Abstract: Embodiments disclosed herein describe an AC power line instruction system which can control the operation of electrical device through the AC power line. The AC power line instruction system comprises a modulator to modulate the timing of the leading edges of the voltage of the AC power line to transmit a command signal, and a receiver to measure the timing of the leading edges of the voltage of the AC power line, to decode the command signal based on the relationship between the timing of the leading edges, and the operation of the electrical device can be controlled by the command signal decoded by the receiver.