A high-precision digital long delay circuit

The general long delay circuit usually relies on electrolytic capacitors or high impedance circuits. This type of delay circuit has poor stability and low delay accuracy. Here is a digital long delay circuit that completely abandons large electrolytic capacitors and high impedance circuits and has high delay accuracy.

1. Circuit Working Principle

The circuit principle is shown in the figure below.

The core of the circuit is the integrated block MCI4521B, which is a 24-level frequency division circuit, including an inverter that can form an oscillation circuit. If the trigger input is grounded or no signal is added, the circuit enters the delay state, and the delay time is adjusted by the range switch X and the 100KΩ potentiometer.

If X is connected to point A, the delay is 1 minute 40 seconds to 18 minutes 30 seconds, and if X is connected to point B, the delay is 13 minutes 20 seconds to 2 hours 28 minutes. When X is connected to point C, the delay is 1 hour 47 minutes to 20 hours. The specific delay time is adjusted by the 100KΩ potentiometer. If a longer delay is required, a large capacitor can be used instead of the 39nF capacitor. In this case, the delay can reach more than one week.

When a positive signal is added to the trigger output, the divider in 4521B is reset.

2. Component Selection and Production

IC uses MCI4521B integrated circuit; Rl~R4 use 1/4W metal film resistors; RP uses organic solid variable resistors. C1 uses ceramic chip capacitors. VD1 uses IN4004 silicon rectifier diodes; VD2 uses IN4148 silicon switch diodes. VT uses BC337 silicon triodes; VS uses 1W, 15V silicon voltage regulator diodes. Connect the circuit as required, and it can work normally without debugging. The delay is reliable and stable, and it is recommended to be powered by a 6~15V voltage regulator.