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24
+12V and GND rails to prevent a lot of current flowing. Lets work
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this out (lets suppose that R is 1 ohm).
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V = IR, therefor I = V/R
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V = IR, therefore I = V/R
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P = VI = V(V/R) = (V^2)/R = ( 12 ^ 2 ) / 1 = 144W
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P = VI, therefor I = P/V = 144/12 = 12A
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P = VI, therefore I = P/V = 144/12 = 12A
31
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That's a lot of current and a lot of power, flowing continuously, just
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| +-------+-------'
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Arduino | ___ ,-|
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Here, R3 = R4 = R5 = 340Ω, as usual. The numbher of LEDs is limited
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only by the current that can be drawn from the power supply.
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An RC (resistor capacitor) curcuit is a basic low-pass filter. Here
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we're talking about giving it a pulse wave signal (a voltage that
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oscillates between 0V and approx. 5V).
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Vin o----[___]----+-----o Vc
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0V o-------------+-----o
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If the resistor weren't there, the capacitor on it's own would act
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like an open circuit to a pulse wave (or to an AC power supply). This
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is because the capacitor would charge to the voltage supplied across
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it and discharge almost immediately. The purpose of the resistor is to
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limit the current that is available to charge the capacitor, so it
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charges slowly and it takes some time before Vc becomes almost equal
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With a pulse wave on Vin, Vc would look something like this:
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,'' \ ,'' \ ,'' \ ,''
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0V / ',,_/ ',,_/ ',,_/
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If you were to draw the tangent to the curve at time=0 (so, the
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initial rate of change of charging), and you note the time that this
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line crossed 5V, then this time is T. That is to say, time T is the
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time it would take the capacitor to charge to the target voltage if it
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charged constantly at the rate it actually initially charges at. Then
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this equation applies
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When using an NPN transistor as a switch, a typical set up might look
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o--------------+---- 5V
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o---[___]--b(|↘ ) T (NPN)
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You would typically connect the pin to 5V to turn on the transistor.
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The current between the emitter and base turns on a larger current
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between the emitter and collector. The resistor, R, limits the
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turn-on current and prevents a short (effectively) across the
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transistor (and whatever the pin is connected to, such as an
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PNP transistors were used in the days when a -5V rail was typical
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instead of a 5V rail. In this case, a typical set up would be exactly
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the same as above, but with -5V used for the top rail and a PNP
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transistor. The thing to note here is that the direction of the
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current would also have changed. Now imagine flipping this diagram
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up-side-down and offsetting both rails by +5V (so that -5V becomes GND
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and GND becomes 5V, respectively). Then you'd have this set up, which
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is a modern-day typical usage scenario for a PNP transistor.
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o---[___]--b(|↙ ) T (PNP)
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o--------------+---- GND
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In this set up, as before, a current is required between the base and
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emitter to turn on a larger current between the collector and emitter.
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But the difference this time is that the pin must be connected to
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ground to achieve this.
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Two capacitors in parallel are equivelant to one capacitor whose
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value is the sum of the two.
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COMMON PARTS LIST AND USEFUL VALUES
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===================================
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BC548/BC547, 5V switch
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BC327/BC328, -5V switch
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With 5V across them, a 560Ω resistor is required.
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Electrolytic Capacitor (Radial, 4700uF 16V)
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Maplin part no. VH57M
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Adruikno-powerable relay (DPDT, gold contacts, 5V, 27mA)
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Maplin part no. N05AW
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