555 TIMER IC

THE 555 TIMER IC

INTRODUCTION:

The 555 timer IC is an amazingly simple yet versatile device. It has been around now for couple of years now and has been reworked into a number of different technologies. The two many versions today are the TTL and the more recent CMOS equivalent. The differences are just the amount of power they require and their maximum frequency of operation but they are still pin compatible and functionally interchangeable.

This post contains only a description of the Almighty 555 Timer IC itself. Functional circuits and a few of the  very wide range of its possible applications will be posted in our subseqeunt post.

The figure  below shows the pin-outs and configurations of the 555 timer IC. The IC is available in either an 8-pin round TO3-style or an 8-pin mini-DIP package. In either case the pin connections are as follows

 

• Pin1 – Ground
•   Pin2 – Trigger
•   Pin3 – Output
•   Pin4 – Reset input
•  Pin5 – Control voltage
• Pin6 – Threshold
• Pin7 – Discharge
• Pin8 – Vcc. +5 to +15V

PRINCIPLE OF OPERATION

The operation of the 555 timer revolves around the three resistors that form a voltage divider across the power supply, and the two capacitors connected to this voltage divider. The IC is quiescent so long as the trigger input (pin 2) remains at +VCC and therefore inactive, and the threshold input (pin 6) is at ground. Assume the reset input (pin 4) is also at +VCC and therefore inactive and that the control voltage input (pin 5) is unconnected. Under these conditions, the  output (pin 3) is at ground and  the discharge transistor (pin 7) is turned on, thus grounding whatever is connected to this pin.

The three resistors in the voltage divider all have the same value (5k in the TTL version of this IC), so the comparator reference voltages are 1/3 and 2/3 of the supply voltage, whatever that may be. The control voltage input at pin 5 can directly affect this relationship, although most of the time this pin is unused.

The internal flip-flop changes state when the trigger input pin 2 is pulled down below +VCC/3. When this occurs, the output (pin 3) changes state to +VCC and the discharge transistor (pin 7) is turned off. The trigger input can now return to +VCC; it will not affect te state of the IC.

However, if the threshold input (pin 6) is now raised above (2/3) of +VCC , the output will return to ground and the discharge transistor will be turned on again. When the threshold input returns to ground, the IC will remain in this state, which was the original state when we started this analysis.

The easiest way to allow the threshold voltage (pin 6) to gradually rise to (2/3) of +VCC is to connect it to a capacitor being allowed to charge through a resistor. In this way we can adjust the R and C (external resistor and capacitor) values for almost any time interval we might want.