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Electronics A general electronics guide i did for another forum

Discussion in 'Modding' started by g0th, 21 Apr 2005.

  1. g0th

    g0th What's a Dremel?

    12 Apr 2005
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    Most of this stuff has probably already been covered here, but i hope there's something that you find useful. Perhaps this could be added to the other sticky, i don't know.

    You might have seen a nice effect using a single diode where the power LED turns off whenever the HDD activity LED comes on.

    But, let's say you want to run a whole array of LED's. The motherboard will not run these adequately.

    Now, using an optocoupler to switch the current is nothing new, but can we combine both ideas sucessfully?


    What we've got here, is 2 4N28 optocoupler chips, 2 TIP31 Transistors, a diode, anything like 1N4148 or 1N4004 will be good, and a 150ohm 1/4W resistor.

    Those 2 other resistors, need to be chosen to suit the particulars of your LEDs or LED arrays, just the same way you would always calculate the resistor needed for running off 5V.

    The left one is for power, and the one on the right is HDD.

    Note the 5V line connection up the top, and the 2 connections for the positive sides of the LED's. The ground rail, with the LED cathodes directly connected there as well, can be noted towards the bottom.

    This is a moderatly difficult thing to wire up, especially compared to general work with switches and LED's, without some sort of board it will be difficult. The best bet is just to get a little matrix board, and use that rather than making a PCB, although that would make for a very professional job.

    The transistors won't need heatsinks, but you'll need to ensure everything is connected up the right way.

    General LCD Guides


    Adding a backlight:


    Using the cheap Oatley Electronics LCD


    Wiring up a 40x4 LCD with 2 En lines


    Details on character tables, timing diagrams etc Very useful for programming and design, generally not important but a good read:


    Serial port interfacing for Hitatchi chipset LCD's



    Heatsinking Luxeons and other Power Semiconductors

    As with any power semiconductor device, we can calculate the required heatsink thermal resistance once we know the maximum junction temperature, ambient temperature and power dissipated.

    As only about 10% of the input power to the LED is emitted as light, it is disregarded in the following calculations. Assuming a nominal LED forward voltage of 3.6V, power dissipation can be found using Ohms law:
    An overclocked 1W star, perhaps

    PD = VI = 3.6V * 700mA = 2.52W

    Let's say the maximum junction temperature allowable is 70°C (Actually i think it's 135°C, which frightens me) and an ambient temperature of 25°C, the junction to ambient thermal resistance is:

    RTHJ-A = TJ - TA / PD

    = (70°C - 25°C) / 2.52W

    = 17.86°C/W

    Next, subtract the junction to board resistance (RTHJ-B) listed in the datasheet to find the board to ambient thermal resistance. For most board-mounted Stars, this is 17°C/W:


    = 17.86°C/W - 17°C/W

    = 0.86 °C/W

    The result is the maximum allowable heatsink resistance needed to keep the LED junction temperature at or below the maximum rating at 25°C ambient.

    Which is about equivalent to a cheap and nasty Socket A i think...

    Pumps, Relays and Geese

    A good way to conceal your pump relay is to conceal it in the PSU, this simplifies wiring too.

    Here are some parts you will need:

    Jaycar SY-4050 Relay


    IEC female panel socket Jaycar PS4002


    Coil: 12V/0V | = = |
    | |
    NO Active In | = = |
    COM: Active Out | = = |
    NC | = = |
    | |

    Just check the pin markings for your particular relay to be sure

    you can just solder the mains connections to the back of the input IEC socket, where L and N should be marked on the terminals, but take the active to the PSU side of the main switch if you have one. The neutral just goes straight to the new socket, mounted on the inside side (you know what i mean) so your pump can easily connect up, and the active goes via the relay as shown.

    L, N and E are also marked on the new socket - connect the earth via a crimp lug to the existing PSU case earth.
    It's good practice to insulate all mains connections with heatshrink tubing. Use mains rated wire - an old scrap of power cable maybe.

    Solder the 12V connections into the main PCB, maybe via a pluggable header or something. While the soldering iron is hot, add a 68 Ohm, 0.5W resistor in series with the fan to reduce noise, without compromising cooling.

    Hopefully it should all work - the relay contacts should click, and the pump should start. Also note that that fan is quieter than the Red October.

    You can do the same thing on the outside, for your monitor too. If you don't have enough room, like me, move the main switch to where the voltage selector switch used to be (desolder the wires and unscrew) and now you can fit the socket.

    Now, just to dispel the myth that you'll end up resembling a a big smoking pile of seaweed if you open that PSU, let's just assume the input filter electrolytics are 470uF and the associated discharge resistors are 330K:
    (That's based on the 250W unit in front of me)

    T=RC=155.1 seconds

    Discharge time = 5RC = 13 minutes. And because the charge decays exponentially, the time for which a risk is present is probably much less than that.

    Dismantle the PSU, wait for the soldering iron to heat, and you'll be fine.

    LED's have a certain fixed voltage drop, which depends on the chemical composition of the PN Junction, eg 2V for red LED's or thereabouts, up to 4.0V for UV LED's.

    These are non-ohmic conductors, meaning that the resistance of the semiconductor material decreases as it heats up, thus increasing the current, and thus, power dissipation. Thus, the condition of Thermal Runaway can destroy LED's without current limiting, which we normally provide in the form of resistors.

    Applying Ohm's law, we get

    R= V/I
    R = V(across Resistor)/Current through circuit

    V across resistor = Vsupply - Vforward (forward voltage of semiconductor)

    R = (V-Vf)/I

    As this uses SI units, we can say R = (1000*(V-Vf))/I so we can use Milliamps, the simplest unit for current in our case.

    Now, there is the matter of Power dissipation.

    P = I*(V-Vf)\1000, again I is in mA as opposed to the more conventional A

    if i have 20 of the above LEDs in parallel from the 5v rail, i'd want 3.3 ohms or so, and power dissipation is about 0.6W, so a 1W resistor might be a good idea.

    A good trick to keep in mind, is if you replace a red or green, yellow etc LED in a 5 Volt circuit with a High intensity White, blue or green LED you will roughly halve the resistor value.

    Remember, in parallel, add the currrent draw, and in series, add the voltage drop across the diodes. You do not need a seperate resistor for each LED. Also keep in mind that using a resistor to drop the voltage for a piece of equipment other than a LED that you want to install in your PC is a very bad idea, don't try it!

    Your best bet for these situations are the 78xx linear regulator IC's such as the 7809. Simply hook up 12V in, ground, and you have 9V out with respect to the ground (0V) rail. Remember that you are limited to 1A, but for most basic applications this won't be a problem.

    When working out the color codes, the only possible trap is when you have 1%, 5-band resistors. There are 3 bands before the multiplier band. Also note that there are much more values in the E24 Series, which is what this system accomodates for.

    The band nearest one end is the first digit. Next is the second digit. The third band represents the multiplier, or a further digit, and the final band is the tolerance.
    To determine the value of a resistor, combine the first and second digits together, then add zeros as indicated by the multiplier.

    <image goes here>

    Say you have a resistor, which is yellow-purple-orange-gold

    We get 4-7-*1000-5%

    47,000 Ohms or 47K

    The same value could also be yellow-purple-black-red-brown for a 1% device.



    Driving lights and relays and things from your mobo
    Last edited: 21 Apr 2005
  2. g0th

    g0th What's a Dremel?

    12 Apr 2005
    Likes Received:
    Damn post length limit...

    As you can see, most of my so called guides are just rough ideas and schematics, but i hope you benefit from them, and feedback would be good too.

    Using a Nokia 3315 or 3310 LCD with LCDinfo


    I noticed someone wanted a stepper motor controller in the wanted guides thread, so here's a good one (external site, not my work)


    HDD activity chaser display

    PWM control for Luxeon or LED arrays

    Boosting the output current of a 3-terminal regulator

    A basic baybus


    Basic Mobo power switch using a Qprox chip.
    Last edited: 21 Apr 2005
  3. SteveyG

    SteveyG Electromodder

    23 Nov 2002
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    Some of your links don't work, and some towards the bottom - e.g. the Luxeon PWM driver don't match up.
  4. g0th

    g0th What's a Dremel?

    12 Apr 2005
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    Should have fixed most of that now.
  5. mandy_modded

    mandy_modded Minimodder

    12 Mar 2005
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    Hi Goth,

    nice to see another oz modder :rock:

    i saw the oatley link, just one question (or 2), what software is needed to drive the display.

    haven't seen any displays irl yet, they do seem popular in many of the mods i see online, but the threads don't often mention what info the lcd/vfds get used to display.
  6. g0th

    g0th What's a Dremel?

    12 Apr 2005
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    It's your standard Hitatchi chipset Alphanumeric LCD, usually connected to the parallel port. So there's heaps of software. CrystalControl, LCDsmartie, and heaps of others that a bit of research will turn up.

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