On an AND logic gate, how can you proven triggering a high when no input is recieved?

Question

for example on a 7408 Integrated circuit if pins 1 and 2 are not connected to anything, then this will trigger a high. How con this be provented without altering the general function of the gate?

Answer 1

Use "pull up" or "pull down" resistors to control the desired state of your inputs when there is no signal.

Pull up means connecting the input to Vcc by a large resistor.
Pull down means connecting the input to the ground by a large resistor.

Answer 2

You should never leave unused inputs unconnected or you will get all sorts of strange effects. Set them to high or low logic level as appropriate by connecting them to +5V or ground through a resistor -- 10kohm is about right.

If you still have trouble with your logic circuits, remember that if you are clocking at above a few MHz you should decouple every chip with a small capacitor from +5V to ground placed physically close to the chip. This prevents activity on other chips feeding back through the supply rail, which can cause very puzzling effects. A 10nF ceramic will usually do the job.

Answer 3

With an AND gate or a NOR gate you can simply tie unused inputs to the nearest used input. For OR gates and NAND gates you definitely need pull-up or pull-down resistors.

Answer 4

Use a pull-down resistor.

This is generally a 1kohm resistor connected between the input and ground.