12V Beam Break Sensor

The objective of this project is to provide a simple 12V powered, long-range, infra-red (IR) beam break sensor with a digital ouput, so that one or more sensors can be connected to our Home Control System (HCS). The circuit design used here can work with the transmitter and receiver being up to 10m apart.

Research

The first thing we did was search to see if we could just buy something suitable off the shelf. Whilst there are many 12V powered beam break sensors, they are expensive and also tend to be quite bulky.

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We bought some of these beam break detectors from Maplin a few years ago. They work well but, they are bulky, battery powered (6V dc) and there is no easy way to interface them to our Home Control System (HCS).

Design

Beam break sensors have two parts:

  1. A transmitter sends a stream of light pulses at infra-red frequencies. We use infra-red because it is not visible to the human eye and is thus not distracting.
  2. A receiver picks up the stream of pulses and looks to see if some are missing due to the beam being broken by an object between the transmitter and the receiver.

We want our receiver design to work with our optically isolated input board, so it works from a 12V dc power supply and connects using just three wires. Both parts need to be very compact, so that they can be easily hidden in the fabric of our home.

Packaging

A key feature of our design is the small packaging. We will be designing the final PCB so that it can be inserted into a hole drilled in walls, door frames, etc. Ideally we are aiming for a package that will fit into a 15mm hole.

Implementation

Having searched around for suitable circuits, we have settled on using these designs:

Transmitter

The transmitter uses the LM556N/NE556N (datasheet) dual precision timer IC to create a 38Khz carrier signal onto a 32Hz pulse train with 30mS pulse width. The original design is powered via 5V dc powered, so we have modified the design to work at 12V dc. The IR LED is driven using a 2N2907 transistor. n

Receiver

The original receiver circuit is based around the TSOP1838 IR receiver module that works at 38KHz. We have modified the circuit to work with the improved TSOP38238 (datasheet). This device operates at 5V dc and so we have a local regulated 5V supply to power the circuit. The final output stage (transistor) provides a 12V signal though.

Testing

We are currently purchasing a few missing components for our design and will be building and testing our circuit very soon.

Applications

Beam break sensors are not widely used in current smart homes. The main reasons for this are that they are quite big things to have installed in your home and they power (more than you really want to supply using batteries) to both the transmitter and the receiver. This typically means that they need a wired installation.

We are planning to install wires for them in our next home and are prepared to run them in our currrent home. This project also delivers a compact transmitter and receiver that can be installed almost invisibly into the fabric our our home.

We have built a solution with a good range, so that it can be used along the length of a corridor or stairway and not just across each end.

Where To Use Them

  • Beam break sensors are really useful in situations where you have an open walkway between two zones, e.g. an archway leading through from the one room to another or a corridor off of a room.
  • Beam break sensors can also be used in door frames where doors are often left open but, they obviously need to be installed such that an open door will not break the beam.
  • Beam break sensors can be very useful outside of your home, to activate lighting and detect intruders along pathways, on drives and in gardens. You can expect the occasional 'false' activation by wildlife if used outside though.

Summary

Beam break sensors are a great way to collect additional data to enable accurate and timely occupancy and presence in a smart home. Careful consideration must be given to where they should be installed and installation can be challenging. Our view is that the hurdles are justified by the benefits that this technolgy brings and we are now installing them in several locations in our current home.

Progress

Dec 2015

We have the circuit design completed. We are now building and testing prototypes.

We plan to get a PCB for the transmitter and receiver parts professionally manufactured very soon.

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