The full bridge in the input enables the operation of the SCR with both the negative and positive semicycles of the power supply voltage. The diodes must be rated for the voltage and current required by the load. The switch can be replaced by a sensor.
Figure 1 Full-wave ac switch I.
(*) Indicated to currents up to 1 A. For heavier loads use the 1N4004 and the 1N4007 for the 220/240 AC power grid.