws a current of only a few microamps, so theoretically at least a 9V dry battery (PP3, 6AM6, MN1604, 6LR61) should have a lifespan of up to 10 years. Such low current is not possible when using an integrated circuit and hence the circuit has a discrete design. Every four seconds, a measuring bridge activates the Schmitt trigger, which is activated for 150 ms by the clock generator. During this 150 ms period, the resistance of the NTC thermistor, R11, is compared with the fixed resistor.
If the first number is less than the second, the alarm will be triggered. When the circuit is activated, capacitor C1 is uncharged and transistors T1 to T3 are blocked. After power on, C1 is gradually charged through R1, R7 and R8 until T1's base voltage exceeds the bias threshold. Transistor T1 then turns on and also turns on T2 and T3. Then, C1 is charged through the current source T1-T2-D1, until the current from the source becomes lower than the current flowing through R3 and T3 (about 3 µA). This results in T1 being cut off, so due to coupling to C1 the whole circuit is turned off. Capacitor C1 is (almost) fully charged, so D1's anode potential drops below 0 V. Only when C1 is recharged can a new cycle begin.
Apparently most of the current is used to charge C1. IC1a port functions as an impedance inverter and feedback stage, and turns on the measuring bridge R9 – R12-C2-P1 periodically for a short time. The bridge is terminated by a differential amplifier, which although low current (and resulting in low conductivity of the transistor) still gives large amplification and therefore high sensitivity. Resistors R13 and R15, through a type of hysteresis, provide a Schmitt trigger input to the differential amplifier, making it possible to obtain fast and clear measurement results. Capacitor C2 compensates for the capacitive effect of long cables between sensor and circuit and thus avoids false alarms.
If the sensor (R11) is integrated in the same package as the rest of the circuit (for example in an ambient temperature monitor), C2 and R13 can be omitted. In this case, C3 will absorb all interfering signals and thus avoid false alarms. To prevent the residual charge in C3 from causing false alarms when the bridge is balanced, the capacitor rapidly discharges through D2 when this occurs. Gate IC1c and IC1d form the oscillator to drive the buzzer (AC type). Since the meter has a very high impedance, an epoxy resin plate (not pertinax) should be used to generate the alarm. For the same reason, C1 must be a very low leakage current type.
If alarm operation is required when the resistance of R11 is greater than the fixed resistance, reverse the connections of the bridge elements and thus be effective for the inverting and non-inverting inputs of the micro amplifier. wrong. An NTC thermistor like R11 has about ten times more resistance at –18°C than at room temperature. It is therefore advisable, even necessary, when correct operation is required, to consult the technical sheet of the instrument or make certain test measurements. For a current circuit, the resistance at –18°C should be between 300 and 400 kΩ. The value of R12 must be the same. The P1 preset allows fine tuning of the feedback threshold. Note that although the prototype uses an NTC thermistor, another type of sensor can also be used, as long as its electrical specifications are known and suitable for the current circuit.