555's 18 unit circuits

Here we classify and summarize the 555 circuit according to its structural characteristics, and divide the 555 circuit into 3 categories, 8 types, and a total of 18 unit circuits. In addition to drawing the standard diagram of each circuit, pointing out their structural characteristics or identification methods, we also give the calculation formula and their uses. It is convenient for everyone to identify and analyze the 555 circuit. The following will introduce these 3 categories of circuits respectively.

Monostable circuit

Monostable working mode, it can be divided into 3 types. See the figure.

The first type (Figure 1) is a manually started monostable, which is divided into two different units due to the different positions of the timing resistor and timing capacitor, and is coded as 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-start monostable, which can also be divided into two different units. Their input characteristics are both "RT-7.6-CT", and both are input from two terminals. The 1.2.1 circuit has no 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 only divide it into two different units. The circuit without any auxiliary devices is 1.3.1; the circuit using auxiliary devices such as transistors and op amps is 1.3.2. The figure lists two commonly used circuits.

Bistable circuit

Here we will summarize and generalize the working mode of 555 bistable circuit. 555 bistable circuit can be divided into 2 types.

The first type (see Figure 1) is a trigger circuit, which has two units: a dual-ended input (2.1.1) and a single-ended input (2.1.2). The single-ended comparator (2.1.2) can be a 6-terminal fixed, 2-stage input or a 2-terminal fixed, 6-terminal input.

The second type (see Figure 2) is the Schmitt trigger circuit, which has two unit circuits: the simplest form (2.2.1) and the input terminal resistor to adjust the bias or add a control voltage VCT at the control terminal (5) to change the threshold voltage (2.2.2).

The input voltage 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. C1 in the 2.2.2 unit circuit only plays a coupling role, and R1 and R2 play a DC bias role.

Astable circuit

The third type is the astable working mode. The astable circuit is the multivibrator circuit, which is the most widely used type in the 555 circuit. The circuit has the most variations. For simplicity, it is also divided into three types.

The first type (see Figure 1) is a direct feedback type, where the oscillating resistor is connected to the output terminal VO.

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

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

The input end of the unstable circuit generally has two oscillating resistors and one oscillating capacitor. The circuit with only one oscillating resistor can be considered as a special case. For example, unit 3.1.2 can be considered as the result of omitting RA. Sometimes, we may encounter an unstable circuit with three terminals in parallel in 7.6.2 and only one resistor RA. In this case, we can regard it as a variation of the circuit in unit 3.2.1 with RB omitted.

The above summarizes 3 categories, 8 types and 18 unit circuits of 555. Although they cannot cover all 555 application circuits, as the old saying goes: No matter how things change, they remain essentially the same. I believe it is still very helpful for us to understand most 555 circuits.