Minecraft wiki entry for Pulse, associated text: An observer watching the input signal (redstone dust, button, repeater set to 1 tick, etc) will produce a pulse on each of the edges of the input, producing two 1-tick pulses on each edge of the input pulse, providing the input pulse is sufficiently long (3 redstone ticks minimum). If the pulse is shorter than this, a redstone lamp can be put infront of the observer to remedy this issue.