The 555 Timer Chip



   The 555 timer chip is a highly stable device for generating 
accurate time delays or oscillation. This chip has terminals 
for triggering and resetting, which can be a convenient feature.
In the delay mode (monostable mode), the delay time is precisely 
controlled by one external resistor and capacitor. In the 
ocsillator mode (astable mode), the frequency and duty cycle 
are controlled with two external resistors and one capacitor. 
The chip can be triggered and reset using a falling waveform 
(low). The output circuit for most brands of 555 timer chips 
can source or sink up to 200ma or drive TTL type circuits.

      FEATURES:
      Timing from microseconds to hours.
      Operates in both astable and monostable modes.
      Adjustable duty cycle.
      Output and supply are TTL compatible.
      Very temperature stable.
      Can have both normally off and normally on output.

      APPLICATIONS:
      Precision timing
      Pulse generation
      Sequential timing
      Time delay generation
      Pulse width modulation
      Pulse position modulation
      Linear ramp generation

      The following are the typical parameters for a LM555 which
is a 555 timer chip that is made by National Semiconductor 
(one of my favorite companies for IC's that are fun and easy to
work with). Several companies make a 555 type chip and for the 
most part they are interchangable. There are however some small 
differences from one brand to another, so if you are unsure 
about the parameters for the brand you are using, get the data 
from the manufacture.

Supply Current= 3ma with no load on the output which is pin 3 
and operating at 5V, 10ma with no load on the ouput and 
operating at 15V.

The trigger voltage is about 5V when the chip is operating 
with a supply voltage of 15V, and about 1.67V when operated 
at 5V. The current drawn by the trigger (pin 2) is typically 
.01uV

The reset voltage is typically .5V, and the current that the 
reset draws (pin 4) is about .1ma.

The control voltage (pin 5) usually needs about 2/3 of the 
supply voltage.


     
   -----MONOSTABLE OPERATION (ONE SHOT TIMER)

        In this mode of operation, the timer works as a one-
shot timer. See FIG1.BMP (or print it). The external 
capacitor is initially held discharged by a transistor inside
the IC. When a negative pulse (less than 1/3 of the Supply 
Voltage Vcc) is applied to pin 2, the internal flip-flop is 
set which both releases the short circuit across the external 
timing capacitor and drives the output high. The voltage 
across the external timing capacitor then increases 
exponentially for a period of time equal to t = 1.1 x Ra x C, 
at the end of which time the voltage equals 2/3 of the 
supply voltage Vcc. Teh comparator then resets the flip-flop
which in turn discharges the external timing capacitor and 
drives the output to its low state. Since the charge and the 
threshold level of the comparator are both directly 
proportional to the supply voltage, the timing interval is
independent of the supply voltage VCC.
        During the timing cycle when the ouput is high, the 
further application of a trigger pulse will not effect the 
the circuit so long as the trigger input is returned high at 
least 10 micro-seconds before the end of the timing interval. 
However, the circuit can be reset during this time by the 
application of a negative (a voltage less than 1/3 supply 
voltage Vcc) to the reset pin (pin 4). The output will then 
remain in the low state until a trigger pulse is again 
applied. When the reset function is not being used, it is 
recommended that it be connected to Vcc to avoid the 
possibility of false triggering.



   
   -----ASTABLE MODE   (Oscillating)

        If the circuit is connected as shown in FIG2.bmp (pins
2 and 6 connected) it will trigger itself and run as a free 
running multivibrator. The external capacitor charges through 
Ra and Rb and discharges through Rb. Because of this the duty 
cycle can be precisely set by the ratio of these two resistors. 
In this mode of operation, the capacitor charges and discharges 
between 1/3 Vcc and 2/3 Vcc. As in the triggered mode, the 
frequency is independent of supply voltage Vcc.
     
The charge time (output high) can be found with the formula:
                      T1 = 0.693 x (Ra+Rb) x C
     
The discharge time (output low) can be found with the formula:
                      T2 = 0.693 x Rb x C

And if you need to be able to find the total period the formula is:
                      T = T1 + T2 = 0.693 x (Ra + (2 x Rb)) x C

The frequency of oscillation is found with the formula:
                      f = 1/T = 1.44 /(Ra + (2 x Rb) x C)





  THE "556" TIMER IC IS ACTUALLY JUST TWO 555 TIMERS IN THE SAME 
  IC AND CAN BE TREATED AS SUCH, THE PINOUT OF THE IC IS DIFFERENT 
  OF COURSE.