This project uses a 555 IC (chip) as an oscillator and CD4017 IC as a
scanner to produce 3 different types of display.
The first is a SCANNER, made popular by the KNIGHT RIDER TV
The Scanner - called Kitt Scanner
Kitt Scanner is produced on the smallest board in this project with a 6-LED
display to produce a
scanner very similar to David Hasselhoff's K.I.T.T Car from
To remind you here are some of the photos of the car and similar cars:
The other two displays are EXPAND and COMPRESS. here is the EXPAND
HOW THE 555 WORKS
The way the 555 works is very hard to understand.
Here are the first 10 facts:
Pin 8 connects to the supply.
Pin 1 connects to 0v.
Pin 4 connects to the supply rail and the chip will work. When it is
taken to 0v, the chip will stop working. The chip will start working
when pin 4 is 0.7v or higher.
Pin 3 is the output pin.
It will deliver (source) 200mA to a LOAD and sink 200mA. But the chip
will get very HOT when doing this and so the output pin normally drives
a buffer transistor and the transistor drives the LOAD.
Pin 7 goes LOW when pin 3 goes LOW and pin 7 goes "open" when pin 3 goes
In other words, pin 7 will sink about 20mA but it will not SOURCE.
Pin 5 has 2/3 of the supply voltage on it and by changing the voltage on
this pin, the frequency of an oscillator can adjusted slightly.
An oscillator works by charging and discharging a capacitor via a
When the frequency of an oscillator is higher than one cycle per second,
it is called an oscillator and the frequency is measured in cycles per
second (called Hertz).
When the frequency is less than one cycle per second, the oscillator is
called a TIMER or DELAY circuit. And the timing is measures in
The 555 can be used as an OSCILLATOR or TIMER (Delay circuit).
It contains all the components to detect when the capacitor is charged
and when it is discharged.
It also charges and discharges the capacitor via one of the pins.
This pin is normally pin 7, but because pin 7 and pin 3 are LOW at the
same time, you have a choice of which pin to use.
The only pins left are pins 2 and 6.
A capacitor is added to the circuit with a series resistor.
These two components are called a TIMING CIRCUIT.
If we connect pins 2 and 6 to the join of these two components, the chip
will be able to detect when the capacitor is charged and when it is
Pins 2 and 6 do not have any effect on the charging and discharging.
They are called HIGH IMPEDANCE pins or lines.
If we connect the TIMING CIRCUIT between the 0v rail and the supply
rail, the capacitor will charge.
To make it discharge we have to remove it from the supply rail and
connect the timing resistor to 0v.
That's what pin 7 does.
In one of the states, pin 7 effectively disappears and the capacitor
charges. In the other state, pin7 takes the charging resistor to 0v.
We need to add a resistor between pin 7 and the supply to allow Pin 7 to
The designer of the 555 chip allowed the capacitor to charge to 2/3 of
the voltage on the supply rail and pin 6 detects this level. He allowed
the capacitor to discharge to 1/3 of the voltage on the supply and pin 2
detects this level.
This has been done because the charging of a capacitor is not linear but
between 0 and 66% it is very linear.
This has been the simple explanation of how the 555 works.
It gets more complicated when you have a circuit that can have a high or
low voltage on pin 6 and at the same time a high or low voltage on pin
PINS 2 AND 6
Now we come to the "first-in best-dressed" situation with pins 2 and 6.
Is there is an "over-ride" situation for these two pins? NO.
That's why you have to be very careful.
Pin 2 must be LOW to make the output HIGH
Pin 6 must be HIGH to make the output LOW.
Pin 2 has control over pin 6.
If neither pin is "controlling the chip" at start-up, what is the
There are some circuits where Pin 6 is LOW and Pin 3 is HIGH, waiting
for an input to make it LOW.
In this situation no pin is sending a "set" or "reset"
message and the output can be either HIGH or LOW, depending on the
imbalances within the chip.
But for all the other cases, the output is known.
If pin 6 is HIGH and pin 2 is LOW - the
output will be HIGH - pin 2 is sending a "set" message
If pin 6 is HIGH and pin 2 is HIGH - the output will be LOW
- pin 6 is sending a "reset" message
If pin 6 is LOW and pin 2 is LOW - the output will be
HIGH - pin 2 is sending a "set" message
If pin 6 is LOW and pin 2 is HIGH - the output will be
HIGH or LOW - this is because no pin is sending a "set"
or "reset" message.
| Pin 2
||Output pin 3
board can be wired to produce all sorts of 555 circuits,
The circuit can be a TIMER, OSCILLATOR, DELAY, or TRIGGER (Schmitt
Trigger), depending on
You can firstly make the circuit on breadboard and when it is operating
correctly, transfer the parts to this PC board. Actually it is best to
use a completely new set of parts, just in case you make a mistake and
have to go back and locate the problem.
Whenever you are designing, building or making anything, you need to do
things in small steps and stages so you can go back to a previous stage,
if a fault develops.
This board is designed to reduce and remove frustration, so work slowly
and double-check everything you do.
Designing a circuit is not easy and even copying a circuit from a book
needs careful attention.
You need to know that the circuit works in the first place and you are
not overloading the output of the chip or forgetting to connect any of
There are over 100 circuits on Talking Electronics website in the 50
- 555 Circuits folder and most of them can be modified and changed to
suit different needs. But you need to build the original circuit first
and change one value at a time. And the value must be changed in small
steps. You cannot change 10k to 1M in some circuits as it
will fail to work and if you do this with 3 components, you will never
get the the result you expect. Only by checking each change,
can you be sure the final circuit will work.
KIT OF PARTS:
555 + 4017 DISPLAY
3 - 330R
1 - 22k
1 - 10k mini pot
1 - 2u2 electrolytic
10 - surface mount diodes
44 - 3mm High-bright red LED
1 - 555 timer chip
1 - 4017 IC
1 - 8 pin IC socket
1 - 16 pin IC socket
1 - 11 pin machine socket (strip)
3 - 11 pin double ended machine pin strips
1 - 9v battery snap
1 - mini slide switch
1 - 555+4017 Display PC board
1 - EXPAND PC board
1 - COMPRESS PC board
1 - SCANNER PC board
A kit of components is available from
plus $4.00 postage.
Talking Electronics has a number of pieces of TEST EQUIPMENT to help in
the design and testing of projects.
Of course you can use a multimeter for most of the testing but some of
the "tricky" faults need a special piece of equipment.
You may only need a LOGIC PROBE once a month, but the project you are
designing will come to a
stand-still if you can't locate a problem.
We designed all these projects because we needed them ourselves.
Add one of them to each order you place with Talking Electronics and eventually you will have the whole