Analysis and Application of 555 Time-Base Circuit

Figure 1 555 integrated circuit internal structure diagram
555 integrated circuit is an 8-pin package, dual in-line type, as shown in Figure 2 (A), according to the arrangement of input and output, it can be seen as shown in Figure 2 (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 (TR), which is the input of the lower comparator; pin 3 is the output terminal (Vo), which has two states of 0 and 1, which is determined by the level added to the input terminal; pin 7 is the discharge terminal (DIS), which is the output of the internal discharge tube, and has two states of floating and grounding, which is also determined by the state of the input terminal; pin 4 is the reset terminal (MR), which can make the output low when a low level is added; pin 5 is the control voltage terminal (Vc), which can be used to change the upper and lower trigger level values; pin 8 is the power supply terminal, and pin 1 is the ground terminal.

Figure 2 555 integrated circuit package diagram
We can also make the 555 circuit equivalent to an RS trigger with a discharge switch, as shown in Figure 3 (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, and the trigger terminal (TR) can be regarded as the set terminal S, which requires a low level. There is an 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 Q=1, the DIS terminal is grounded, and when Q=0, the DIS terminal is suspended. In addition, there are a reset terminal MR, a control voltage terminal Vc, a power supply terminal VDD, and a ground terminal GND. This special 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 set terminal S, i.e., the trigger terminal (TR), requires a low level.
(2) The trigger levels of the two input terminals also have different threshold voltage values ​​for the output to flip. When the V c terminal is not connected to the control voltage, for the TH (R) terminal, >2/3VDD is a high level 1, and for the TR (S) terminal, >1/3VDD is a high level 1.

Figure 3 555 circuit equivalent R-S trigger
555 integrated circuits are bipolar and CMOS. The advantages of CMOS 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 advantages of bipolar are high output power, and the drive current is up to 200 mA, but other indicators are not as good as CMOS. There are
many application circuits for 555. As long as the external additional circuit of 555 integrated circuit is changed, hundreds of application circuits can be constructed, which can be roughly divided into three categories: 555 monostable, 555 bistable and 555 unstable (i.e. oscillator). 2 555 monostable circuit
The monostable circuit has a steady state and a quasi-stable state. It uses the charge and discharge of the capacitor to form the quasi-stable state. Therefore, its input ends are equipped with a timing resistor and a timing capacitor. There are two common 555 monostable circuits:
1) Manual start type
Connect the 6 and 2 pins of the 555 circuit in parallel and connect them to the RC timing circuit. Connect the button switch SB at both ends of the timing capacitor CT to become a manual start type 555 monostable circuit, as shown in Figure 4 (a). Replace the 555 with an equivalent trigger and omit the parts that are not related to the monostable operation as shown in Figure 4 (b). The following is an analysis of its working principle:
Steady state: After connecting the power supply, the capacitor CT is quickly charged to VDD. From Figure 4 (b), we can see that the trigger input R=1, S=1, and the output Vo=0 from the function table. This is its steady state.
Quasi-stable state: Press the switch SB, and the charge on CT is quickly reduced to zero, which is equivalent to the trigger input R=0, S=0. The output immediately flips to Vo=l, and the quasi-stable state begins. After the switch is released, the power supply charges CT again. After time TD, when the voltage on CT rises to >2/3VDD, the output flips to Vo=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.

Figure 4 Artificial start type 555 monostable circuit
2) Pulse start type
Connect the 6th and 7th pins of the 555 circuit in parallel and connect them to the timing capacitor CT. Use the 2nd pin as the input to become a pulse start type monostable circuit, as shown in Figure 5 (a). The 2nd pin of the circuit is usually connected to a high level. The circuit is started when the input is connected to a low level or a negative pulse is input. After replacing the 555 with an equivalent trigger, see Figure 5 (6). The following is an analysis of its working principle:
Steady state: After the power supply is connected, R=1, S=1, the output Vo=0, the DIS terminal is grounded, the voltage on CT is 0, that is, R=0, and the output still maintains Vo=0. This is its steady state.
Temporary steady state: After the negative pulse is input, the input S=0, the output immediately flips to Vo=1, the DIS terminal is open, the power supply charges CT through RT, and the temporary steady state begins. After time TD, when the voltage on CT rises to >2/3VDD, the input becomes R=1, S=1 again. At this time, the negative pulse has disappeared, and the output flips to Vo=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.

Figure 5 Pulse start monostable circuit
3 555 bistable circuit
There are two common 555 bistable circuits:
1) RS trigger type bistable
uses the 6 and 2 pins of the 555 circuit as two control input terminals, and the 7 terminal is not used, which becomes an RS trigger. Note that the trigger level and threshold voltage of the two input terminals are different, as shown in Figure 6 (a). Sometimes there may be only one control terminal. At this time, the other control terminal must be connected to the power supply terminal. According to the circuit requirements, the R terminal can be connected to the power supply terminal, as shown in Figure 6 (b), or S can be grounded and the R terminal can be used as the input.
Bistable circuits with two input terminals are often used for motor speed regulation, power supply upper and lower limit alarms, etc. Bistable circuits with one input terminal are used as single-ended comparators in various detection circuits.

Figure 6 555 constitutes RS trigger
2) Schmitt trigger type bistable
Connect the 6 and 2 pins of the 555 circuit in parallel to form a trigger with only one input terminal, as shown in Figure 7 (a). The relationship between the output voltage and the input voltage of this trigger is a rectangular loop, as shown in Figure 7 (b). From the curve, it can be seen that when the input V1=0, the output Vo=1, when the input voltage rises from 0 to >2/3VDD, Vo flips to 0, when the input voltage drops from the highest value to
4 555 unstable circuit (oscillator)
The multivibrator composed of 555 timer is shown in Figure 9 (a), and its working waveform is shown in Figure 9 (b).
After the power is turned on, the power supply VDD charges the capacitor C through R1 and R2. When Uc2C, it is determined by the discharge time of the capacitor C; TH=0.7(R1+R2)C, which is determined by the charging time of the capacitor C, and the pulse period T≈TH+TL.

Figure 9 555 multivibrator
The above only discusses several typical application examples of 555 timer. In fact, due to its high sensitivity and flexible functions, 555 timer is widely used in electronic circuits.