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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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