The majority of the sensors and devices used in our home automation are 12V and provide 12V signals back to our Home Control System (HCS). Most of the input boards and computing equipment work on 5V dc logic levels though. To bridge this gap, we are using a number of techniques and circuits to simplify the connection of sensors:

Capacitive Switch

Capacitive switches have application in places where a safe switching capability is required, with no possibility of people being exposed to dangerous voltages. They make particular sense in bathrooms and other areas where their are high moisture levels.

We are currently investigating suitable switches or switch circuits.

Optical Isolation

Optical isolation provides a means to keep 12V signals (and any interference or voltage spikes) away from the sensitive computing side of our Home Control System (HCS). This is achieved using ILQ74 quad-opto isolator. The input side requires a resistor to limit the current through the LED. We have used a 5K6Ω resistor to limit the input current to around 2mA at 12V.

In this circuit to our USB IO board we use the opto-isolator in pull-down mode, which means the input is basically inverted. This is actually a benefit because the PIR sensors and reed relays used in door sensors have normally closed contacts.

1-Wire DS2405

The Dallas DS2405 can be used as an input or output device. This diagram shows it configured in input mode, with the photo-transistor pulling the PIO line high when active. This assumes a local 5V supply is present. The 1MΩ resistor is used to pull the input low when not active.

The 100Ω resistor in series on the data line is optional and only required when there are many devices loading the 1-Wire network.

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