Analysis of Several 555 Time-Base Control Circuits

　　Simple temperature controller using 555 time base circuit: A temperature automatic controller composed of 555 time base integrated circuit and few peripheral components. Because the voltage at each point in the circuit comes from the same DC power supply, it does not require a high-performance voltage regulator, and can work reliably using the capacitor voltage reduction method. The circuit components are low-priced, small in size, and easy to make under amateur conditions. The temperature automatic controller made by this circuit can be used for industrial production and household electric heating control with good results.

　　Working principle of the circuit: When the temperature is low, the negative temperature coefficient thermistor Rt has a large resistance, the potential of the 2nd pin of the 555 time base integrated circuit (IC) is lower than 1/3 of the Ec voltage (about 4V), and the 3rd pin of the IC outputs a high level, triggering the bidirectional thyristor V to turn on, and connecting the electric heater RL for heating, thereby starting the timing cycle. When the temperature of the thermistor Rt placed at the temperature measurement point is higher than the set value and the timing cycle has not yet been completed, the heater RL is cut off after the timing cycle ends. When the temperature of the thermistor Rt drops below the set value, it will trigger the bidirectional thyristor V to turn on again, and connect the electric heater RL for heating. In this way, the purpose of automatic temperature control can be achieved.

　　Selection of components: Thermistor Rt can be MF12 or MF53 with negative temperature coefficient, or different resistance values ​​and other types of negative temperature coefficient thermistors, as long as the relationship Rt + VR1 = 2R4 is satisfied under the temperature conditions to be controlled. A larger potentiometer VR1 can obtain a larger adjustment range, but the sensitivity will decrease. Bidirectional thyristor V can also be selected according to the size of the load current. There are no special requirements for other components, and they can be selected according to the parameters given in the circuit diagram.

　　Automatic temperature controller using 555 time-base circuit

　　Working principle of the circuit: The operating state of electrical equipment can be controlled by temperature changes. IC1555 integrated circuit is connected as a self-excited multivibrator, Rt is a thermistor. When the ambient temperature changes, the oscillation frequency composed of resistor R1, thermistor Rt and capacitor C1 will change. The frequency change is sent to pin 3 of frequency decoding integrated circuit IC2LM567 through pin 3 of integrated circuit IC1555. When the input frequency falls exactly on the center frequency of IC2 integrated circuit, pin 8 outputs a low level, making relay K conduct and contacts close, thereby controlling the on and off of the equipment and forming the function of temperature control circuit.

　　Selection of components: IC1 uses NE555, μA555, SL555 and other time-base integrated circuits; IC2 uses LM567 frequency decoding integrated circuit; VD uses IN4148 silicon switch diode; R1 uses RTX-1/4W carbon film resistor. C1, C2, C3 use CT1 ceramic capacitors; C4, C5 use CD11-25V electrolytic capacitors; K uses JZC-22F small medium-power electromagnetic relay with working voltage of 9V; Rt can use 51KΩ negative temperature coefficient thermistor at room temperature; RP can use WSW organic solid fine-tuning variable resistor.

      The whole circuit of the simple temperature controller can be installed on a circuit board, and generally no debugging is required. The time interval is 1.1R2×C3, which should be selected smaller than the thermal time constant of the heating system, but not too small, otherwise it will cause excessive radio frequency interference due to the rapid conduction or shutdown of the bidirectional thyristor V. After installation and debugging, it can be placed in a small plastic box, and the thermistor Rt can be led out to the temperature measurement point. As long as the circuit is correct during the process, this circuit is easy to implement. If the component performance is good, it can be used without debugging after installation.