Introduction to the Multipurpose 555 Time Base Integrated Circuit

The internal circuit of the 555 circuit contains two voltage comparators, a basic RS flip-flop, and a discharge switch T. The reference voltage of the comparator is provided by a voltage divider composed of three 5K resistors. They make the reference levels of the non-inverting input of high-level comparator A1 and the inverting input of low-level comparator A2 to 2/3VCC and 1/3VCC respectively. The output terminals of A1 and A2 control the RS flip-flop state and the discharge tube switch state. When the input signal is from pin 6, that is, the high level trigger input exceeds the reference level 2/3VCC, the flip-flop is reset, and the output pin 3 of the 555 outputs low level, and the discharge switch is turned on at the same time; when the input signal is from 2 When the pin input is lower than 1/3VCC, the flip-flop is reset, pin 3 of 555 outputs high level, and the discharge switch is turned off at the same time. 555 timer RD is the reset terminal. When RD=0, 555 outputs low level. Usually the RD terminal is open or connected to VCC.
We know that 555 circuits can generally be classified into three categories in terms of application and working methods. Each type of working method has many different circuits. In practical applications, in addition to a single type of circuit, many different circuits can also be combined, such as: multiple monostables, multiple bistables, monostable and astable, bistable and astable combinations, etc. As a result, the circuit becomes more complex. In order to facilitate our analysis and identification of circuits and better understand the 555 circuit, here we classify and summarize the 555 circuit according to its structural characteristics. The 555 circuit is divided into 3 categories, 8 types, and a total of 18 unit circuits. In addition to drawing its standard diagram and pointing out their structural characteristics or identification methods, each circuit also provides calculation formulas and their uses. It is convenient for everyone to identify and analyze the 555 circuit. These three types of circuits will be introduced below.
The monostable
working mode of monostable circuits can be divided into three types. See illustration.
The first type (Figure 1) is a manually activated monostable, which is divided into two different units due to the different positions of the timing resistor and timing capacitor, and are codenamed 1.1.1 and 1.1.2 respectively. The form of their input terminals, that is, the structural characteristics of the circuit, are: "RT-6.2-CT" and "CT-6.2-RT".

The second type (Figure 2) is a pulse-started monostable, which can also be divided into two different units. Their input characteristics are all "RT-7.6-CT", which are input from terminal 2. The 1.2.1 circuit does not have any components at both ends and has the simplest form; the 1.2.2 circuit has an RC differential circuit.

The third type (Figure 3) is a voltage controlled oscillator. There are many monostable voltage controlled oscillator circuits, all of which are relatively complex. For simplicity, we just divide it into 2 different units. The circuit without any auxiliary devices is 1.3.1; the circuit using auxiliary devices such as transistors and operational amplifiers is 1.3.2. Two commonly used circuits are listed in the figure.

Bi-stable circuit
Here we will summarize and summarize the working method of the 555 bi-stable circuit. 555 bistable circuit can be divided into 2 types.
The first type (see Figure 1) is a trigger circuit, which has two units: double-ended input (2.1.1) and single-ended input (2.1.2). The single-ended comparator (2.1.2) can be 6-terminal fixed and 2-stage input; it can also be 2-terminal fixed and 6-terminal input.

The second type (see Figure 2) is a Schmitt trigger circuit, which has the simplest form (2.2.1) and the input terminal resistor adjusts the bias or adds the control voltage VCT to the control terminal (5) to change the threshold voltage ( 2.2.2) A total of 2 unit circuits.

The input voltage terminal of the input terminal of the bistable circuit generally does not have a timing resistor and a timing capacitor. This is the structural feature of the bistable working mode. 2.2.2 C1 in the unit circuit only plays a coupling role, and R1 and R2 play a DC bias role.

The third category of unstable circuits
is the unstable working mode. The astable circuit is a multivibrator circuit, which is the most widely used type of 555 circuit. There are also the most variations in circuits. For simplicity, it is also divided into three types.
The first type (see Figure 1) is the direct feedback type, and the oscillation resistor is connected to the output terminal VO.

The second type (see Figure 2) is the indirect feedback type, and the oscillation resistor is connected to the power supply VCC. Among them, the first unit circuit (3.2.1) is the most widely used. The second unit circuit (3.2.2) is a square wave oscillation circuit. The third and fourth unit circuits are both pulse oscillation circuits with adjustable duty cycles. They have the same function but slightly different circuit structures, so they are coded 3.2.3a and 3.2.3b respectively.

The third type (see Figure 3) is the voltage controlled oscillator. Since the circuit variation form is very complex, for the sake of simplicity, it is only divided into two units: the simplest form (3.3.1) and the one with auxiliary devices (3.3.2). The figure shows two application examples.

The input terminal of an astable circuit generally has two oscillating resistors and an oscillating capacitor. Those with only one oscillating resistor can be considered a special case. For example: Unit 3.1.2 can be considered as the result of omitting RA. Sometimes we encounter an unstable circuit with 7.6.2 three terminals connected in parallel and only one resistor RA. At this time, it can be regarded as a deformation of the 3.2.1 unit circuit without RB.
The above summarizes the 3 categories, 8 types and 18 unit circuits of 555. Although they cannot cover all 555 application circuits, as the old saying goes: nothing can change without them. I believe it is still very helpful for us to understand most 555 circuits.