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 !!
4a. The RESET
Pin 4 is the reset pin. When it is taken to the
positive power rail, the chip "works." When it is taken to 0v rail, the chip
"freezes" and pin 3 instantly goes LOW, (even if it is halfway through a timing
cycle). But the current taken by the chip remains at 10mA and you cannot use
this pin to "turn off the current taken by the chip."
If pin is unconnected, it will allow the chip to operate as the internal
circuitry puts a "High" (950mV) on the pin.
But the Reset Pin is more complex.
When nothing is connected to pin 4, it has a voltage of about 950mV on it and it
will deliver about 350uA. This is due to the circuitry inside the chip.
It must be taken below 500mV to turn the chip off.
A 2k7 resistor connected to pin 4 will turn the chip off.
A 3k3 resistor connected to pin 4 will NOT turn the chip off.
This means pin 4 is generating a voltage (and current) and we can use this
feature to turn the chip OFF via a Light Dependent Resistor (LDR).
If we connect an LDR to pin 4, it will require a very bright light to reduce the
resistance of the LDR to less than 3k3.
By adding a 10k to the circuit, we help the LDR produce a combined resistance
less than 3k3 and the circuit will work in a normal lounge-room or bedroom. When
the LDR sees illumination from a room-light, it will turn the chip off.
You can try values as low at 4k7 to make the chip MORE sensitive.
In the following circuit, both pin 2 and pin 6 activate the chip to make the
output HIGH and LOW "at the same time" but Pin 2 has control over pin 6 and pin
2 makes pin 3 (the output pin) HIGH.
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
5a. The Schmitt
Here is a circuit with the 555 wired as a Schmitt Trigger. It is an oscillator
with a square-wave output and operates by detecting 33% and 66% of rail voltage
on the capacitor to change the state of the output.
The first diagram shows the rectangular 555 and the second diagram shows it as a
triangular shape which is the same shape for a 74c14 (40106) Hex Schmitt
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.
In this circuit we can see pin 2 is taken LOW when the power is
applied and quickly goes HIGH.
Pin 2 has control over pin 6 as explained in THE 555 PINS
The action of Pin 2 going LOW "activates the 555" and makes pin 3 HIGH. This
only applies because pin 6 is LOW.
Because pin 2 goes HIGH after 0.1secs, it has done its job of turning ON the 555
and now we wait for pin 6 to go HIGH to turn the chip OFF. If pin 2 remained
LOW, the chip would NEVER turn OFF.
8. LONG Duration Timer:
The 555 can be used as a timer up to 10 minutes, but the accuracy of
this duration is not reliable and will vary enormously according to the
temperature of the day and the leakage of the electrolytic. If you make the time
too long, the output may never go HIGH.
Reliability stops at about 1 minute, so to get a long duration we connect the
555 to a chip called a "divider-chip" or "counter-chip."
They are called divider chips because they are designed to take a high
frequency and reduce it to a lower frequency by using a number of flip flops.
A flip-flop has the ability to divide a signal by 2. In other words, the output
oscillates at half the input frequency.
If you connect a chain (or train) of flip flops, the signal gets divided by 2, 4, 8, 16,
32, 64, 128 etc.
Thus 1 minute will turn into 2 minutes . . . 128 minutes.
This type of chip is called a BINARY DIVIDER CHIP.
You can also connect a 555 to a divide-by-10 counter (CD4017) to get 2 minutes,
3, 4, 5, 6, 7, 8, 9, 10.
If your timing is 61 seconds, the 10 minutes will be 10 minutes and 10 seconds.
Here are 2 LONG DURATION TIMING CIRCUITS:
LONG DURATION TIMER -1 will produce a HIGH on pin 3 after 1
minute. Or pin 11 after 10 minutes. The timer start when the power is applied,
providing the 220u is fully discharged. If not, the 1st minute will not be
accurate. The timer will keep cycling.
CD 4024 LONG Duration Timer:
You have to test the circuit to see if the counter-chip advances on the HIGH or
LOW of the 555 and the actual timing for each pin may be different to the
circuit above. The circuit is just a sample of how to connect the two chips.
The circuits above will CYCLE. In other words, the outputs will "start all over
again" after the longest time-interval has occurred.
To FREEZE the output and make it stay HIGH, you need the following modification:
Delay before turn-on:
Pin 3 comes on LOW and goes HIGH after a few seconds:
In other words, Pin 2 "turns the chip ON" when it sees a LOW
Delay before turn-off:
Pin 3 comes on HIGH and goes LOW after a few seconds:
In other words, Pin 6 "turns the chip OFF" when it sees a HIGH
Don't forget, the chip takes 10mA
ALL THE TIME
because it is connected to the supply.
These two circuits show how pins 2 and 6
control the chip.
11. HIGH for 3 seconds every 60 seconds
Here's a very simple
circuit to create a HIGH for 3 seconds and a LOW for 60 seconds:
12. Level Shifter
This circuit produces
a maximum 5.6v on the output and minus 0.7v when the output is LOW. The
electro on the output charges to about 8v when the output is HIGH and when
it goes low, this 8v would produce a negative 8v on the output, but when the
output goes below 0.7v, the zener turns into an ordinary diode and conducts
to discharge the electro.
This guarantees 0v on the output as pin 3 of the 555 does not go to 0v and
sometimes the device being driven by pin 3 does not fully turn off due to
13. PIN 3 - THE OUTPUT PIN
The output of the 555
does not rise to rail voltage or fall to 0v.
The actual value of the HIGH and LOW will depend on the supply voltage and
the load on the output. The output can be as much as 2v less than rail
voltage and up to 1v above the 0v rail.
Because of the it can fail to turn OFF an NPN transistor connected to the
output and it will fail to turn OFF a PNP transistor.