Making a 3 to 8 decoder, grounding problems.

Discussion in 'The Projects Forum' started by Dakkan, Jun 10, 2008.

  1. Dakkan

    Thread Starter New Member

    Jun 10, 2008
    3
    0
    Hello everyone.

    This is my first serious attempt at any sort of electronic work with a plastic breadboard, and my supplies are limited so I've been working around some issues.

    When it comes to logic gates, a 3 to 8 bit decoder is no problem, but I'm having serious problems translating into electrical work. My biggest problem currently is working with ground. As of right now I'm using a NAND IC to make inverters, except it hardly works. I'm getting incorrect charges when I shouldn't, and I'm sure I'm doing something inapropriate with the mysterious ground that my teacher never really talked about. I've drawn up a example of what I have so far.

    Am I supposed to send the ground back somewhere? Or jab it into the dirt or something? :D As of right now, here's what my multimeter tells me: When all three switches allow current to flow into the circuit, the gate works properly and outputs low voltage (0). When you start to mix and match however, the one that is supposed to give me a high output, seems to return the full 6 volts, however the other's give me about 1 or 2 volts less, I think a website called that a Hi-Z error, relating to grounding issues. I'm not certain at all really! And when all 3 switches are open I get power through all three wires like I'm suppose to, but given I'm always getting power through them unless their all closed, I don't know if it's "correct" power, I guess.

    Any thoughts? I have no resistors or capacitors hooked into this, and I hope I won't need them, though I think that's a fantasy haha. :D

    Also the "switches" are for now me pulling the wires out of the breakboard to break the flow, not exactly a real switch.
     
  2. kiwisparky

    New Member

    Jun 7, 2008
    2
    0
    Might I politely suggest you take the ground to the other terminal of your 6 volt power supply, this being the negative (symbol - as opposed to + being positive). That IS your ground. If you wish you could drive a copper rod into the ground outside your house, and connect that also to negative (-) but it won't make a scrap of difference to your project.
     
  3. Dakkan

    Thread Starter New Member

    Jun 10, 2008
    3
    0
    Okay, Well I sent back the ground to the negative side of the battery, but now when I use my multimeter to test from one of the 3 inverted outputs to see whether I have a 0 or 1, where do I put the other test rod to see if current is passing through? I tried testing from an output to the ground, which THEN goes back to the battery, but all I get now is no power passing through at all.
     
  4. kiwisparky

    New Member

    Jun 7, 2008
    2
    0
    Got any more chips ? I have millions of them.

    Do you have a logic probe in your possession ? If not and you want to play with these chips, grab one, it will save you hours. Much better than a meter.

    The way your drawing looks all outputs will be logic High, and change to logic Low as the switches are closed. Don't expect to draw too much current through these devices, they are designed to switch more grunty devices downstream.

    You should be testing between neg and any other position in the circuit.

    Almost certainly if you tested using your multimeter current measuring feature and put you probes on the chip output and the other probe to negative, it would have put a dead short on the output of the chip.

    Hope this helps.
     
  5. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Please also be aware that a floating input to your gates counts as a "high" input. So your switches are switching inputs from "1" to "1" instead of from "0" to "1." You can solve this by adding "pull-down" resistors from the gate inputs to ground.

    Ground, in this instance, is the negative terminal of your power supply.

    For now, always take your voltage measurements referenced to ground. This means putting your black lead on the one of the conductors connected to the negative terminal of your power supply.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If your CMOS IC is a 4011 or 4093, Vdd is pin 14, and Vss is pin 7. Connect your supply + and - voltages respectively to those pins. I consider a 4093 generally more useful, particularly in your application, because it has Schmitt-trigger inputs. This type of input helps a great deal in "squaring up" output signals from slow rising or falling input signals.

    You must never leave a CMOS input "floating"; that is, you must always have a current path to either Vdd (your + supply voltage, also called Vcc for transistor circuits) or Vss (ground, or your - supply voltage).

    If your inputs are left to "float", the CMOS IC may oscillate at high frequencies, or may overheat due to the internal CMOS circuits being in a partially conducting state.

    Thingmaker3 mentioned "pull-down resistors" - he's absolutely correct.
    I suggest using resistors in the range of 5k Ohms to 10k Ohms wired from the IC side of the switches to Vss/ground/- supply. That way, when a switch is closed (or "on"), there will be a current path to Vdd/+ supply, and when the switch is open (or "off"), the pull-down resistors provide a current path to Vss/ground/- supply.

    It is also customary (IOW, good practice) to use a "bypass capacitor" across the power supply pins. For CMOS and TTL IC's, these are typically 0.1uF (100nF) ceramic or tantalum capacitors. For your circuit, this is not much of an issue since it is for such a low speed. It will become a big issue when you start using clock signals.
     
    Last edited: Jun 10, 2008
  7. Dakkan

    Thread Starter New Member

    Jun 10, 2008
    3
    0
    Alright, I'm on www.allelectronics.com looking at resistors, and there are 3 categories. 1/2, 1/4, and 1/8 watt resistors. They all have the same resistors in said categories, so which ones do I get for this?

    Also if anyone has any other good electronics webpage I'd like to hear them too! :)
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Calculating power:
    P = EI (Power in Watts = Voltage x Current), also IsquaredR (R=resistance in Ohms) or Esquared/R
    Calculating current:
    I = E/R = P/E
    Calculating voltage:
    E= IR = P/I
    In general, you want to use resistors rated at least twice the power you expect to see flowing through it.
    At low voltages, you can use 1/4 Watt resistors for most purposes, unless you're in the lower resistance values (below 2k or so) - then you need to keep an eye on the power dissipation.

    I've never used AllElectronics.
    http://www.Mouser.com - Mouser Electronics
    http://www.Jameco.com - Jameco, Inc. - discounter
    http://www.goldmine-elec.com - The Electronics Goldmine - lots of surplus stuff
    http://www.MPJA.com - good source for power supplies, PCB's
    http://www.AlliedElect.com - Allied Electronics, a major supplier
    http://www.Newark.com - Newark, another major supplier
    http://digikey.com - Digi-Key, another major supplier
    and others - yes.
    Note that many of the major suppliers have a minimum order requirement.
    You can always visit your local Radio Shack and pick up a resistor assortment. It's more expensive that way, but you'll have a reasonable selection of resistors on hand rather than having to order stuff every other day, and having to pay a lot in shipping charges.
     
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