Colour selector
Behind door number 5 you will find
a second cable. Use it to convert your
circuit so that the red LED is not switched
on until you press the push button. At
the same moment the green LED will go
out. With this switch you can change the
colour: pressed = red, released = green.
Once the switch is closed, both LEDs will
be connected in parallel. One might think
that current would flow through both and
that both would glow. This is actually the
case when the same LEDs are used. But
there’s a big difference here. The green LED
needs more current than the red LED. If
the red LED is now switched on, the LED
voltage drops to such an extent that the
green LED can no longer shine.
Voltage, resistance and current
You may already know that the
electrical voltage is measured
in volts (V). The battery is 9 V.
Resistance is measured in ohm
(k
Ω
) or kilo-ohm (1 k
Ω
= 1,000
Ω
). But there is another very
important measurement: The
electrical current is measured in
amperes (A) or in milliamperes
for small currents (mA = 1/1000
A). All these names come from
famous researchers who were the
first to research electricity about
200 years ago: Alessandro Volta,
Georg Simon Ohm and André-
Marie Ampère.
A measuring device could be
used to measure how much
current flows through the LED. But
you can also calculate it, if you
know the voltage of the battery
and the voltage of the LED. If the
battery is still new, it will have a
voltage of 9 V. The green LED
needs about 2 V. That leaves 7V
for the resistor. And then you can
do the calculation like this:
Current = voltage/resistance
Current = 7 V/10,000
Ω
Current = 0.0007 A =
0.7 mA
That’s not much, there is a flow
of only 0.7 mA, although the LED
can handle a current of 20 mA.
But the battery lasts for a very long
time! It usually has a capacity of
500 mAh (500 mAh), so it could
deliver 500 mA for one hour or
1 mA for 500 hours. Your lamp
will therefore glow at 0.7 mA for
about 700 hours, which is about
a month long.
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