Only the 1st Lesson in Electronics 2 is available for free viewing.
with just three components.
a touch sensor.
In this circuit,
touching two wire connectors (finger probes)
to your finger lights an LED.
for the Touch Sensor
with the first three parts:
the LED,
the resistor
and the battery.
Meet the transistor --
an electronic switch
with NO moving parts!
for each part to read
the circuit diagram
for your touch sensor.
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.
transistor
resistor
within the Breadboard.
the Breadboard.
Strip
Strip
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.
insert its three prongs
into any three adjacent holes
on the breadboard,
along a column, as shown.
into a hole directly behind the collector (c) side of the transistor.
This connects the resistor
to the transistor’s collector.
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.
directly to these same components
in the Touch Sensor Circuit Diagram.
into the breadboard.
The longer positive lead is inserted into the breadboard,
as shown.
positive terminal on the battery.
when you are finished.
POSITIVE lead to the LED.
The disconnected end of
this wire will become one
of the finger probes.
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.
to this diagram.
that your zero ohm resistors
act as wires!
You will use these as wires on your breadboard.
The RED-lined power strip is positive.
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.
are pushed firmly
into the breadboard.
of transistor
onto the 9V battery.
Connect the red and black wires from the clip to the power strip on the breadboard.
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.
firmly into the breadboard.
your connections,
before you add power!
You made
a touch sensor!
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.
that uses a transistor.
What does it do in this circuit?
button
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!
as electronic switches which can be either
in an "ON" or an "OFF" state.
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.
your transistor acted like an “ideal switch.”
finger
probe
finger
probe
When ON, it acts as a short circuit or wire.
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.
A Simulation
then LARGE current from C to E.
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.
then NO current from C to E.
are touched to your finger,
the base IS connected
to the battery through your
finger and current (i) can
flow into the base (B).
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.
come into a transistor
in your (NPN) transistor
does the larger current flow
transistor have moving parts
to pass a small current from the battery to the base of the transistor
describe a transistor
to the emitter
controlled switch


















































