Lesson 3

1 2 LED FLIP-FLOP Flasher This circuit combines
components you have already used.
On to making two LEDs flash,
first one and then the other,
back and forth.
3 PART LED Flip-Flop Circuit Diagram SYMBOL + You are familiar
with all of these:
the LED,

the resistor,

the battery

the transistor
Use these SYMBOLS,
for each part to read
the circuit diagram
for your flip-flop circuit.
and the electrolytic
capacitor
Remember, the capacitor stores electrical charge.
An electrolytic capacitor is a ONE-WAY part.
The transistor acts as an electrically controlled
switch with no moving parts.
4 Two BC547
transistor
(not provided) breadboard battery clip 9 volt battery LEDs Two each of
470 & 10K ohm resistors
To build the touch sensor, you will need: Two 100 microfarad electrolytic capacitor c b e
5 1 Connect your circuit,
as shown in the circuit diagram.

You can use the breadboard
layouts in steps 2-8 to position parts,
as shown on the next slides.

There are many ways to layout
a circuit board.
REMEMBER: The electrolytic capacitor
has a PLUS and MINUS lead.
The shorter MINUS lead is marked
with a bar that is labeled
with a minus sign.
6 Insert one lead
from each of
the four resistors
(470k, 10k, 470k, 10k)
into a hole in the
positive power strip.

Insert the other lead
of each resistor
into a breadboard hole, as shown.
Insert the three prongs
of the BC547 transistors.
The base of each transistor
should be connected
through the breadboard
to a 10k resistor,
as in the circuit diagram.
NOTE: These breadboard diagrams
were created using Fritzing,
free software you can use, too.
power strip 10k 10k 3 2
7 Use a zero ohm resistor
to wire the emitter (e) prong
on each transistor
to the negative power strip.
A black wire is used
to indicate a (-) or
ground connection.
Connect the positive lead of
one LED to a 470 ohm resistor.
Insert the shorter, negative lead
of the LED to the collector (c)
of the nearest transistor.

Connect the second LED in the
same way to the other transistor.
+ + 4 5
8 Connect the clearly-labeled
negative lead of the capacitor
to the base of one transistor.
Connect the other lead of the
capacitor to the collector (c)
of the other transistor (blue wire).

Use the remaining capacitor
to make a mirror image connection.
That is, connect its negative lead
to the base of the remaining transistor
and its positive lead to the collector (c)
of the other transistor.
6
9 - + Connect the 9 volt
battery to
the power strip
on the breadboard.
Do the two LEDs flash in sequence,
first one, then the other, back and forth?
If not, check your connections.
8 7 Check your connections!

The POSITIVE lead
on the LED
connects 470 ohm
resistor.
NOTE: The negative lead on
each electrolytic capacitor
must connect to the base
of each transistor.
10 Use your
multimeter
to check
the resistor
values.
Replace the two 10k resistors
with two 1k resistors. What changes?

Try two 47k resistors, instead.
How is this different?
9
11 10 Replace each 470 ohm
resistor with an LED.
Be sure that the + end
of each LED is connected to
the + terminal of the battery.

What happens now?
Add a third LED.
Repeat step 9 to vary the
frequency of the flashing.
11
12 WHAT MAKES
THIS CIRCUIT
WORK?
13 Your capacitor plays an important role in this circuit.
Remember, it is an electrolytic capacitor and
can be connected in only one direction.

Two transistors are present.
As one turns on, the other one turns off.
This give you the flip-flop or alternating
flashes of the LED.
The video on the next slide
gives a detailed explanation of how this circuit works.
Don't worry if you do not understand the explanation at once. Much of learning is like that.
It makes more sense as you repeat it.
14 The Flip-Flop Oscillator 15