Elenco Green Projects - Manual (Page 29)

0.5mA
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Project #29 Long Light
Build the circuit and set the meter (M6) to the 5V setting. Set
the slide switch (S5) to position C and watch the voltage on
the meter for a while as the battery runs the yellow LED (D5).
How quickly does the voltage drop? Try placing the circuit in
darkness, changing the temperature, or varying the wind.
If your battery was recently recharged then you probably
found the voltage drops very, very slowly, and thought this
was boring. That was the idea - batteries can run things for
a long time and (unlike solar or wind power sources) are
hardly affected by changing weather conditions. Batteries
can provide power whenever you need it - but, eventually,
they do run out.
Project #30 LED Currents
Build the circuit, set the slide switch (S5) to
position B, and set the meter (M6) to the 0.5mA
setting. This circuit has the LED in series with a
10K
Ω resistor in the pivot stand. 10KΩ is a high
resistance, so the meter measures a small
current and the red LED (D1) is dimly lit. It is
easier to see the red LED if you take the circuit
into a dark room. If the LED does not light at all
then the battery needs to be recharged.
Switch the motor to the 50mA setting. Now set
the switch to position C, replacing the 10K
Ω
resistor with a smaller 47 Ω resistor in the pivot
stand. The LED is brighter now and the current is
higher.
Now push the press switch to bypass the 47 Ω
resistor. The current and LED brightness are
even higher now.
Replace the red LED with the yellow LED (D5)
and see how the brightness and current change.
5V
Adding resistors to a circuit is
like partially blocking a water
pipe, reducing the flow of water.
High currents can damage
LEDs, so resistors are usually
used with them to limit the
current. Your D1 and D5 LEDs
have internal resistors of
around 50
Ω to protect them.
50mA
See projects 1 & 3 if you need to recharge the battery (B4).