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Discussion in 'General Electronics Discussion' started by kap, Sep 10, 2013.

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  1. kap


    Jul 15, 2013
    Hi guys,
    I am experimenting & trying to learn Logic IC's, I have few with me and verifying the working on bredboard, but non of them giving me correct output.

    For example I am working on "DM7408n" it is a quad 2-input AND gate,
    1)Power Supply --->I connected pins Vcc + Gnd,

    2)Data Input--->I connected pin A1,B1 to high(Vcc) & I placed switches between these two inputs so that I can give the different inputs.

    3)Data Output--->I connected LED between Y1 & Gnd.

    when I turn On A1 & B1,LED dose not works, in fact LED never lights up.

    I checked the power supply voltage,it is 5V & it is OK,

    so what could be the reason any suggestions or help?

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    Last edited: Sep 10, 2013
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    there are a number of problems.

    1) TTL pulls down hard, but kinda floats high. A high output can't really drive much of a load. A red LED might light dimly, a white one won't. Green LED... maybe (is it around the right way?). Alternatively you can use a transistor to drive the LED.

    2) you need to use a resistor with your LED. If you've already "tested" it with your 5V supply without a resistor, it's probably damaged.

    3) A TTL input will float high if disconnected. I would recommend pulling the inputs high with a 1k resistor and using a switch to bring them down to 0V (gnd). The way you have them now, maybe a 470 ohm pull-down resistor might help.

    Now, if you have your LED and resistor connected between the output and +5V, and the inputs connected as I suggest, you will be using inverted logic. This will turn your AND gate into an OR gate.

    It might be simpler if you used CMOS (74C08, 74HC08, etc). But beware that if you do, all inputs must be connected somewhere. Connecting all unused inputs to ground.

    CMOS outputs can drive a LED (use a 2k2 resistor with a red LED and the output will remain valid with most chips)

    Pull CMOS inputs low with a 100k resistor and use a switch to pull them high.
  3. kap


    Jul 15, 2013
    thank you 'Steve' for the details.
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