I²C-homebus: Switch power outlets with your PC 1

Published:2011/8/16 3:20:00 Author:Amy From:SeekIC

Paul Goossens

This unit, in combination with the USB/I2C interface described elsewhere in this issue, may be used to switch four mains-powered devices on and off via the PC. Eight of these modules may be connected to a single interface, allowing up to 32 loads to be controlled. The system is ideal for a small home bus!

Triggered by other articles on home automation in this issue, you may feel like investigating the possibilities of automating, or at least remote control­ling, a couple of electrical devices in and around your home. If you do not want to rush off to the high street stores to buy a box full of ready-made modules working on a more or less familiar protocol, then home construc­tion of a small system is a perfectly viable alternative.

An I2C system can be used without problems for a small home bus comprising a couple of features for control and switching. Provided extra bus drivers are applied, distances of the order of tens of metres are easily cov­ered. The connecting cable between the modules is reasonably simple as we employ 6-wire UTP cabling with RJll plugs. Also, there is a wide choice of ICs with I2C compatibility, serving an impressive number of appli­cations.

At the PC side we already have a suit­able circuit in the form of our USB/I2C interface. Thanks to its design, this interface is capable of covering large distances and so forms the perfect starting point for a simple I2C-based Home Bus. At the ’device’ side there are a great many possibilities when it comes to creating an I2C connection. In this article we describe a circuit that allows four mains-powered devices to be switched (TV, lighting, coffee machine, etc.).

Several modules may be connected to the bus, so quite a lot of electrical equipment in and around the home may be ’bused’ in this way.

Keep it simple

As you can see in the circuit diagram in Figure I, the active part of the cir­cuit consists of a remote 8-bit I/O expander type PCF8574 and an I2C bus extender type P82B715PN.



The PCF8574 in position IC3 contains an 8-bit bidirectional port and an I2C bus interface. The eight open-drain outputs can ’source’ sufficient current to drive LEDs directly. Thanks to inter­nal latches, the IC remembers the most recent configuration. Four output pins control the four status LEDs in the cir­cuit; the other four connections each serving a mains output. Actually, the LEDs and triacs are controlled sepa­rately via the bus, but the software written for the system will automati­cally ensure each LED will always indi­cate the status of the associated mains connection (i.e., triac).

Three pins (wired to DIL switch SI) allow one of up to eight module addresses to be set; meaning that up to eight of these circuits may be con­nected to a single bus — and addressed individually. It should be noted that the -A version of the PCF8574 has a different slave address range. If you use the PCF8574A in another eight modules, up to 16 mod­ules may be connected to the bus. The bus extender IC, IC1, acts as a ’booster’, increasing the currents through the I2C bus and so making it less susceptible to external interfer­ence. If, as in our example, UTP cable is used, then a distance of about 200 metres may be covered between PC interface and module — more than enough to ’wire’ your average home we’d say.

Note that the extender IC has to be applied in all modules connected to the bus if it is also present on the USB/I2C interface board (or similar modules of your own design).

The connections with the I2C bus are routed via two RJ11 connectors on the board.

The mains-powered loads are switched on and off using a combina­tion of an optoisolator and a triac. In this way we have a good electrical iso­lation between the mains on the one hand and the circuit and the Home Bus, on the other. The MOC3043 is an optoisolator with an internal diac and zero-crossing detector, which make it ideal for driving a triac.

The triacs used are the common-or-garden type TIC206D. They require a so-called snubber network to protect the device against voltage surges across the anodes. In this case, we’ve added a series network of a 39-_ resis­tor and a 10-nF capacitor for each triac. If you want to use so-called snubberless triacs, then resistors R15/R18/R21/R24 and capacitors C8/C9/C10/C11 may be omitted.

The low-voltage logic circuitry is pow­ered from the mains using a trans­former, a bridge rectifier, a reservoir capacitor and a 7805 voltage regulator. This power supply affords electrical isolation between the circuit and the mains network.

Reprinted Url Of This Article: http://www.seekic.com/blog/project_solutions/2011/08/16/Isup2;C_homebus__Switch_power_outlets_with_your_PC_1.html

Print this Page | Comments | Reading(3471)