Safety Photocells for Automatic Gates

Photocells or Photo Electric Beams are basically a point to point beam of invisible infra red light that passes from a transmitting unit to a receiving unit and if an object blocks the light you know something's there. These are used with automatic gates on either side of a driveway as a safety device to prevent a gate closing on a vehicle. Normally used when a gate is set to close by itself after a time delay automatically (auto-close).

how they work

3D graphic of car blocking photocells beam of infra red light

Point to Point verses Reflector 

Photocells that have a point to point infra red light beam from a transmitter to a receiver at either side of a gateway are preferred for use with gates than ones with a reflector because reflector types tend to not work as well in the rain or with sun shining on them. Although there is a reflector type that works with an Infra red Laser that is a lot better. 

Alignment

The most important thing when installing Photocells is to make sure they are facing each other. The beam of light coming from the transmitter is quite narrow so if not aimed at the receiver properly it may not work or worse still only just works and then as soon as there is some rain will stop working leaving the gate open.

Height of the Photocell

photocells at different height to cover all vehiclesThe height of the Photocell is important, if too high it may pass through car windows or go right over an empty trailer, if too low it may go under a vehicle, which may cause the gate to operate erratically. Around 600mm from the ground is a good height for cars, vans, utes, small trucks and trailers. 

If a gate is shared by larger trucks then two sets of photocells at different height are necessary.  One at 600mm and another at 900mm usually works well.

Swing Gates Require Two Sets of Photocells

For swing Gates you need at least two sets of Photocells to get proper coverage. Double swing gates are better than singles when using Photocells because the distance between the inside and outside set of photocells is less, where as if you have Photocells on a 4m swing gate, the distance between Photocells must NOT be more than 4m, so if you get a really short car stop between Photocells, the gate still could close on it. It's best to place the Photocells as close to the gate as possible to reduce the chance of this happening.

swinging gates require two sets of photocells viewed from aboveswinging gates require two sets of photocells viewed form front

Photocells can Control Closing of the Gate 

With Commercial Gate Motors the Photocell can also control closing of the gate that is once a vehicle has passed through a gate it will close behind it to help prevent other vehicles following through.  This closing only occurs once a vehicle clears the photocell and can either close the gate instantly or be delayed.  The Photocell delay may be different to the auto-close delay.

Installing Photocells

For point to point photocells you do need a low voltage cable across the driveway.  With sliding gates this is usually simply a matter of installing the cable inside the gate track, if it is hollow, although care must be taken that the cable doesn't get damaged where it enters the track and that the track doesn't have any gaps between it and the driveway large enough for the cable to slip into and be crushed by the next car that drives over it. The photocells can be installed on the gate posts and if these are hollow metal posts cabling can be run inside the post.

For swing gates low voltage cable will need to be run along each side of the driveway as well as across the driveway.  One set of photocells will need stand alone posts.

... more info on installing low voltage cabling

Vandal Proofing Photocells

If photocells are exposed to a lot of walk past traffic the odds that they will be vandalised increases, although if they are small unobtrusive photocells that have a thicker casing this will keep this to a minimum. A metal cover can also be used with a hole in it for the infra red light.  A mesh cage is not so good because vandals can still poke things through the holes and damage the photocells.  A metal box is best because the vandals can't see what's inside reducing their interest level because they like to see it break.

Photocells for Solar Powered Automatic Gates

Photocells can be used with Solar Powered Automatic Gates so long as the power to the photocells can be turned off when the gate is not in use otherwise they can flatten the battery particular with swing gates that have two sets of photocells.  If the controller for the automatic gate has a light output this can be used to provide power for the photocells when the gate is opened so long as it stays on if the gate is held open for any reason.