RF Control Interface: Parallel interface for remote appliance switching

Published:2011/8/8 4:56:00 Author:Amy From:SeekIC

Karl Kockeis

There are a number of ’home automation’ systems available that use a hand-held RF control unit to switch household electrical appliances remotely. This article describes some simple interface hardware to link the control unit to your computer’s parallel port and allow automatic control from your PC.

The vast majority of printers man­ufactured today connect to the USB port on your computer. This means that the parallel printer port is often left just gathering dust. The interface described in this article connects to the paral­lel port and uses very simple and inexpensive hardware to convert a commercially available RF appliance controller for automatic control from a PC.

The computer parallel port has an eight-bit wide data bus (DO to D7) available on pins 2 to 9 of the 25-pin sub-D printer port socket. Each of these eight signals is used in this circuit to independ­ently switch eight transistors. These transistors are in open-col­lector configuration and wired in parallel to the pushbuttons on a commercially available control unit so that when the control soft­ware switches one of the transis­tors for approximately 0.5 s, if has the same effect as if that but­ton were pressed manually. Resis­tors Rl to R8 serve to limit the transistor base drive current.

A standard printer cable (make sure the connectors are not the moulded-on type) is adapted to connect the PC to the remote con­trol handset. The 25-way sub-D connector housing is first disman­tled and each wire connection desoldered. Any cables not required can be clipped off to give more space inside the hous­ing for the eight transistors and resistors. With reference to the cir­cuit diagram, solder the eight resistors and transistors between the 25-way connector and the cable. Ensure that the finished assembly v/ill fit inside the con­nector housing without shorting when it is reassembled. Next sol­der the 9-way sub-D connector at the other end of the cable. After testing, the connector covers can be reassembled.

It is necessary to fit a 9-way con­nector to the hand-held unit to provide a connection to the com­puter cable. Whenever manual operation is required it is a sim­ple matter to just unplug the unit from the PC cable and use it as normal. The unit must be opened-up to fit the 9-way sub-D connec­tor as shown in the circuit dia­gram. First locale the earth track on the PCB (the battery minus connection) and solder a wire connecting this point to pin 9 on the connector. Next identify the connections for the eight switches in the unit and solder eight wires lo the PCB pads to connect these with the correct pins on the con­nector. The unit will be ready for use once the cables have been tidied and the unit reassembled. The connector will now be acces­sible via the battery compartment.

So much for the simple hardware, control software for the unit is also available and will be fea­tured in an upcoming edition of Elektor Electronics so be sure not to miss out!

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