Campus radio station automatic broadcast control circuit

The working principle of the circuit: As can be seen from the figure, S is the power switch; S2 is
the forward and reverse switch of the AC motor M, which can control the forward and backward movement of the sealed items; S3 is
the cold mounting and heat sealing switch, that is, when S3 is closed When the machine is open, it is heat-sealed, and when it is disconnected, it is cold-mounted (one machine
can be used for two purposes); S. It is a temperature measurement and constant temperature switch. When it is turned upward, it is in the temperature measurement state. At this time,
the current temperature of the machine can be measured. When it is turned downward, it is in the constant temperature state (preset constant temperature state).
The measured and fixed temperatures are read through the thermometer port on the panel. RP is a constant temperature adjustment potentiometer
, used to set the required operating temperature. S. Both S4 and RP are designed on the panel
to facilitate operation. VT is a current amplifier tube, used to drive the pointer thermometer port.
The constant temperature control circuit is composed of thermistor Rr, LM741 , etc. When starting up, the voltage of pin 3 of LM741
is greater than the voltage of pin 2 (U3, U3), then pin 6 outputs high level , VTz is saturated and turned on, relay K is closed, contact K1 is switched, and the power
of quartz electric heating tube RL is turned on , and the machine
Start heating. As the temperature increases, the RT resistance becomes smaller and the potential of pin 3
decreases. When U3 < U2, pin 6 becomes low level, VT2 is cut off, K is released, and
after RL is cut off, heating stops. After the temperature decreases, the resistance value of RT becomes larger, and then reaches > 0, then
pin 6 outputs a high level and heating starts again. By repeating this process over and over again, the goal of constant temperature is achieved
. The red light and green light are power and constant temperature indicator lights.