i don`t get why when iam using a spst switch with these logic gates the circut dont implies voltage and i have to use that logic gates (shown in the image)? and another thing how i can make 2 pads in the pcb layout for an out source without determining that it is a battery ? thanks


I'm not sure I understand your question - however...

The 7404 is a bipolar TTL part. TTL inputs source current, and will appear as a logic High if not connected. They must be connected to ground, either directly, or through a low value resistor, to be seen as a logic Low, so you should connect your switch between the input pin and ground.

Also, a TTL output can sink much more current than it can source, so it is best to connect your LEDs and the required series resistor between the output pin and the positive supply rather than between the output pin and ground. You must have a resistor in series with the LED to limit the current through the LED - 300 - 400 ohms would do.

Your circuit should be something like:


simulate this circuit – Schematic created using CircuitLab

R1 is optional, but recommended.

I don't know Proteus, but it is likely that you have to connect the 7404's Vcc pin to a +5 volt power source, and connect its ground pin to the circuit ground, in order for the simulation to work.

  • \$\begingroup\$ i didnt get what you exactly mean i tried to connect switch with 5v inputs with a resistance and the led and other terminal i tried to connect it to ground like wht i get from you but i found that the first term of invert i wont able to connect it anymore you also said "connect 7404`s vcc pin to 5v etc.. " yes that would work for the ic but in the simlution there is only 2 terminals in and out no vcc or ground any way thanks for take a time to answer this \$\endgroup\$ – Mai Ehab Dec 12 '15 at 23:41
  • \$\begingroup\$ That didn't make any sense at all apart from the "thank you", What is the world coming to? \$\endgroup\$ – Andy aka Dec 13 '15 at 0:40
  • \$\begingroup\$ @peter thanks for adding the schematic it helped and the led now respond.. thanks so much \$\endgroup\$ – Mai Ehab Dec 14 '15 at 20:45

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