555 timer circuit
555 timer oscillator
The 555 timer IC was first introduced around 1971 by the Signetics Corporation as the SE555/NE555 and was also the very first and only commercial timer ic available. It is after 30 years still very popular and used in many schematics. Although these days the CMOS version of this IC, like the Motorola MC1455, is mostly used, the regular type is still available, however there have been many improvements and variations in the circuitry. But all types are pin-for-pin plug compatible.
Specifications
Supply voltage 4.5-18V
Supply current 3-6 mA
Output current 225mA (max)
Rise/Fall time 100 ns
Operation
external capacitor determines the off-on time intervals of the output pulses
time taken by capacitor to charge to 63.7% of the applied voltage = time constant, t = RC
Modes
one shot (monostable)
oscillator (astable)
One-shot
generates a single pulse of a fixed time duration each time it receives and input trigger pulse
used for turning some circuit or external component on or off for a specific length of time
also used to generate time delays
duration of pulse dependent on RC
Oscillator
generates a continuous stream of rectangular off-on pulses that switch between two voltage levels
frequency and duty cycle (ratio of time-on to time-off) are dependent upon RC
frequency can be calculated with the formula:
f = 1/(.693 x C x (R1 + 2 x R2))
pulse is on for t1 seconds, then off for t2 seconds, total period (t) is t1 + t2.
duty-cycles can be approximately 55 to 95%:
D = t1/t = (R1 + R2) / (R1 + 2R2)
t1 = .693(R1+R2)C
t2 = .693 x R2 x C
Pin functions
1: Ground (if split-rail power supply is being used) or -Vcc for single rail
2: Trigger
a voltage level of < .3Vcc (or, .5 Vpin 5) for at least 1us is sufficient to trigger
trigger input is momentarily taken from a higher to a lower level (output is inverse of trigger)
trigger pulse must be of shorter duration than the time interval determined by RC
if this pin is held low longer than that, the output will remain high until the trigger input is driven high again
must not remain lower than 1/3Vcc for a period of time longer than the timing cycle
if this is allowed to happen, timer will retrigger itself upon termination of the first output pulse
minimum monostable output pulse width should be in the order of 10uS to prevent possible double triggering
a dc current of typically 500nA, the trigger current, must also flow from this terminal into the external circuit
so R must be < 3 MW
3: Output
provides a high-state output voltage about 1.7 volts less than the V+ supply level used
rise and fall times of the output waveform are quite fast - switching times being about 100nS
output pin is inverse of the input trigger
output can also be made to go low by taking the reset to a low state near ground [see "Pin 4 - Reset"]
4: Output Reset
voltage threshold level is 0.7 volt with sink current of 0.1mA for 0.5 µS required
reset is an overriding function
it will force the output to a low state regardless of the state of either of the other inputs
may thus be used to terminate an output pulse prematurely
to gate oscillations from "on" to "off", etc.
delay time from reset to output is typically on the order of 0.5 µS
when not used, it is recommended that the reset input be tied to V+ to avoid any possibility of false resetting.
5: Reference
allows direct access to the 2/3 V+ voltage-divider point
use is optional
by applying a voltage to this pin, it is possible to vary the timing of the device independently of the RC network
control voltage may be varied from 45 to 90% of Vcc in monostable mode
possible to control the width of the ouput pulse independently of RC
in astable mode, the control voltage can be varied from 1.7V to the full Vcc
produces a frequency modulated (FM) output
if not used, ground via a capacitor of about 0.01uF (10nF) to eliminate false triggering.
6: Reset latch
causes the output to go low
a dc current, the threshold current of 0.1µA must also flow into this terminal from the external circuit
R < 16 Mega-ohm
7: Timing capacitor
"on" (low resistance to ground) when the output is low and "off" (high resistance to ground) when the output is high
8: +Vcc
positive supply voltage +4.5 volts (minimum) to +16 volts (maximum)
chip generates a big (about 150mA) supply current during each output transition
therefore recommended to use a large decoupling capacitor as near to this pin as possible
even so, the 555 may have a tendency to generate double output transitions
More Examples
schmitt trigger
10 minute timer
metronome
power alarm
555 timer circuit
555 timer circuit
Introduction
555 timers and their circuits on the
The 8-pin 555 timer must be one of the most useful ICs ever made and it is used in many projects. With just a few external components it can be used to build many circuits, not all of them involve timing!
A popular version is the NE555 and this is suitable in most cases where a '555 timer' is specified. The 556 is a dual version of the 555 housed in a 14-pin package, the two timers (A and B) share the same power supply pins. The circuit diagrams on this page show a 555, but they could all be adapted to use one half of a 556.
Low power versions of the 555 are made, such as the ICM7555, but these should only be used when specified (to increase battery life) because their maximum output current of about 20mA (with a 9V supply) is too low for many standard 555 circuits. The ICM7555 has the same pin arrangement as a standard 555.
The circuit symbol for a 555 (and 556) is a box with the pins arranged to suit the circuit diagram: for example 555 pin 8 at the top for the +Vs supply, 555 pin 3 output on the right. Usually just the pin numbers are used and they are not labelled with their function.
The 555 and 556 can be used with a supply voltage (Vs) in the range 4.5 to 15V (18V absolute maximum).
Standard 555 and 556 ICs create a significant 'glitch' on the supply when their output changes state. This is rarely a problem in simple circuits with no other ICs, but in more complex circuits a smoothing capacitor (eg 100µF) should be connected across the +Vs and 0V supply near the 555 or 556.
The input and output pin functions are described briefly below and there are fuller explanations covering the various circuits.
source :http://www.kpsec.freeuk.com/555timer.htm
tft lcd
tft lcd
A thin film transistor liquid crystal display (TFT-LCD) is a variant of liquid crystal display (LCD) which uses thin-film transistor (TFT) technology to improve image quality (e.g., addressability, contrast). TFT LCD is one type of active matrix LCD, though all LCD-screens are based on TFT active matrix addressing. TFT LCDs are used in television sets, computer monitors, mobile phones, handheld video game systems, personal digital assistants, navigation systems, projectors, etc.
Construction
A diagram of the pixel layout
Small liquid crystal displays as used in calculators and other devices have direct driven image elements—a voltage can be applied across one segment without interfering with other segments of the display. This is impractical for a large display with a large number of picture elements (pixels), since it would require millions of connections—top and bottom connections for each one of the three colors (red, green and blue) of every pixel. To avoid this issue, the pixels are addressed in rows and columns which reduce the connection count from millions to thousands. If all the pixels in one row are driven with a positive voltage and all the pixels in one column are driven with a negative voltage, then the pixel at the intersection has the largest applied voltage and is switched. The problem with this solution is that all the pixels in the same column see a fraction of the applied voltage as do all the pixels in the same row, so although they are not switched completely, they do tend to darken. The solution to the problem is to supply each pixel with its own transistor switch which allows each pixel to be individually controlled. The low leakage current of the transistor prevents the charge applied to the pixel from leaking away between refreshes to the display image. Each pixel is a small capacitor with a layer of insulating liquid crystal sandwiched between transparent conductive ITO layers.
The circuit layout of a TFT-LCD is very similar to that of a DRAM memory.[citation needed] However, rather than fabricating the transistors from silicon formed into a crystalline wafer, they are made from a thin film of silicon deposited on a glass panel. Transistors take up only a small fraction of the area of each pixel; the rest of the silicon film is etched away to allow light to pass through.
The silicon layer for TFT-LCDs is typically deposited using the PECVD process from a silane gas precursor to produce an amorphous silicon film.[citation needed] Polycrystalline silicon (frequently LTPS, low-temperature poly-Si) is sometimes used in displays requiring higher TFT performance. Examples include high-resolution displays, high-frequency displays or displays where performing some data processing on the display itself is desirable. Amorphous silicon-based TFTs have the lowest performance, polycrystalline silicon TFTs have higher performance (notably mobility), and single-crystal silicon transistors are the best performers.
