Temperature range control circuit

Circuit working principle: 220 V mains power is stepped down by transformer T, VD2-VD5 bridge
rectifier, C2 filtered and then supplied to the entire circuit. After the switch S is closed, since
the potential of point M is lower than 1/3 Vcc at this time, the 555 circuit is set, the output pin 3 is high level,
the relay K is closed, and the contact K-1 is turned on to control the heating. The circuit makes the temperature rise, and at the same time
the light- emitting diode LED indicates.