XOR via Nands with Transistors

Discussion in 'The Projects Forum' started by brandini, Mar 18, 2008.

  1. brandini

    Thread Starter Member

    Feb 25, 2008
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    All voltages are 12V but the site where I got the circuit from was using 6V and I'm not sure what to do to pick the right resistor values. The two Ls in the picture are logic inputs and when high are 12V. Ignore from the left of the 6K resistors to the right. This point is the output and will either need to drive a relay or be able to output 1A at 12V.

    So my questions are:
    1) Do I really need a relay or by answering 2 and 3 can you meet the requirements?

    2) What is a correct resistor value for all the 10K resistors

    3) suitable transistor model numbers

    Please include all 3 with your answer to fight confusion between posts, Thanks!
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Why not use a CD4070 (XOR) or CD4077 (XNOR) CMOS IC, driving a Darlington (TIP120) or power MOSFET (like an IRF510, IRFZ24) instead of trying to build it from discrete components?

    BTW, the TIP120, IRF510 or IRFZ24 will be able to SINK 12V @ over an amp, but they cannot SOURCE 12V. Your circuit will be simplified if you can supply the ground to your load, rather than have to source current.

    If this is a homework problem, it's posted in the wrong area.

    No.

    10k can be used for 12v; it will result in a maximum of 12mA current flow when 12v is placed across the circuit.

    See the attached. Note the changes, in particular the output section. It was necessary to add an inverter to get the P-channel MOSFET to work as a source.
     
  3. brandini

    Thread Starter Member

    Feb 25, 2008
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    0
    Man with software like that I sorta wish I didn't have a mac...

    So the output changes you made were to be able to drive the 5watt load correct?

    And if I were to try to wrap my head around what you have suggested would be taking my inputs into the XOR, amplifying them with the Darlington, and the mosfet to allow greater output it be describing it correctly? Would this be cheaper and/or simpler?


    And no it's not homework, I'm putting this logic together to use inputs in a car turn signal to output a parking light/turn signal combo since my side-marker on my car is useless to traffic beside me and it's starting to piss me off now that I live in a more urban area.
     
  4. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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  5. SgtWookie

    Expert

    Jul 17, 2007
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    Don't regret - I frequently dislike Windoze... ;)

    If you examine the output of the P-channel IRFI9Z14G power MOSFET, you will see a green circle with a "C" inside of it. That green-circled "C" test point is represented by the green trace on the simulated O-scope, below. "Rload" is a simulated 12 Ohm resistor. Since I = E/R (Current = Voltage / Resistance), and the voltage at point C is approximately 12, there is approximately 1 Ampere of current flowing through Rload.

    I added an inverter to the basic XOR circuit to change it to an XNOR (inverted XOR) in order to drive the P-channel MOSFET gate.
    A CMOS 4077 IC contains four XNOR gates. This will save you a considerable amount of wiring. You will need to connect all of the unused inputs to either ground/Vss or Vdd though, or the IC may oscillate on it's own, causing overheating and other undesirable things.

    Instead of the IRF9Z14G, you could use a TIP125, TIP135, TIP145, which are PNP Darlington pairs with increasing current capability (TIP125=4A, TIP135=8A, TIP145=10A). You would need to connect the output of the 4077 gate to the base of the Darlington via a 2.7K resistor to limit current. This would allow up to about 2.2A of output current from the Darlington at an hFE of 500. You would likely need to use a heat sink.

    I see. Have you considered using wide-angle super-bright LEDs? They consume less power and last far longer than incandescent (filament) bulbs do. If you want to use red, you actually need to use a red-orange color in the States; a "pure red" is not street legal. You could also use amber.
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    You can save a few parts...
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Even fewer parts.

    Ignore the signal generators on the left; they're just to simulate input signals.
     
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I got the impression from his previous thread (Brandini, why did you start a new thread?) that he didn't want to use CMOS, perhaps because of the vehicle's power supply noise. I posted a similar circuit using a CMOS IC with circuitry added to protect the IC.
    Below is a bipolar version with only three BJTs and a PMOS driver.
     
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