Circuits timers with the 555
The 555 are the best example of than in spite of the constant development of new falsified devices more they exist, nevertheless, some integrated circuits that continue being irreplaceable, in spite of the passage of time, for the accomplishment of some assemblies like monostable timers and.
The disposition of the legs is as it is in figure 1:
figure 1
The function of each leg is:
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Its diagram of functions is as it is detailed in figure 2:
figure 2
| Technical properties of the 555 | |||
| Power consumption: | 1 mW to 5 V of feeding (CMOS) | Frequency of operation: |
2 MHz (CMOS) 500 KHz (Bipolar) |
| Exhaust stream: | 100 mA drained or 10 mA in state under (CMOS) | Impedance of entrance: | 1012 ohm (típ.) (CMOS) |
| Band of operation tension: | 2 and 18 V (CMOS) | ||
We will continue our analysis for an integrated technology CMOS like the TLC555 of Texas Instruments that as already we saw owns better frequency response and also it has better characteristics than a bipolar standard, the disposition of the legs is the same for both technologies and also it is it his diagram of functions.
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Absolute maximums |
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| Tension of feeding (Vdd): | 18 V | Band of entrance tension (any entrance): | - 0.3 to 18 V |
| Dissipation (25º C): | 600 mW | Swept of temperatures of operation: | -55 to + 125 º C |
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Electrical characteristics for Vcc from 5 to 15 V |
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| Threshold voltage in percentage of the feeding tension: | 66.7% | Current of threshold for Vcc= 5V: | 10 pA |
| Tension of firing like percentage of the feeding tension: | 33.3% | Current of firing for Vcc= 5V: | 10 pA |
| Tension of reset: | 0,7 V | Current of reset for Vcc: | + 10 pA |
| Tension of control (open circuit) like percentage of the feeding tension: | 66.7% | Tension of exit in the low level (5 V, 5 mA): | 0,1 V |
| Tension of exit in the high level (5 V, 1 mA): | 4,5 V | Current of feeding: |
Vdd= 15 V: 360 uA, Vdd= 5 V: 170 uA |
| Maximum frequency in the astable way (Ra= 470 ohm, 200 CL= pF, Rb= 200 ohm): | 2.1 MHz | ||
Configurations in which we can use the TLC555:
Figure 3
The operation frequency is given by the following formula:
f = 1,44
(Ra + 2 Rb) C1
The maximum frequency of operation of this configuration is of 2.1 MHz, consequently we can place 200 a minimum capacity of pF. C1 = C2 and the capacitor C2 will be placed optionally to obtain the function of by-pass.
Monostable configuration:
Figure 4
The time is given by the following formula:
t = 1.1 Ra C
Minimum temporisation is given by a capacitor of 200 pF and one resistance nonsmaller than 200 ohm. In this case also the capacitor C2 will be optional fulfilling the same mission that in previous circuit.
From these two basic circuits that we have seen we can have diverse variations, with which we will be able to obtain alterations of the active loops, excitation of loads of greater power, modulation, etc.
Astable with 50% of active loop:
Figure 5
With this configuration a perfect square signal with active loop 50% is obtained when coming out, which did not happen with basic the astable configuration (Figure 3)
This cycle is possible because the routes - as much for the unloading as for the load of capacitor- they are the same. It is important that the used resistance is not minor who 10 K not to overload the exit.
The waveform of the entrance and the exit will be cone is seen in figure 6.
Figure 6
The excitation of one loads external of greater power, like for example a lamp or a relay, can take control of the disposition of the figure 7, which will be loaded the week that comes.
