im trying to build a half/voll-adder with XOR and AND IC's. (For learning reasons)

When i use one switch i should have

1 0 = 1 and the LED is on

When i use both switches i should have

1 1 = 0 and the LED is off.

I tried both and nothing happend to the LED :(

Whats wrong with my circuit?

Thanks for the answers?

74LS86 Datasheet: Quad 2-Input Exclusive OR Gate http://www.alldatasheet.com/datasheet-pdf/pdf/46213/SLS/74LS86.html

My breadboard

I think the LED is round the wrong way.

Sorry, i don't understand :( ? What do you mean?

LED's are diodes (hince the name Light Emitting Diode), and will only let electricity flow in 1 direction and not the other (in a perfect world), if the led is wired up backwards then it wont light

try flipping it around

You can put the LED in two ways, you want to have it so the long leg is connected to the resistor and the short leg to the ground.

I flipped the LED but nothing happened.

The LED is working and i tried a AND ic and got the same problem.

Something is wrong wired and i cant find it :(

I tired to make a circut diagram to find the error:

IC Pin 7 to GND IC Pin 14 to 5V

if the inputs on the 74LS86 float high, then flipping the switches S1 and S2 will have no effect. They will go from "floating high" to "forced high". For an XOR this means: both inputs are high, so output is low. Output is low means the LED is off. So, to fix it, put a "pull down" resistor on the two inputs of the XOR gate (e.g. 10K ohm or so) so that if the switch is open, they are pulled low, but of the switch is closed the switch forces the input to high.

I'm not sure why the AND gate failed to work however. If both inputs are high with the AND gate, then the AND output should be high and the LED should have illuminated.

hope this helps.


thanks for the reply.

I replaced the XOR gate with an AND gate and turned both switches on. But nothin.

Here a picture:

a NEGATION IC (74LS04) also dont work :(

The ic has GND and V5

You need to listen to the advice, I think.

TTL Logic is very literal. I can clearly see why your states are not changing and other people here have also told you.

The INPUT on lots TTL logic gates is actually the emitter of an NPN transistor. You clearly are attempting to PULL them high, since your switches are connected to the +5 bus. In essense, like was already said, you are changing a FLOATING UNTERMINATED (aka high input) to a PULLED high input which = no change.

You need to HOLD your input pins (at least on the XOR or NAND gate) at logic 1 by attaching a 1K resistor from the gate pin to +5V.

To make the pin change state, you need to pull the pin to GROUND potential with the switch. So, move the Yellow and green wires on the horizontal +5 buss to the GND bus and try again.

I think you are assuming that unterminated = GND (or low) and that’s not quite true.