Elenco Circuit Maker Sound Plus 200 - Manual (Page 33)

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Project #55 Transistor Amplifiers
When you place one or more fingers across the two snaps marked X &
Y you will notice the LED (D1) turns on. The two transistors are being
used to amplify the very tiny current going through your body to turn on
the LED. 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 from your fingers first, then the NPN amplifies it
more to turn on the LED.
Use the circuit from Project #55 shown above.
When you placed your fingers across the two snaps marked X & Y you
noticed the LED (D1) came on in Project #55. 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 #56
Pressure Meter
Project #57
Resistance Meter
Use the circuit from Project #55 shown above.
When you placed your fingers across the two snaps marked X & Y you
noticed the LED (D1) came on in Project #55. 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 100KW resistor (R5) across the R & Z snaps and note the
brightness of the LED. Next, press the 5.1kW resistor (R3) across R & Z.
Notice how the LED gets brighter when the resistance is less. This is
because the NPN amplifier (Q2) gets more current at its input when the
resistance is lower. The PNP amplifier (Q1) is not used in this test.
CM-200_Manual_031914.qxp_CM-200_Manual_031914 4/2/14 12:04 PM Page 33