Photocells or Photo Electric Beams are basically a beam of invisible infrared light that passes from a transmitter unit to a receiving unit and if broken 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 with auto-close function.
There are three types of Photocells including:
- Reflector Beam - this has the transmitter and receiver at one end with a special reflector at the other. The advantage of the reflector beam is you only need to run wiring to one side of the driveway. REFLECTOR BEAMS ARE NOT SUITABLE FOR OUTSIDE USE as any rain on the reflector can stop it working.
- Point to Point Beam - this is best for Automatic Gates as there is a transmitter on one side of the gateway and the receiver on the other. These tend to be very reliable when used used outside.
- Dual Beam - which is basically two point to point beams built in the one unit. You must break both beams before it will work. Ideal if you have a problem with the sun shining on the receiver at certain times of the day, which isn't normally a problem with Automatic Gates because a vehicle going through the gate will block the sun as well as the transmitter beam. These are normally used for outdoor security perimeter alarms.
Alignment
The most important thing when installing Photocells is to make sure they are lined up with 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 work and then as soon as there is some rain will stop working leaving the gate open.
Height
The height of the Photocell is also 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 single lane gateway is also used by larger trucks you can put the Photocell at 600mm at one end and 900mm at the other so you get a better coverage, but you do need to line them up properly other wise they may not work. An adjustable angle Photocell is best used here.
For 2-way gateways that are used by larger trucks you really need to install two sets of Photocells at different heights to get proper coverage. One should be set at 600mm and the other 1000mm from the ground.
Swing Gates Two Photocells Needed
For swing Gates you need 2 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.
Photocell control closing of the gate
The PC input of an e-Gate control board can be set to control closing of the gate. Basically once a car clears the Photocell the gate will start closing straight away rather than waiting for auto-close delay to time out. This is very handy for preventing unauthorized vehicles from following vehicles through the gate.
If Photpcell closing control is ON and a few pedestrians walk through the Photocell can cause the gate to reverse back and forth unnecessarily. To prevent this happening a delay can be set so once the Photocell is clear the gate won't start closing until the delay has timed out. This delay can be set from 1 second to 60 seconds and is independant from the auto-close delay.
There is also the issue of when an in ground loop detector is used with a 2 way gate, which opens the gate automatically for vehicles exiting a property. For vehicles Exiting the photocell will close the gate as soon a vehicle is safely through but for incoming vehicles they may also be detected by the in ground loop detector so the gate will be held open for longer. The e-Gate control board passes the closing control onto the in ground loop detector if the Photocell is activated first, this reduces the time the gate will be open to once a vehciles clears the in ground loop rather than waiting for the auto-close delay.
Vandal Proofing
If the Photocells are in an area where they could be vandalised then a protective cover needs to be added with a small hole in it to let the light beam through. An aluminium diecast metal box is normally ideal for this with security screws being used to secure it.