Liquid flow[]
Liquid flows are "natural" transmission components. It can trigger a wash-out detector.
Minecart track[]
Minecart tracks are pathways that minecarts can run along.
Greenstone wire[]
A "greenstone" wire uses cactus mechanics that cause them to break when there's a solid block adjacent to them.
Variations: Cactus height can be increased to increase the "circuit" delay. Also, torches can be used, but they seem not to work in Pocket Edition.
TNT wire[]
TNT wire uses primed TNT to ignite other TNT, to form a chain reaction. TNT can destroy blocks, so it can be used to trigger a block destruction detector. However, this wire is laggy, expensive, has a random delay, non-renewable (in Survival) and can destroy blocks around it, unless encased in blast-resistant blocks.
Variations: This wire can be made one-way only by using advantage of primed TNT obeying gravity:
Sign chain[]
When a sign is broken, attached signs are broken too. This chain can be used as an instant circuit.