Micropower CMOS electronic timer

Micropower CMOS electronic timer

Using CMOS circuits, its input impedance is very high, so it is possible to use small-capacity and high-precision polyester or polycarbonate capacitors, which are easy to obtain. Timers have many uses. The timer described in this article has special advantages. It has almost no static current and can always remain in a waiting state. A typical application is to install such a switch on the street lights in the corridor. Another application is to install such a switch on the headlights of a car so that you can light up the path in the middle of your garden for a few minutes to let you enter the apartment smoothly. A timer of more than ten minutes usually has a large electrolytic capacitor and a resistor to form a charging and discharging circuit. There are many disadvantages of using electrolytic capacitors. First, it has a large leakage current, so the charging and discharging time is often affected. Second, its own capacitance error is also large. Therefore, the delay accuracy is very poor.

This article uses a CMOS circuit, which has a very high input impedance, so it is easy to use small-capacity, high-precision polyester capacitors or polycarbonate capacitors. Comparing the two, the latter is smaller. So it is best to use polycarbonate capacitors in this circuit. This circuit uses a 2 Hz oscillator. Changing the output pulse width can change the charging rate, thereby changing the timing time. This timer is indeed very reliable and accurate.
As shown in the figure:

A and B are connected to form an oscillator. The operating frequency is 2 Hz. C2 is a charging capacitor. The input voltage of D is very low, so its output presents high voltage. When the input of A is high potential, the oscillator starts to work. When the output of A is positive voltage, D1 is turned on, and VR1 controls the charging current entering C1. The discharge current of C1 can only flow out through R1. In this way, the charging voltage of C1 is a series of positive pulses. Its width is adjusted by VR. The positive pulse charges C2 through R6 and D2. When its voltage reaches the gate voltage of D, D flips. Output low voltage. Cause the output of A to become high potential. Thus, the charging of C2 will be completed quickly. E also flips. Power amplification is carried out through TR2 and TR3 to drive the relay to operate and achieve the purpose of timing. In addition, when the relay operates, it may cause battery voltage fluctuations to cause the relay contacts to shake. In order to eliminate this phenomenon, D3, C3, and R7 are added. Transistor TR1 is a device used to discharge capacitor C2 when the switch is started.

Component R1 uses 10MR. R2 uses 2.2KR. R3 uses 330KR. R4, R5, and R8 all use 10KR. R9 uses 10KR. R6 uses 1MR. R7 uses 4.7MR. Potentiometer VR1 uses 10KR. Capacitors use 1uF polyester capacitors C1C2C3. C4 uses 470uF electrolytic capacitors with a withstand voltage of 10 volts. Semiconductor tubes D1, D2, D3, and D4 all use 1N914. TR1 and TR2 use BC184 and BC2142 respectively. TR3 uses BFX29. 1C1 uses CMOS4011B. Transistor BC184 can be replaced by domestic 3DG130B. BC214L can be replaced by domestic transistor model 3CG20A. BFX29 can be replaced by domestic 3CK9D. Quad-NAND gate CMOS4011B. Can be replaced by domestic C036.