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- Committer: Tim Marston
- Date: 2012-03-10 13:04:29 UTC
- Revision ID: tim@ed.am-20120310130429-310w5ejo968mc6mo
cleaned-up notes
- files added:
- .bzrignore
- project
- src
- src/util
- test
- test/fan-test
- test/fan-test/Makefile
- test/led-test
- test/led-test/Makefile
- test/phantom-button-presses
- test/phantom-button-presses/Makefile
- test/rtc-test
- test/rtc-test/util
- files removed:
- propeller-clock.sch
- files renamed:
- Notes => notes
- Makefile => src/Makefile
- files modified:
- electronics-information
added
removed
electronics-information
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+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
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That's a lot of current and a lot of power, flowing continuously, just
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to provide 6V!
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voltage in the range 6-20V (although 7-12V is recommended).
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Wiring up multiple LEDs in series to a single arduino pin
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Wiring up multiple LEDs in series to a single Arduino pin
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=========================================================
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First, lets think about a single LED.
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| R D
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|
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The arduino pin, when raised high, is at 5V and no more than 20mA can
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The Arduino pin, when raised high, is at 5V and no more than 20mA can
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be taken from it. The LED will take about 10mA and wants about 1.5V.
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You can think of this as a potentiometer arrangement, with the
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You can see that beyond three or perhaps, at a push, four LEDs you're
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not going to get the required 1.5V across each LED. So three (or four)
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is the limit to how many LEDs you can drive in series from one pin on
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the arduino.
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Wiring up multiple LEDs in parallel to a single arduino pin
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===================================--======================
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Image we have
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the Arduino.
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Wiring up multiple LEDs in parallel to a single Arduino pin
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===========================================================
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Imagine we have this
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|
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Arduino | ___ ,,
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o-----|___|---+---►|---.
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| R | D1 |
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| R | D1 |
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| | |
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| | ,, |
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| '---►|---+--- GND
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So, imagine we have this instead
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|
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Arduino | ___ ,,
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Arduino | ___ ,,
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o----+---|___|----►|---.
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| | R1 D1 |
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| | |
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| '---|___|----►|---+--- GND
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| R2 D2
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This is ok. R1 and R2 are just the usual 340 ohms. But we have to
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This is OK. R1 and R2 are just the usual 340 ohms. But we have to
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bare in mind that each LED requires 10mA. So the total current drawn
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from the arduino pin will be 20mA, which is the most you're allowed to
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from the Arduino pin will be 20mA, which is the most you're allowed to
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draw. So two LEDs is the most that we can drive, in parallel, directly
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from a pin.
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Using a transistor to drive multiple LEDs
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=========================================
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In both of the following diagrams, the resistor on the arduino pin,
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In both of the following diagrams, the resistor on the Arduino pin,
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R1, just needs to be something suitably high to provide a small
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current on the base of the transistor. So, R1 could be 1kΩ.
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▼ D1
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¯``
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|
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| |
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|
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| ▼ D2
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| ¯``
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| |
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| ¯``
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| |
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Arduino | ___ ,-|
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o----|___|--(|< ) T
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o----|___|--(|↘ ) T
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| R1 `-|
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| |
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| '----- GND
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| +-------+-------'
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| |
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Arduino | ___ ,-|
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o----|___|--(|< )
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o----|___|--(|↘ ) T
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| R1 `-|
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| |
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| '----- GND
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Here, R3 = R4 = R5 = 340Ω, as usual. The numbher of LEDs is limited
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Here, R3 = R4 = R5 = 340Ω, as usual. The number of LEDs is limited
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only by the current that can be drawn from the power supply.
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RC CIRCUITS
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===========
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An RC (resistor capacitor) circuit 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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___
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Vin o----[___]----+-----o Vc
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R |
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|
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===
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| C
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|
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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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to Vin.
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With a pulse wave on Vin, Vc would look something like this:
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5V
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_ _ _ _
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,'' \ ,'' \ ,'' \ ,''
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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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T = RC
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TRANSISTORS
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===========
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When using an NPN transistor as a switch, a typical set up might look
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something like this:
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o--------------+---- 5V
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|
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[]
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( ) some load
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[]
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|
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___ ,-|c
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o---[___]--b(|↘ ) T (NPN)
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R `-|e
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|
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----+---- GND
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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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Arduino!).
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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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----+---- 5V
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|
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___ ,-|e
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o---[___]--b(|↙ ) T (PNP)
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R `-|c
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|
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[]
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( ) some load
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[]
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|
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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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CAPACITORS
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==========
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Two capacitors in parallel are equivalent 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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NPN transistors
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BC548/BC547, 5V switch
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PNP transistors
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BC327/BC328, -5V switch
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LEDs
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~1.5V, ~10mA
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With 5V across them, a 560Ω resistor is required.
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DIODE
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1N4001
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Electrolytic Capacitor (Radial, 4700uF 16V)
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Maplin part no. VH57M
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Arduino-powerable relay (DPDT, gold contacts, 5V, 27mA)
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Maplin part no. N05AW
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