1-Wire Input Board

This project provides monitoring capability via opto-isolated inputs using a Dallas 1-Wire network for control by our Home Control System (HCS). The project is used to monitor PIR sensors, magnetic contact sensors on doors and our twilight sensor.

As with the rest of our Home Control System (HCS), this project will be powered from our Home Power System (HPS), to ensure reliability, security and resilience.

Design

The circuit provides input capability to our Home Control System (HCS) using our 1-Wire and a Dallas DS2405 devices. The basic circuit/layout is shown below and supports 8 optically isolated input using the ILQ74 quad opto-isolator. The inputs are configured to support a switched 12V signal via a PIR or door switch and can be active high or active low. To limit the current into the ILQ74 we are using 5.6KΩ resistors, which limits the input current to about 2mA at 12V.

Our 1-Wire network uses Cat5 cable and one of the wires provides 5V power. This is used to provide an input to the DS2405 devices, via the transistor on the opto-isolators. The circuit is covered in more detail in the input design section.

The basic board supports two ILQ74 ICs and eight DS2405 devices, supporting 8 input lines but, the boards can be daisy chained to provide many more inputs if required but, we are mainly using USB I/O boards in our Home Control System (HCS) as they provide better response times.

This is our circuit layout (Microsoft PowerPoint) using Vero board.

Connectors

All the devices in our home like PIRs and door sensors are connected using standard 6-wire alarm cable and are terminated using 0.1 Inch straight 4-way sockets to fit onto matching PCB mount headers. This means we have a high degree of flexibility in swapping I/O interfaces and technologies. All of the PIRs could plug into our 1-Wire network, an Ethernet IO board or a USB IO adaptor for example.

Status

This project was completed and is in use in our current home.

Conclusions

  • The 1-Wire technology provides a secure and reliable means to control devices like this at very low cost. It is not a fast sensing technology so is better suited to devices where there are not rapid changes of state such as dusk/dawn sensors, door contact sensors etc. We have moved the PIR sensors onto our USB I/O board.
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