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Bypass photoelectric alarms

Photoelectric beams are another form of motion detection…sort of. Sort of? This type of motion detector is somewhat different than the rest. You have probably seen many forms of them and not known it. An example would be the annunciation device used in commercial establishments to let the proprietor know when someone has walked in the front door. Some dry cleaners happen to have one of these. Most of these are nothing more than a set of photoelectric detectors hooked into a control and a buzzer.
The detector itself is quite a simple device consisting of a transmitter and a receiver. The transmitter emits a beam of light that is sent to the receiver. The light in the older models is quite visible; the newer models, however, use more of an invisible source and are virtually impossible to see. Please see the picture at left for the various photoelectric protection patterns.
When the photoelectric receiver no longer sees the source of light, it opens it’s internal switch and signals an alarm. This is done very quickly. A break in the light source for as little as a second will trigger an activation of the alarm system. So, guess what happens when the intruder walks into the invisible beam of light? You guess right-the beam of light is broken and sensed by the receiver, which activates a small switch inside the unit and causes a break in the protection loop. The alarm master control senses this and, yes, the alarm system activates all of the bells and whistles that are attached to it.
This type of detection device is designed to operate over both short and long distances. Some models are designed to go a few hundred feet and some a few thousand. They are primarily used in two applications. The first is a large warehouse situation where a motion detector simply would not reach far enough. For instance, if you were trying to protect a long row of roll-up doors or a long aisle in a trucking company building, a photoelectric detector could be used to send an invisible beam across the entire length for excellent protection at a minimum price.
The second application might be in an outdoor area such as a car dealer lot or an open storage yard. Outdoor units generally are placed in special enclosures that are tamper-resistant and equipped with small heaters so that the cover of the detector does not fog up or collect morning dew. What I mean by tamper-resistant is that the unit may have a smaller tamper switch connected to the alarm master control that will sound an alarm when the cover is removed. If the detector is so equipped, it is normally in operation only when the alarm system is on.
Now we’re ready for the fun part. Circumventing is fairly simple. Let’s review. We have two units to be concerned with-the transmitter and the receiver. Since the transmitter normally only transmits light, we don’t need to be concerned with it at this time. The receiver, on the other hand, is where the critical pair of wires are. As in the other circumvention techniques, the system needs to be in the off position.
The key is to identify which unit is the transmitter and which is the receiver. Not a difficult task. Look at both units. The transmitter is probably only going to have one pair of wires running into it. That pair is used to power the light source. On the other hand, the receiver should have four wires running into it-two wires for power and two going to the protection loop. It may also have a meter or monitor light on the front to let the service technician or alarm user know whether it is working or not.
As with the other motion detectors that we’ve previously looked at, you must identify the two wires for power and the two for the alarm loop connection. You don’t want to disturb the power wires. Instead, you want to carefully disconnect the pair that connect to the alarm protection loop. Strip back about an inch of insulation and simply twist wires together. Don’t forget to replace the cover. All done. It’s out of service, but since the power is still connected, it will look like it’s operating perfectly.

This entry was posted on Friday, September 5th, 2008 at 11:27 am and is filed under Alarms. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.