The OR gate
The OR gate gives 1 when at least one of its inputs is 1. The output is 0 only when every input is 0. It is the logic version of “or”: the alarm rings if the door or the window is open, or both.
Formula Y = A + B
Truth table
| A | B | Y |
|---|---|---|
| 0 | 0 | 0 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 1 |
Try it Click the inputs to change them.
Y0
Where it is used
OR gates combine signals: any one of several sensors can raise an alarm, any key on a keypad can wake a device. In the full adder, an OR joins the two possible carries into one.
Careful with everyday language: when people say “tea or coffee” they usually mean one of the two, not both. That “one or the other, but not both” is a different gate, XOR.
Build it yourself
An OR gate can be made from three NAND gates, thanks to De Morgan’s law. Try it in the OR from NAND challenge.