Several major features of 555 integrated time base circuit

Some major features of the 555 integrated timing circuit, the component composition of the 555 integrated circuit, and the two common 555 monostable circuits: manual start monostable and pulse start monostable, are provided for reference by beginners.

Several major features of 555 integrated time base circuit

When the 555 integrated circuit first appeared, it was used as a timer, so it was called a 555 timer or a 555 time base circuit. However, after development, in addition to timing delay control, it can also be used for various controls such as dimming, temperature regulation, voltage regulation, speed regulation, and measurement and detection; it can also form pulse oscillation, monostable, bistable and pulse modulation circuits, as an AC signal source, and complete power conversion, frequency conversion, pulse modulation, etc.

Because of its reliable operation, easy use and low price, it is currently widely used in various small household appliances.

There are dozens of components inside the 555 integrated circuit, including voltage dividers, comparators, triggers, output tubes and discharge tubes. The circuit is relatively complex and is a mixture of analog circuits and digital circuits. Its performance and parameters can only be found in the manual of nonlinear analog integrated circuits. The 555 integrated circuit is an 8-pin package. Figure 1 (a) is a dual in-line package. According to the arrangement of input and output, it can be drawn as Figure 1 (b). Among them, pin 6 is called the threshold terminal (TH), which is the input of the upper comparator. Pin 2 is called the trigger terminal (), which is the input of the lower comparator.

Pin 3 is the output terminal (VO), which has two states: 0 and 1. Its state is determined by the level applied to the input terminal. Pin 7 is the discharge terminal (DIS), which is the output of the internal discharge tube. It also has two states: floating and grounded, which are also determined by the state of the input terminal. Pin 4 is the reset terminal (), which can make the output low level when a low voltage (< 0.3 volts) is applied. Pin 5 is called the control voltage terminal (VC), which can be used to change the upper and lower trigger level values. Pin 8 is the power supply, and pin 1 is the ground terminal.

For beginners, the 555 circuit can be equivalent to an R-S trigger with a discharge switch, as shown in Figure 2 (a). This special trigger has two input terminals; the threshold terminal (TH) can be regarded as the zero terminal R, which requires a high level; the trigger terminal () can be regarded as the set terminal, which is valid at a low level. It has only one output terminal VO, which can be equivalent to the Q terminal of the trigger. The discharge terminal (DIS) can be regarded as a contact controlled by the internal discharge switch, which is controlled by the Q terminal of the trigger: when =1, the DIS terminal is grounded; when =0, the DIS terminal is suspended. In addition, this trigger has a reset terminal, a control voltage terminal VC, a power supply terminal VDD and a ground terminal GND.

This special R-S trigger has two characteristics: (1) The trigger levels of the two input terminals require one high and one low: the zero terminal R, i.e. the threshold terminal TH, requires a high level, while the low terminal S, i.e. the trigger terminal, requires a low level. (2) The trigger levels of the two input terminals, i.e. the threshold voltage values ​​that cause them to flip, are also different. When the VC terminal is not connected to the control voltage, for the TH (R) terminal, > 2/3 V DD is a high level 1, and < 2/3 V DD is a low level 0; while for the () terminal, > 1/3 V DD is a high level 1, and < 1/3 V DD is a low level 0. If the control voltage VC is added to the control terminal (VC), the upper trigger level becomes the VC value, and the lower trigger level becomes 1/2 VC. It can be seen that changing the control voltage value of the control terminal can change the upper and lower trigger level values.

After simplification, the 555 circuit can be equivalent to a trigger, and its function table is shown in Figure 2 (b).

 There are two types of 555 integrated circuits: bipolar and CMOS. The advantages of the CMOS type are low power consumption, low power supply voltage, and high input impedance, but the output power is small, and the output drive current is only a few mA. The advantage of the bipolar type is high output power, and the drive current is up to 200 mA, but other indicators are not as good as the CMOS type.

In addition, there is a 556 dual-time base circuit with a 14-pin package, which contains two identical time base circuit units. There are many application circuits of 555, which can be roughly divided into three categories: 555 monostable, 555 bistable and 555 unstable. 555 monostable circuit The monostable circuit has a stable state and a temporary stable state. The 555 monostable circuit uses the charging and discharging of the capacitor to form a temporary stable state, so its input end has a timing resistor and a timing capacitor. There are two common 555 monostable circuits.

1. Manual start monostable

Connect the 6 and 2 terminals of the 555 circuit in parallel to the RC timing circuit, and connect the push button switch SB to the two terminals of the timing capacitor CT to form a manually started 555 monostable circuit, as shown in Figure 3 (a). Replace the 555 with an equivalent trigger, and omit the parts that are not related to the monostable operation to draw the equivalent Figure 3 (b).

Working status:

 ① Steady state: After the power is connected, the capacitor CT is quickly charged to V DD . As can be seen from Figure 3 (b), the trigger input R=1, =1, and the output Vo=0 is found from the function table. This is its steady state.

 ② Temporary steady state: When the switch SB is pressed, the charge on CT is quickly reduced to zero, which is equivalent to the trigger input R=0, =0, and the output immediately flips to V o =1, and the temporary steady state begins. After the switch is released, the power supply charges CT again. After time td, when the voltage on CT rises to > 2/3 V DD, the output flips to V =0 again, and the temporary steady state ends. td is the timing time or delay time of the monostable circuit, which is related to the value of the timing resistor RT and the timing capacitor CT; td=1.1RTCT.

2. Pulse start monostable

Connect the 6th and 7th terminals of the 555 circuit in parallel to the timing capacitor CT, and use the 2nd terminal as the input to form a pulse-start monostable circuit, as shown in Figure 4 (a). The 2nd terminal of the circuit is usually connected to a high level, and the circuit is started when the input is connected to a low level or a negative pulse is input. After replacing the 555 circuit with an equivalent trigger, it can be drawn as Figure 4 (b). This circuit uses the discharge terminal to enable the timing capacitor to discharge quickly.

Working status:

 ① Steady state: After power is turned on, R=1, =1, output Vo=0, DIS terminal is grounded, voltage on CT is 0, that is, R=0, output still maintains Vo=0, this is its steady state.

 ② Temporary steady state: After the negative pulse is input, the input = 0, the output flips to V o = 1, the DIS terminal is open, the power supply charges CT through RT, and the temporary steady state begins. After td, the voltage on CT rises to > 2/3 V DD, at this time the negative pulse has disappeared, the input becomes R = 1, = 1, the output flips to V o = 0 again, and the temporary steady state ends. At this time, the internal discharge switch is turned on, the DIS terminal is grounded, and the charge on CT is quickly reduced to zero, preparing for the next timing control. The timing time of the circuit is td = 1.1RTCT.

These two monostable circuits are often used for timing delay control.