Lesson 1

1 2 Simple Touch Sensor This circuit is built
with just three components.
Begin by building a very simple circuit,
a touch sensor.

In this circuit,
touching two wire connectors (finger probes)
to your finger lights an LED.
3 PART Circuit Diagram
for the Touch Sensor
SYMBOL + You are familiar
with the first three parts:
the LED,
the resistor
and the battery.

Meet the transistor --
an electronic switch
with NO moving parts!
Use these SYMBOLS,
for each part to read
the circuit diagram
for your touch sensor.
Ω c b e Click to link to a lesson on circuit diagrams. OPENs IN A NEW TAB. This transistor is one of the great inventions of the last century.
It is a key component in practically all modern electronics.

Before going into how transistors work,
let’s explore what happens when you put it into this circuit.
4 BC547
(not provided) breadboard battery clip 9 volt battery LED 470 ohm
To build the touch sensor, you will need: jumper wires c b e
5 Interconnections
within the Breadboard.
Top View
the Breadboard.
How the Breadboard is Pre-wired The breadboard allows you to connect parts together
without soldering. Below is a top view and an inside view of the connections. The breadboard has a “power strip” running lengthwise along both sides, as shown by the two sets of red and blue lines.
6 1 2 3 With the flat side of the transistor facing you,
insert its three prongs
into any three adjacent holes
on the breadboard,
along a column, as shown.
c b e Bend down the two resistor leads. Insert one resistor lead
into a hole directly behind the collector (c) side of the transistor.

This connects the resistor
to the transistor’s collector.
Insert the other lead of the
resistor into an adjacent row in the breadboard. The five holes in
row 50 are wired together inside
the breadboard.
This is true of every row.
7 + + c b e The placement of these three components relates
directly to these same components
in the Touch Sensor Circuit Diagram.
4 Now insert the LED
into the breadboard.
The longer positive lead is inserted into the breadboard,
as shown.
NOTE: The positive lead of the LED will be connected to the
positive terminal on the battery.
Be sure to turn OFF the meter
when you are finished.
8 5 6 Insert one end of the red jumper wire into an empty hole in the same row as the
POSITIVE lead to the LED.

The disconnected end of
this wire will become one
of the finger probes.
Insert one end
of the black jumper
wire into an empty
hole in the same row
as the base (b), the
center prong of the
transistor, as shown.

The disconnected end of this black wire will become the other finger probe.
Ω Touch Sensor Circuit Diagram
Compare your circuit
to this diagram.
9 CONTINUITY! * Use your multimeter to test
that your zero ohm resisters
act as wires!
Now locate two zero ohm resisters.
You will use these as wires on your breadboard.
10 Use a ZERO ohm resistor to connect the POSITIVE end of the LED to the red power strip marked on the breadboard.
The RED-lined power strip is positive.
11 Use a ZERO ohm resistor
to connect the transistor’s
emitter (e), the third prong
on the right, to the negative (blue line) power strip
on the breadboard.

REMEMBER: To connect
to the emitter, the zero ohm
resistor has to go into
a hole in the same ROW
as the transistor’s emitter (e).

Use ANY hole along the NEGATIVE power strip.
Be sure that all leads and connectors
are pushed firmly
into the breadboard.
8 finger probes back view
of transistor
e b c
12 9 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!
13 Congratulations!
You made
a touch sensor!
Now touch the open ends
of the two finger probes
to your finger, as shown.

Watch the LED.

Does it light each time
you touch the probes to your finger?

The skin on your finger
acts as a resistor that allows
current to flow through the circuit.
14 You have just built a circuit
that uses a transistor.
What does it do in this circuit?
15 toggle switch push
toggle switch Mechanical
You use wall switches
to turn lights on and off.

You may have used
a toggle switch, too.

These types of switches
have moving parts.

A transistor IS --
an electronic switch
with NO moving parts!
wall switch plate
16 Transistors are often used in digital circuits
as electronic switches which can be either
in an "ON" or an "OFF" state.
Your transistor has three terminals:
collector (c), base (b) and emitter (e).
ONE of your finger probes is connected
to the base (b) of your transistor.
The OTHER finger probe is connected
to the plus terminal of the battery.
In the circuit you just built,
your transistor acted like an “ideal switch.”
Ω When OFF, the circuit is an open circuit.
When ON, it acts as a short circuit or wire.
17 A transistor acts like a switch.
It is not a switch that you press directly with your finger. Instead, the switch works automatically in response to electrical signals.

In your touch sensor, the transistor turns on an LED when the probes touch your finger. That is, the LED is turned on when 0.6 to 0.7 V is applied to the base(B).

Turn ON the base current in the simulation to see how this works.
Explore Transistors
A Simulation
18 If a small current from B to E,
then LARGE current from C to E.
How the Transistor Works in the Touch Sensor WHEN the finger probes
are NOT touching your finger,
the base is NOT connected
to anything.

NO current can flow from the battery
through your finger into the base (B).

With NO current going to the base,
NO current will flow
from the collector (C) to the emitter (E).
THAT is how the transistor works.

The circuit is OPEN.
The switch is OFF.
The LED has no power.
It does not light.
ON If NO current from B to E,
then NO current from C to E.
WHEN the finger probes
are touched to your finger,
the base IS connected
to the battery through your
finger and current (i) can
flow into the base (B) to
the emitter (E).

This small voltage/current (i)
into the base (B) CAUSES
a much larger current (i)
to flow from the collector (C)
to the emitter (E).

The circuit is CLOSED.
The switch is ON.
The LED lights.
19 The Transistor What IS a Transistor? From Transistors to Bits and Bytes 20 Where does a "small voltage"
come into a transistor
Through which leads
in a transistor does
the larger current flow
Does the ON/OFF "switch" in a
transistor have moving parts
What resistor did you use
to pass a small current from the battery to the base of the transistor
1 2 3 4 5 How would you
describe a transistor
your finger! the base from the collector
to the emitter
an electrically
controlled switch