Redstone circuits/Logic/OR

A device where the output is on when at least one of the inputs are on. Note that since the OR operation is associative and commutative, OR gates can be combined freely: You can OR huge numbers of inputs by using small OR gates to collect groups of inputs, then OR'ing their results. The result will not depend on the arrangement of the inputs, or on which ones were combined first.

The simplest version of the OR gate is design  A : merely a wire connecting all inputs and outputs. However, this causes the inputs to become "compromised", so that they can only be used in this OR gate. The introduction's example, using a solid block instead of wire, does not suffer the same hazard.

If you need to use the inputs elsewhere, the inputs need to be "isolated", by passing them through a device such as a torch or repeater. Torches yield version  B . Note that this is in fact a NOR gate with an inverter on the output.

Version  C  isolates the inputs with repeaters. It can be expanded horizontally up to 15 inputs. Besides the isolated inputs, it is one tick faster than B. Additional repeaters can be used to add new groups of inputs, or to to strengthen the output signal. This design is more expensive, as each repeater costs 3 redstone dust to craft (along with smooth stone).

Version  D  is designed for vertical use, such as in walls. Though its inputs are not necessarily isolated from its outputs, this can be easily fixed with a one-tick repeater immediately after the lower input. This is the only version that can only take two inputs, but it is only 1 block wide.

Version  E  utilizes the properties of light-transparent blocks, like half-slabs and glowstone. These send signals up, but not down. It is expandable, like design  C .