Temperature and humidity over-limit alarm circuit diagram using LC179

CD4538 is a precision monostable flip-flop in DIP-16 and SMD-16 packages. It has the characteristics of stable performance, few external components required, strong logic function, and two independent monostable triggers integrated inside the chip. It is widely used in fields such as counting, frequency division and temperature control. The typical circuit of the infusion heating controller composed of CD4538 is shown in the figure. When the temperature difference between the liquid temperature and the body temperature of the human body is too large, the controller can use the pulse width temperature control method to heat the infusion set to alleviate the pain, chills and other uncomfortable symptoms that occur during patient infusion. It is safe and efficient.

Temperature and humidity over-limit alarm circuit composed of LC179

Circuit structure and main component selection:

As can be seen from the figure, the infusion heating controller consists of a power supply circuit, an astable multivibrator, a temperature control circuit and a thyristor control circuit. Among them, the power circuit consists of power switch S, AC fuse FU, power transformer T, rectifier bridge stack UR, filter capacitors C1, C7 and current limiting resistor R6 and other components. In practical applications, S often uses a 5A, 250V power switch; T uses a 5-10W power transformer with a secondary voltage of 15V; UR uses a 1A, 50V rectifier bridge stack. After the 220V AC voltage is stepped down, rectified and filtered, one channel provides nearly 18V DC voltage for the light-emitting diodes inside the optocoupler VLC; the other channel provides working power for IC1 and IC2 after being current-limited by R6 and filtered by C7.

The astable multivibrator is composed of time base integrated circuit IC1 and peripheral components such as resistors R2 and R3, capacitors C2 and C3. In practical applications, IC1 often uses the NE555 time base integrated circuit.

The temperature control circuit consists of precision monostable trigger CD4538 (IC2) and peripheral components such as thermistor RT, potentiometer RP, resistors R4, R5, R7~R9, capacitors C4~C6, electronic switches VD1, VD2. In practical applications, negative temperature coefficient thermistors are often used for RT; small synthetic carbon film potentiometers are used for RP; IN4148 silicon switching diodes or ZAK10 germanium switching diodes are used for VDI and VD2.

The thyristor control circuit consists of the transistor VT, the photoelectric coupler VLC, the bidirectional thyristor VS and the heating appliance EH. In practical applications, VT often uses S9013 or S8050 silicon NPN transistors; VLC uses 4N25 photocoupler; VS uses bidirectional thyristors above 1A and 100V; EH uses PTC heaters.

working principle:

After the circuit is powered on, the astable multivibrator is powered on and the NE555 pin 3 outputs a square wave pulse signal. After differential processing by C4 and R4, the pulse signal is input from the CD4538 pins 4 and 12, triggering the internal pulse of the chip. Two monostable circuits and perform pulse width comparison. When the 6th and 9th pins of CD4538 output high level at the same time, the electronic switches VD5 and VD6 are cut off at the same time, making VT in the conductive state, the light-emitting diode inside the photoelectric coupler VLC lights up, and the photosensitive transistor conducts, making the bidirectional intergranular Tube VS is triggered and conducts, and EH is energized and starts heating. As the temperature of the liquid medicine rises, the resistance of RT begins to decrease. When the heating temperature reaches the set temperature of RP, pin 6 of CD4538 changes from high level to low level, VD5 is turned on, and VT, VS, and VLC are all cutoff, EH stops heating. When the temperature of the liquid drops to a certain value, pin 6 of CD4538 outputs high level again, VD5 is cut off, VT, VS and VLC are turned on again, and EH starts to heat again. The above process is carried out over and over again to keep the temperature of the liquid medicine constant near the set value of RP.