555 timer menu
Connect the high trigger terminal TH and the low trigger terminal TR together, and the above-mentioned 555 menu becomes the following menu.
Because of its flexibility and convenience, 555 timer has been widely used in waveform conversion and generation, measurement and control, household appliances, electronic toys and other fields.
(1) constitutes a Schmitt trigger, which is used for the interface of TTL system, shaping circuit or pulse amplitude discrimination.
(2) forming a multivibrator and a signal generating circuit;
(3) A monostable trigger is constructed for timing delay shaping and some timing switches.
Types and performance of 555 timer There are two kinds of 555 timer products: TTL type and CMOS type. The last three digits of TTL products are all 555, and the last four digits of CMOS products are all 7555. Their logical functions are exactly the same as the arrangement of external leads.
Performance comparison between bipolar timer and CMOS 555 timer: They have the same pin arrangement, mutual compatibility, the same function and can be interchanged, but attention should be paid to the differences in use.
Circuit structure and working principle of Schmitt trigger of 555 timer;
When the fifth pin is connected to DC voltage VI, then VT+=VI, VT-= 1/2VI. Therefore, changing the voltage of the voltage control terminal CO (pin 5) can change the return voltage. Generally, the higher the CO at the voltage control terminal, the greater the δ U, and the stronger the anti-interference ability, but the sensitivity decreases accordingly.
When not using 5 feet, it can be suspended; You can also connect a capacitor of 0.0 1uF to bypass high-frequency interference.
Reasons for the poor income:
Because the reference voltages of C 1 and C2 are different, the reset signal of basic RS-FF and the reset signal of 1 must appear at different levels of the input signal vi. Thereby forming a voltage transmission return difference.
The monostable trigger composed of 555 timer has only one steady state. Before the trigger signal is added, the trigger is in a stable state. After being triggered, the trigger changes from a stable state to a transient state. After the transient state is maintained for a period of time, it will automatically turn back to the original stable state. Monostable flip-flops are usually used in delay and pulse shaping circuits.
There are many forms of monostable flip-flop circuits. Figure (a) shows a monostable trigger composed of a 555 timer, R and C are external components, and the trigger pulse u 1 is input from two terminals. When terminal 5 is not in use, it is generally grounded with 0.0 1uF capacitor to prevent interference. The following analysis is made with reference to Figure (b).
(1) steady state
Turn on t to turn on, and the capacitor c discharges, and then UC; , u0 remains in the 0 state. The circuit will always be in this stable state.
(2) Transient state
At t=t 1, a negative pulse appears, that is, u 1
When t=t2, the trigger negative pulse disappears (u1>; ), if UC
When t=t3, when uc rises slightly, /RD=0, /SD= 1, the basic RS flip-flop is reset, and the output u0=0 returns to the initial steady state. At the same time, the transistor T is turned on, and the capacitor C is rapidly discharged through T until uc is 0. At this point, /RD= 1, /SD= 1, and the circuit is ready for the next flip.
The output pulse width tp is the duration of the transient, that is, the time required for the voltage of the capacitor C to charge from 0. allow
According to the above formula:
① Changing the values of R and C can change the output pulse width, which can be used for timing control.
② When the values of R and C are fixed, the amplitude and width of the output pulse are also fixed, which can be used to shape the waveform with non-steep edge and uneven amplitude. Uncle answers for you, I hope you are satisfied! !