A flexible safety interface for a PIC/Logic/PC to mains power controller

This project will enable the constructor to experiment in regulating power to, or simply switching on and off, mains powered equipment, via a PIC or logic circuit, or if used with EPE’s serial interface, a PC, in safety.

The advent of the PIC microcontroller has revolutionised electronics in recent years, and not least to benefit from this has been the hobby sector. The number of components needed for a control circuit has dropped dramatically, and controlling devices that operate at low voltages is relatively straightforward. Controlling mains powered devices, however, is generally not so straightforward, primarily for a very practical reason.

This reason is, of course, safety. Mains safety poses a particular problem for the experimenter – circuits with parts at mains voltage should certainly not be “lashed up” on a patch board, or “bread” board. When working with low voltages, inexperience, or an occasional lapse of concentration, is unlikely to have consequences that are more serious than needing to replace damaged components.

The Safety Interface presented in this article provides a solution to these problems in one relatively simple project. By selecting which components to fit on a single p.c.b., the constructor can build a simple on/off controller, or alternatively a device that will provide either a power regulation controller function or an on/off controller function, by exchanging one socket-based component.

When built and used as described here, this interface can be safely connected to the controlling circuitry via an isolated five-pin DIN connector. Mains power for the interface is obtained by plugging a normal three-pin mains plug into a household mains socket, and the controlled mains output is available on a “flying” household mains socket hard-wired to the interface.

With the interface box cover properly fitted, no part at mains voltage is accessible. This device will allow a relatively inexperienced constructor to experiment safely with mains powered devices, and it should also be very useful to the more experienced user who wants a general-purpose interface to use at the design stage of a project.