I'm trying to make a logical NOR-gate using a 74HCT02 IC (datasheet), but I cannot figure out how to wire it. Below is a schematic of what I tried.


simulate this circuit – Schematic created using CircuitLab

The power source (V1) is coming from a transformer connected to the power net, it gives a 4.5 VDC output at 3.6VA maximum.

What I observe is that the led is always on, with little intensity difference when I press or release either button.

I do not have much experience regarding circuits with IC's, so I might be making some beginners mistakes here.

  • \$\begingroup\$ Add pulldown resistors on 1A and 1B (anywhere from 1megohm down to 1 kilohm will be OK). At the moment these inputs are not 0V with the switches open. \$\endgroup\$ – Brian Drummond Nov 14 '15 at 16:01
  • \$\begingroup\$ Thanks, I added two 3000 ohm resistors (see the updated circuit). Now the led is still lit regardless of the button states, although it does burn much brighter now. I measured 4.5VDC over R1 and D1 together, whereas before the voltage would fluctuate. Did I add the pulldown resistors incorrecly, or is there another problem with my circuit? \$\endgroup\$ – JeroenD Nov 14 '15 at 16:20
  • \$\begingroup\$ What voltage do you get on pin 1A with its button (a) unpressed, (b) pressed? \$\endgroup\$ – Brian Drummond Nov 14 '15 at 16:23
  • \$\begingroup\$ Between pin 1A and the VCC I measure 4.5 VDC, both when pressed and released. I also tried to connect the wires directly to make sure that the button is not broken. \$\endgroup\$ – JeroenD Nov 14 '15 at 16:27
  • \$\begingroup\$ It's normal to use GND not VCC as the reference. This suggests you'd get 0V both times, pointing to a broken or disconnected switch. (should be 4.5V at the other end of the switch...) \$\endgroup\$ – Brian Drummond Nov 14 '15 at 16:35

As drawn, your inputs to the gate will always be low. The resistors R2 and R3 pull it low (to ground/the negative side of the supply). Then when you push the switches they connect the inputs to low again, so they have no effect. Rewire the switches so when you push them they connect the inputs to the high side of the supply, that way you will get low until you push the switch, then they will override the resistors and provide high.

  • \$\begingroup\$ This indeed worked! I updated the circuit such that the buttons use the + of the power supply instead of the -. Thanks a lot. \$\endgroup\$ – JeroenD Nov 14 '15 at 17:10

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