05-05-2012, 04:34 PM
TIMER
TIMER.doc (Size: 346 KB / Downloads: 38)
. Introduction
One of the most versatile linear ICs is the 555 timer which was first introduced in early 1970 by Signetic Corporation giving the name as SE/NE 555 timer. This IC is a monolithic timing circuit that can produce accurate and highly stable time delays or oscillation. Like other commonly used op-amps, this IC is also very much reliable, easy to use and cheaper in cost. It has a variety of applications including monostable and astable multivibrators, dc-dc converters, digital logic probes, waveform generators, analog frequency meters and tachometers, temperature measurement and control devices, voltage regulators etc. The timer basically operates in one of the two modes either as a monostable (one-shot) multivibrator or as an astable (free-running) multivibrator.The SE 555 is designed for the operating temperature range from – 55°C to 125° while the NE 555 operates over a temperature range of 0° to 70°C.
The important features of the 555 timer are :
It operates from a wide range of power supplies ranging from + 5 Volts to + 18 Volts supply voltage.
Sinking or sourcing 200 mA of load current.
The external components should be selected properly so that the timing intervals can be made into several minutes Proper selection of only a few external components allows timing intervals of several minutes along with the frequencies exceeding several hundred kilo hertz.
It has a high current output; the output can drive TTL.
It has a temperature stability of 50 parts per million (ppm) per degree Celsius change in temperature, or equivalently 0.005 %/ °C.
The duty cycle of the timer is adjustable with the maximum power dissipation per package is 600 mW and its trigger and reset inputs are logic compatible.
Basic Timing Concept
Figure illustrates some basic ideas that will prove useful in coming blog posts of the 555 timer. Assuming output Q high, the transistor is saturated and the capacitor voltage is clamped at ground i.e. the capacitor C is shorted and cannot charge.
The non-inverting input voltage of the comparator is referred to as the threshold voltage while the inverting input voltage is referred to as the control voltage. With R-S flip flop set, the saturated transistor holds the threshold voltage at zero. The control voltage, however, is fixed at 2/3 VCC (i.e. at 10 V) because of the voltage divider.