Lesson 2

1 2 Simple LED Flasher This circuit is built
with just five components.
You can make an LED flash
using only one transistor.

You will be adding a new part, a capacitor.
With it, you will be able to make an LED flash
ON and OFF, ON and OFF.
This is called an oscillation.
3 PART Circuit Diagram for the Simple LED Flasher SYMBOL + You are familiar
with the first four:
the LED,

the resistor,

the battery

and the transistor.
Use these SYMBOLS,
for each part to read
the circuit diagram
for your LED flasher.
Now meet the
capacitor:
All capacitors store electrical charge.
An electrolytic capacitor is a ONE-WAY part.
One specific lead, the longer one, is connected
to the positive terminal of your battery.
4 BC547
transistor
(not provided) A farad is a unit of measurement for capacitance.
1 μF (microfarad, one millionth (10−6)
of a farad) = 0.000001 farads
breadboard battery clip 9 volt battery LED 100 & 1K ohm
resistors
To build the touch sensor, you will need: 220 microfarad electrolytic capacitor c b e
5 1 2 Connect your circuit,
as shown in the circuit diagram.
You can use the breadboard
layout to position parts.
There are many ways to layout
a circuit board.
NOTE: The capacitor
stores electrical charge.
This electrolytic capacitor
has a PLUS and MINUS lead.

The MINUS lead is marked
with a bar that is labeled
with a minus sign.
+ - c e 100 ohm 1k ohm
6 + + + Notice that the base of the transistor is connected to NOTHING!
The transistor is still acting as a switch but in a novel way!
Save that discussion for a moment. First get the LED to flash!
3 Check your connections!
The POSITIVE lead
on the LED
connects to the collector
on the transistor.
NOTE: The positive lead on
the electrolytic capacitor
must be connected to the
emitter on the transistor AND
the 1K ohm resister, as shown.
Be sure to turn OFF the meter
when you are finished.
7 Interconnections
within the Breadboard.
Top View
the Breadboard.
Power
Strip
Power
Strip
Power Strip on Breadboard Remember, you need to connect this circuit to a 9 volt battery.
As before, the breadboard has a “power strip”
running lengthwise along both sides, as shown..
8 CONTINUITY! * Use your multimeter to test
that your zero ohm resistors
act as wires!
REMEMBER, use the zero ohm resistors as wires on your breadboard.
9 4 Snap the battery clip
onto the 9V battery.
Connect the red and black wires from the clip to the power strip on the breadboard.
The RED wire connects to the
PLUS terminal of the battery.
Connect this red wire to
any hole next to the red bar.

The BLACK wire connects to the MINUS terminal of the battery.
This black wire connects
to any hole along the blue bar.
Be sure that both wires are pushed
firmly into the breadboard.
Always check
your connections,
before you add power!
10 Does the LED
BLINK
ON and OFF?
Troubleshooting:

1) IF THE LED STAYS ON:
If the resistor that charges the capacitor (10k) has too low a value or if the power supply (battery) voltage is too high, the current out of the capacitor
may not become low enough for
the transistor to turn OFF.

2) IF THE LED DOES NOT LIGHT:
The battery voltage may be too low.
Get a new battery, or increase the voltage by adding alkaline batteries in series (no higher than 12V) or reduce the 10k resistor. The issue is that the capacitor is not charging enough to turn on the transistor.

3) Try higher and lower values for the 1k resistor,
perhaps .5k to 5k.

4) Do not let the base (b) on the transistor touch ANYTHING, even plastic. Try reversing c and e (was the transistor connected correctly?)
Try another transistor, as transistors fry easily.
11 You have just built a circuit
that uses a transistor and
a capacitor to make an oscillator.
What is happening in this circuit?
WHAT IS A CAPACITOR?
12 This circuit is called an Esaki oscillator.

It is named after Leo Esaki (born in 1925), a Japanese physicist. He shared the Nobel Prize in Physics (1973) for the discovery of electron tunneling.

As with chemistry and basic biology,
much of electronics is all about --
Ω WHERE the electrons GO!
13 toggle switch push
button
Mechanical
Switches
You just built Mr. Esaki’s oscillator.
In this circuit, you were again using the switching capability of a transistor to make an LED blink on and off, but this time you were manipulating something called BREAKDOWN voltage to do it.

Breakdown voltage is the voltage where an insulator STOPS being an insulator and becomes a conductor. Air is an excellent insulator, but even air has a point where it becomes a conductor. Though air’s breakdown voltage is quite high, lightning can reach this breakdown voltage. Your transistor requires a much smaller voltage for it to STOP being an insulator and become a conductor, instead.

As long as the voltage does not go too far beyond this breakdown voltage, your transistor will continue to function. Push the voltage too high, however, and the transistor will overheat and be destroyed.
wall switch plate
14 Your capacitor plays an important role in this circuit.

FIRST it has to charge.
That is what capacitors DO.
They build up and store electrical charge.
The charge goes up; the charge goes down.

A capacitor is any two conductors separated by an insulator.
Two squares of aluminum foil separated
by a piece of paper makes a capacitor!
YOU can make such a capacitor
in the ScienceWiz Inventions kit.

Remember, it’s all about
WHERE the electrons GO!
15 Double click on the bulb on the right hand side of the screen. On the next screen you should see a battery, a capacitor with two metal plates and a bulb.

Use the slider ON THE BATTERY to turn up the voltage flowing from the battery. Notice that it charges the plate and stops after the plates are fully charged. Move the switch (kinked wire located above and below the plates) so it points straight up and straight down. Does the charge remain on the plate? YES.

Now use the switch to connect the charged capacitor to the bulb. What happens? Did the capacitor discharge? Does the bulb stay lit. NO! It remains lit only until the capacitor has fully discharged.
Explore How A Capacitor Works
A Simulation
16 So how does your flasher circuit work? Below the threshold
for the breakdown voltage,
the transistor is OFF.

As the capacitor discharges,
the voltage drops below
the breakdown voltage
and current no longer
flows to the LED.

The circuit is OPEN.
The switch is OFF.
The LED has no power.
It does not light.
ON OFF
As the capacitor charges,
it reaches the limit where
the voltage breaks down
across the emitter/collector
junction of the transistor.

Reaching this threshold voltage
allows current to flow
from the capacitor
through the transistor to the LED.
This voltage acts as a switch!

Below the threshold, the transistor
is OFF and above the threshold
it is ON.

As the capacitor recharges,
the cycle repeats.
You have an oscillation
that repeats and repeats,
as long as the battery lasts.
17 In this circuit, did you notice
that the base (b) of the transistor is not connected to ANYTHING. Therefore, this disconnected base cannot supply the small current needed
to switch on the transistor.

While working in transistor quality control,
Mr. Esaki realized that the emitter had
a low breakdown voltage,
far lower than the collector’s breakdown voltage.

That is WHY he connected the capacitor to the emitter (e), NOT the collector.

As the breakdown voltage is exceeded,
the transistor conducts electricity
from the emitter (e) to the collector (c).
The Esaki
Oscillator
18 The Capacitor What is a Capacitor? (Part1 - 3 min) Units of Capacitance (Part2 -1.5 min) 19 The Esaki Oscillator 20 There is a minimum voltage that turns an insulator into a conductor. What is it called On a transistor, which lead has the lowest breakdown voltage What is the unit of
measurement for capacitance
A repetitive variation
or cycle is called an
1 2 3 4 5 What does a capacitor do oscillation the breakdown voltage the emitter stores electrical charge the farad
21