Elenco Electronics 202 - Manual (Page 78)

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Project #176
OBJECTIVE: To learn about the most important component in
electronics.
Transistor Amplifiers
OBJECTIVE: To show how electronic amplifiers can detect skin
pressure on two contacts. OBJECTIVE: To show how electronic amplifiers can detect
different values of resistance.
Use the circuit from project #176 shown above.
When you placed your fingers across the two snaps marked X & Y you
noticed the LED (D1) ame on in project #176. Repeat this process, but
this time press very lightly on the two snaps marked X & Y. Notice how
the brightness of the LED is dependent on the amount of pressure you
use. Pressing hard makes the LED bright while pressing very gently
makes it dim or even flash. This is due to what technicians call contact
resistance. Even switches made to turn your lights on and off have
some resistance in them. When large currents flow this resistance, will
drop the voltage and produce the undesirable side effect of heat.
Project #177
Pressure Meter
Project #178
Resistance Meter
When you place one or more fingers across the two snaps marked X
& Y you will notice the light comes on. The two transistors are being
used to amplify the very tiny current going through your body to turn
on the LED (D1). Transistors are actually electrical current amplifiers.
The PNP transistor (Q1) has the arrow pointing into the transistor
body. The NPN transistor (Q2) has the arrow pointing out of the
transistor body. The PNP amplifies the current through your fingers
first, then the NPN amplifies it more to turn on the LED.
Use the circuit from project #176 shown above
When you placed your fingers across the two snaps marked X & Y you
noticed the LED (D1) came on in project #176. In this project, you will
place different resistors across R & Z and see how bright the LED glows.
Do not snap them in; just press them up against the snaps labeled R &
Z in the diagram above.
First, place the 100k
Ω resistor (R5) across the R & Z snaps and note
the brightness of the LED. Next, press the 5.1k Ω resistor (R3) across R
& Z. Notice how the LED gets brighter when the resistance is less. This
is because the NPN transistor amplifier (Q2) gets more current at its
input when the resistance is lower. The PNP transistor amplifier (Q1) is
not used in this test.