Photobeam types

Photobeams are the staple safety device for gate automation. Are they all the same? We take a look at some new developments.

Author ;     Huw Jones

Photobeams, photocells, magic eyes, infrared beams; we may have different names for them, but we all know where to find them. These familiar devices are often the first sign that a gate has been automated. 

Photobeams are are standard issue in all automation kits though a single beam is seldom enough for a gate to be considered safe. Beams can be used as a sensor to open a gate, but are likely to let your dog or children out on the road.

Problems and diagnosis

The beam is infrared light at about 850nm, so they cannot be seen by the naked eye. Don't be conused by a red light you see in some beams; it is only an indicator. The beam is very low level and quite safe to the human eye.

Although infrared is invisible to our eye, mobile phones often respond to the IR waveband. Set the camera to video. You may only see a pin prick of light. A mist of warm breath on a cold morning also helps. Remember, the beam is only emitted from one side of the driveway.

Photobeams are easy to check. The receiver side may make a click sound as the beam is blocked, and there is often an indicator that will change. 

A photobeam is a nice warm dry place for spiders to nest. A photobeam that doesn't work often needs cleaning out. A spray of WD40 will keep insects out. Other common faults are blown fuses or burned out IR LED (the emitter).

Standard photobeam on a gate post

A common failure of photobeams set between the gate posts is getting clouted by a car. These 180° beams send their beam sideways. They mount on the front faceof the post, out of harm's way.

Battery photobeams

The nature of safety provision is the need to add devices. There is not always enough wires in the right place. Photobeams with battery powered transmitters can be a life saving solution, but be sure to test the batteries regularly!

Retro-reflective beams

You have probably walked through a reflective beam in a convenience store. The transmitter and receiver are housed together. The beam is returned by an omni-directional reflector. 

Retro beams may not be used for safety purposes but make good warning or activation devices.

Photobeam distance

Beam range is determined by the power of the beam and the angle of spread. One could design around an IR emitter with 5° beam angle, but alignment would try the patience of any installer and lead to unreliability. Power delivery is the answer. 

Imagine a 1W bulb with a 25° beam angle. Double the power and you will blow the bulb. Double the power for half the time keeps the average power at 1W. Power the bulb at 100W at 1% duty cycle will increase the range by 100 fold. Happy days.

Standard photobeams

The most common photobeam set has a transmitter with two terminals, and a receiver with five contacts. A standard set could be defined by both sides being powered with 12 to 24Vac. The receiver also has relay contacts.

A simple way to comply with safety requirements is to create a ring of beams around the area within the swing of the gates. Standard photobeams need only power supplied to the post mounted photobeams. The transmitter and receiver are connected as a repeater pair.

Every manufacturer makes a standard set of beams. Standard beams are simple, versatile, cheap, and interchangeable. But what if the receiver is positioned where it can detect two transmitters? Which one to choose?

Simple sychronised beam pairs achieve this in the wiring. They look like standard wiring beams (2 terminals and 5 terminals), but are not interchangeable.

A ring of beams protects the risk zone

Synchronised photobeams

Consider a gate post with 2 photobeams. To prevent confusion, the two beams could go in opposite directions. Fine for 2 beams , but what about 3 or more beams!

Synchronised beams ensure a receiver only accepts the transmitter it is paired with. Simple synchronised beam pairs achieve this by wiring the receiver & transmitter pairs together. The receiver sends out a random pulse, so knows exactly when to expect the beam return.

This wired solution works so long as those random pulses don't overlay. The Came DXR20 can be mounted in goups of 8 beams. A terminal on each pair is used to put those random pulses in an ordered sequence.  

An infrared curtain is an all in one solution. The Came BI8120 is a 2m high curtain with 8 pre-wired synchronised beams, typically used to detect objects as small as a hand getting trapped in a vertical barred sliding gate

A curtain of photobeams

Laser technology

A laser diode is a more refined light source than the humble infrared diode. It is concentrated in a narrower beam so that power is relatively low.

Linear laser sensors are used for detecting objects. They measure distance by timing the sending of a pulse out, to the receiving of the echo back (ToF). Sensors can be selective about signalling the presense of objects based on distance.

Scanning laser sensors use a rotating prism to distribute a linear beam over an area. A 3D picture can be built up. These devices are used on autonomous vehicles, and more recently to identify foreign objects in the path of a gate.

Laser scanner creates a protective curtain