555 the characteristics of time-base circuit
555 timer IC for the application is started, so it is called 555 or 555 timer circuit. But then after the development, which in addition to routine delay control, but also for dimming, thermostats, pressure, speed, and other control and measuring and testing. You can also form pulse oscillation, monostable, bistable, and pulse modulation circuit for AC signal source, power conversion, frequency conversion, pulse modulation. Because it is reliable and easy to use, low cost, now widely used in various electronic products, IC 555, there are dozens of internal components, a voltage divider, comparator, the basic RS flip-flop, and the buffer discharge etc., more complex circuits, analog circuits and digital circuits is a mixture, as shown in Figure 1.
Figure 1 555 IC internal structure
IC is an 8-pin package, 555, dual in-line type, as shown in Figure 2 (A) as shown by the arrangement of input and output can be viewed as shown in Figure 2 (B) below. One 6 foot side, said threshold (TH), is on the comparator input; 2 pin, said trigger terminal ( TR ), is the next comparator input; 3 pin is the output (Vo), which has two kinds of O and 1 state, added by the input level of the decision; 7 feet is the discharge end (DIS), which is the output of the internal discharge tube, a floating and grounding two states, the state is determined by the input; 4 feet is the reset terminal ( MR ), coupled with low enable output is low; 5 feet is the control voltage terminal (Vc), use it to change the value of the upper and lower trigger level; 8 feet is the supply side, a pin is to end.
Figure 2 555 Figure IC Packaging
We can also be equivalent to a 555 with the discharge circuit switch RS flip-flop, as shown in Figure 3 (A) shows, this particular flip-flop has two inputs: the threshold terminal (TH) can be regarded as zero end of R, requires high level, trigger terminal ( TR ) can be set side as S, requires low, there is an output Vo, Vo can be equivalent flip-flop Q terminal, the discharge end (DIS) can be as the discharge switch is controlled by an internal contact, by the flip-flop Q terminal control: Q = 1时grounded DIS, Q = 0时DIS side vacant. There is also reset terminal MR , control voltage terminal Vc, power supply terminal VDD and
Ground terminal GND. This particular trigger has two characteristics:
(1) two input trigger level requires a high and one low, zero client-side R is the threshold (TH) requires high level, and set the trigger end s end ( TR ) requires low power almost;
(2) two input trigger level so that the output overturns the threshold voltage is different, when the client does not take control of V c when the voltage on the TH (R) side is concerned, 2/3VDD is high 1 , 2/3VDD is low 0: while the TR ( S ) side is concerned, 1/3VDD is high 1,1 / 3VDD is low 0. If the control terminal (Vc) on the control voltage Vc, the time on the trigger level to become a Vc value, the next trigger level to become 1/2Vc value, showing that changes in the value of the control side of the control voltage can change the upper and lower trigger fair value. It is the menu shown in Figure 3 (B) below.
Figure 3 555 R-S flip-flop circuit is equivalent to
555 bipolar and CMOS integrated circuit has two kinds. Advantages of CMOS-based low power consumption, low voltage, high input impedance, but low output power, output drive current of only a few mA. The advantages of bipolar output power, drive current of 200 mA, other indicators are not as good as CMOS type.
555 of the application circuit a lot, as long as the changes in the external add-on circuit 555 integrated circuits, hundreds of applications can be formed circuit can be divided into 555 single-stable, 555 and 555 non-bistable stable (ie, oscillator) categories.
2 555 monostable circuit
Monostable circuit has a steady-state and a provisional state, is the use of capacitor charging and discharging of the formation of temporarily stable, so it comes with the input of the timing resistor and timing capacitor, a common 555 monostable circuit, there are two:
1) The start type of artificial
6.2 feet to 555 circuits and pick up in the RC timing circuit connected, in the timing capacitor CT, both ends of the connection button switch SB, to become a type of artificial start 555 monostable circuit, as shown in Figure 4 (a) shows, with equivalent alternative trigger 555 and spent nothing to do with the single-stable part of the work after the Figure 4 (b) below, the following analysis of how it works:
State: connected to power, quickly charge capacitor CT to VDD, from Figure 4 (b) see that the trigger input R = 1, S = 1, from the menu to see the output Vo = 0, which is its steady state.
Temporary state: press the switch SB, CT rapidly into the charge of zero, equivalent to the trigger input R = 0, S = 0, the output immediately turn into Vo = l, temporary steady start. Switch open, the power Youxiang CT charge, after a time after the TD, CT on the voltage up to 2/3VDD, the output and turn into a Vo = O, temporary steady-state end. TD is the timing monostable circuit time or delay time, it and the timing resistor RT and timing capacitor CT values on: T D = 1.1R T C T .
Figure 4 manual start type 555 monostable circuit
2) Pulse start type
6,7 555 circuit will pick up the foot and connected to the timing capacitor CT, and with the 2 pin for input to a single-shot pulse start, shown in Figure 5 (a) shows, the circuit is usually connected to 2 feet high When the input received low or negative pulse when the input starting circuit, replaced with an equivalent flip-flop after 555 Figure 56) shows, the following analysis of how it works:
State: connected to the power, R = 1, S = 1, the output Vo = 0, DIS grounded, CT 0 is the voltage on the R = 0, the output remains Vo = 0, which is its steady state.
Temporary steady-state: negative pulse input, the input S = 0, the output immediately turn into Vo = 1, DIS-side open, power to the CT through RT charge, temporarily steady start. After the elapsed time TD, CT up to 2/3VDD the voltage, the input has become the R = 1, S = 1, then a negative pulse has disappeared, the output and flip into the Vo = 0, end of temporary state. Internal discharge switch is turned on then, DIS grounded, CT rapidly into the charge of zero, as preparation for the next time control. The timing circuit time T D = 1.1R T C T .
Both one-shot delay timer is used as control.
Figure 5 single-shot pulse start
3555 bistable circuit
Common bistable circuit 555, there are two:
1) RS-type bistable flip-flop
6,2 555 feet to the circuit as two control inputs, 7 not in use to be a RS flip-flop. Notice the two trigger level input threshold voltage difference, as shown in Figure 6 (a) shows, sometimes there is only one console, then the other side to try to take control of death, according to circuit requirements can end to the power R side, as shown in Figure 6 (b) below, can also be ground into S, with R-side for input.
There are two input bistable circuit is used as motor speed, power and other uses on the lower limit alarm. A bistable input comparator circuit as a single-ended for a variety of detection circuit.
Chart 6555 form RS flip-flop
2) Schmitt trigger-type bistable
6.2 feet to 555 circuits and pick up only one input connected as a trigger, as shown in Figure 7 (a) shows, this triggers the output voltage and input voltage between the back line is a rectangular, as shown in Figure 7 ( b), we can see from the curve, when the input V1 = 0, the output Vo = 1, when the input voltage from 0 up to 2/3VDD later, Vo flip to 0, the highest value when the input voltage down to 1/3VDD from post , Vo and flip to 1. Because it has two different input threshold voltage, so that this circuit is commonly used in electronic switches, various control circuits, the transformation and shaping the waveform shown in Figure 8.
Figure 7 555 constitute a Schmitt trigger
Figure 8, the transformation and shaping the waveform
4555 no stable circuit (oscillator)
Posed by the 555 timer multivibrator shown in Figure 9 (a) shows the waveform of their work shown in Figure 9 (b).
After power on, power supply VDD through capacitor C R1 and R2 of charge, when Uc1/3VDD, the oscillator output Vo = 1, discharge cut-off. When charged to ≥ 2/3VDD Uc, the oscillator output Vo flip to 0, then discharge tube turns on, the discharge end (DIS) grounded, capacitor C discharges through R2 to ground, so that Uc down. When reduced to ≤ 1/3VDD Uc, the oscillator output Vo and turn into one, this time discharge and cut-off, so that the discharge end (DIS) is not grounded, power supply VDD through R1 and R2 and the capacitor C charged, and also allows Uc from 1/3VDD up to 2/3VDD, flip-flop flip took place, again and again, resulting in changes in the output Vo is continually oscillating pulse waveform. Pulse width TL ≈ 0.7R 2 C, the decision by the capacitor C discharge time; TH = 0.7 (R1 + R2) C, the decision by the capacitor C charging time, pulse period T ≈ TH + TL.
Chart 9555 form multivibrator
The above discussion only 555 timers by several typical examples. In fact, due to the high sensitivity of 555 timer, functional flexibility, thus widely applied in electronic circuits.