Manual

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B
y using the LED display you will see the effect of
electrical signals. An LED is similar to a normal
diode except when current flows through it, it emits
light. One example of the LED display is a power
indicator on your DVD player or your radio that tells
you the power is on.
A seven-segment LED display can show the
numbers 0 through 9 for reading information on a
calculator. Seven is the minimum number of
segments (separate lines that can be each lighted)
that are necessary to clearly distinguish all ten
digits. Two conditions that you must always observe
for the proper LED operation are:
1. Polarity correctness (+ and – LED connections)
2. Proper current flow
LEDs can burn out due to reverse polarity if the
voltage is more than about 4 volts, or if the current
is not limited to a safe value. When the polarity is
reversed the LED will not light.
Series resistors (permanently wired to your kit) are
used with the LED display to keep the current flow
at a proper level. Current flows through these
resistors and the LED to terminal 25, providing a
comparatively constant voltage (approx. 1.7 volts)
to the LED. To make the current flow through the
LED display we need voltages above this value.
The series resistors set how much current flows
from the batteries through the LED.
Now it is time for you to learn about the common-
cathode seven-segment LED digital display. Seven
LED display segments use one contract point –
terminal 25 – as a common negative electrode in a
common- cathode.
To allow current to flow through an LED must have
both (+) and (–) connections. The anode is the
positive side and the cathode is the negative side.
In this kit the LED display is a common cathode
type. You connect any anode segment terminals as
required, to the battery’s positive side and connect
the common cathode segment terminal (terminal
25) to the negative side of the battery.
LEDs operate tremendously fast. An LED can turn
off and on hundreds of times per each second; so
fast that you won’t even see it blink. There is no
warm up time or large amount of heat produced
unlike an incandescent lamp.
D
o the following experiment to experience how fast
the LED operates.
1. Do not close the key but hook up the circuit.
2. Decrease the light in the room to a low level so
that you are able to see the LED light emission
easily.
3. Close the key but only for less than a second.
You will notice that the display goes quickly off and
on. Hold the platform steady but glance quickly at
the LED as you quickly tap the key. It will appear
that the display goes on and off. What occurs in the
persistence of the human eye is much longer than
the LED’s time but without the use of special
instruments this gets the point across.
Notes:
EXPERIMENT #23: LED DISPLAY BASICS
Wiring Sequence:
o 17-18-19-20-21-22-23-24-138
o 25-120
o 119-137
Schematic
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