HOW A 555 WORKS
To design a 555 circuit you must know how a 555 IC
works. Here is an explanation:
The capacitor charges between 33% and 66% of tail
voltage. When the capacitor is 66%, pin 6 detects the voltage and makes pin 7 go
LOW. The capacitor discharges to 33% and pin 2 detects the voltage and makes pin
go HIGH (actually pin7 goes OPEN) and the capacitor charges to 66% to repeat the
1. Connect the 555:
We start with pins 2 and 6 NOT CONNECTED. This is
called the UNKNOWN STATE because the chip is not getting any information and the
output is UNKNOWN.
The inputs are called "FLOATING." Inputs must NEVER be
Connect Pins 2 and 6:
We start by connecting pins 2 and 6 as shown:
This is also called the UNKNOWN STATE because Pin
2 detects a LOW to make the output HIGH and pin 6 detects a HIGH to make the
The output will be HIGH or LOW depending on which pin is detected first.
The output is controlled by a BUILDING BLOCK called a FLIP FLOP or TOGGLE or
SWITCH or BI-STABLE LATCH. It will remain "locked" or in a stable state and it
will take a voltage of about 33% or more of rail voltage to change the state via
pins 2 or 6.
In most cases the output will "flip over" to the state with the output LOW. If
you delay pin 2 by 1uS, the output will go HIGH, indicating pin 2 is detected
first to make the output HIGH.
3. Toggle (or change) the output:
The output can be changed by using pin 2 or 6.
Pin 2 and Pin 6 change output
The output changes immediately, the animation is
4. The output:
If we make pin 2 LOW and Pin 6 HIGH, both
pins are trying to change the output of the chip and pin 2 WINS !!!
The output is HIGH !!
Pin 2 WINS !!
5. The Schmitt
Pin 2 detects 33% of rail voltage and pin
6 detects 66%. This gives a gap of 33% between the two. This gap is
called the HYSTERESIS GAP.
Here is a circuit to show how the gap works:
The Hysteresis Gap
The 555 is wired as a SCHMITT
TRIGGER. A Schmitt Trigger has a gap between HIGH and LOW input so the
signal has to rise and fall a large percentage of rail voltage to make the
output change. This means noise on the input will not alter the output as
noise has a small amplitude. This type of circuit "cleans up" noisy inputs.
This circuit is also called AN INVERTER, A BUFFER and AN
6. One-Shot or Switch Debouncer:
The 555 can take the noisy pulses from a
switch and produce a single CLEAN pulse.
The circuit is also called PULSE EXTENDER.
The 555 as a Delay (Timer):
The 555 can be used as a timer up to 10 minutes. This circuit is also called a
To start timing, the START button is pressed briefly and the output of the chip
goes LOW. At the expiration of 10 minutes, the output goes HIGH and the red LED
A simple application may be for a cooking operation in a shop.
If a product needs to be cooked or heated etc, the button can be pressed and the
LED illuminates when the time has expired.
When calculating the time-duration for the circuit above, the
capacitor charges from 0v to 2/3 rail voltage.