| During the day when the sun is shining on the Solar CD, the
electricity provided by the solar cells flows into the batteries,
recharging them for their night's work.
Anybody might
ask, "What keeps the LEDs from turning on during the
day?"
The answer is that the current provided by the Solar CD seeks out
any open path. Most of the current flows directly from the Solar CD
and into the batteries. But a
small amount of current flows through 2.2k resistor R1, where it
meets the higher resistance of 10k
resistor R2. An even smaller amount of
current flows through R2; the rest of this current flows into the
"base" lead of transistor
Q1, which "turns on" Q1, creating an open path
for current to flow through 2.2k
resistor R3 and then through Q1 from collector
to emitter. This prevents current from flowing into the base
of transistor Q2. If current flowed
into the base of Q2, the LEDs would turn
on.
The width of the red arrows attempts
to denote the amount of current flowing at each location in the circuit.
Please note that all voltage and mA numbers are approximate. The actual
voltage and mA varies depending on conditions.
|
|
Anybody asks,
"What turns on the LEDs at night?"
When the sun sets, the Solar CD stops producing electricity, and the batteries take over
all the work and start pumping their stored current into the
circuit. The current runs into one-way valve, diode D1, and can go
no further. So a small amount of current flows across 2.2k resistor
R3, and since it can't flow across transistor Q1 (which is turned off),
the current flows into the base lead of transistor Q2: this
"turns on" Q2. Now, this creates an open path so that most of the
current can flow across
the LEDs and through Q2, which, of course, makes the LEDs light up!
Anybody says, "I
have a vague understanding of current and electricity. I know it has
something to do with 'bitty particles' moving around, but it still all
seems like magic to me!"
Jeff writes, "Actually,
the 'bitty particles' are all electrons and they all have the
same charge: -1. It only seems like
magic because the part that moves is too
small to see. The
traditional beginner crutch is to think of water flow in pipes. You
can't see that either but you know
it's there because you can see the water
when it comes out the end. Saying
that you have a certain current flow in Amps is just a shorthand
for saying that some (very large)
number of electrons have passed a certain
point in one second. It's the same idea as measuring water flow
in gallons per minute. Keep
in mind that the various currents differ only quantitatively.
One might be 10mA and another 50A (50,000mA) but they're
both just electrons passing a
point, and electrons are all the same."
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