NE555 long time delay timing circuit

Two 555 time base circuits are used to form a timing circuit with a long timing time and an adjustable timing time range.

1. Circuit Working Principle

The circuit principle is shown in Figure 1.

Figure 1 Dual 555 time base circuit long delay circuit diagram

The IC1555 time base circuit is connected to a self-excited multivibrator with adjustable duty cycle. When the button SB is pressed, a 12V DC voltage is added to the circuit. Since the voltage of capacitor C6 cannot change suddenly, the 2nd pin of the IC2 circuit is at a low level, the IC2 circuit is in the set state, the 3rd pin outputs a high level, the relay K is energized, the contacts K-1 and K-2 are closed, the K-1 contact is closed to form a self-locking state, and the K-2 contact is connected to the electrical equipment to achieve the function of controlling the electrical equipment on and off. At the same time, the IC1555 time base circuit begins to oscillate, so the 3rd pin outputs high and low levels alternately. When the 3rd pin outputs a high level, the capacitor C3 is charged through the diode VD3 and the resistor R3. When the 3rd pin outputs a low level, the diode VD3 is cut off, and C3 is not charged. Therefore, C3 is charged only when the 3rd pin is at a high level, so the charging time of the capacitor C3 is longer. When the potential of capacitor C3 rises to 2/3VDD, the IC2555 timing circuit is reset, pin 3 outputs a low level, relay K loses power, contacts K-1 and K-2 are disconnected, and the circuit returns to its initial state, ready for the next timing.

2. Selection of components

IC1 and IC2 use NE555, μA555, SL555 and other timing integrated circuits; VD1~VD4 use IN4148 silicon switching diodes, and the light-emitting diodes can use general light-emitting diodes; R1~R5 use RTX-1/4W carbon film resistors; capacitors C1, C2, C5, C6 use CT1 ceramic capacitors, C4 uses CD11-16V electrolytic capacitors, and C3 uses tantalum electrolytic capacitors with extremely small leakage current; RP can use WSW organic solid fine-tuning variable resistors; relay K uses JRX-13F small electromagnetic relay with two sets of changeover contacts.

3. Production and debugging methods

During debugging, the variable resistor RP can be adjusted to change the duty cycle of the square wave pulse output from the 3rd pin of the IC1555 time base circuit, thereby changing the timing of the timer. This circuit has a simple structure and can work normally as long as it is soldered according to the circuit diagram and the components are selected correctly.