Learning to Annunciate

Any engineer will tell you that one of the most important components of designing a system is the feedback element. The user or operator of a system must be made aware of certain conditions as that system operates. While much of our industry is evolving to more sophisticated LCD-type displays, there is always a need for basic universal annunciators.

This month, we will look at some technical applications using the lonely, yet highly versatile, light emitting diode, otherwise known as the “LED.” We will also briefly look at some audio annunciation as well.

Learning the Lowdown on LEDs
The LED is actually a semiconductor diode. It has two leads; one is called the cathode (negative) and the other is the anode (the positive). In the forward-biased direction, the anode is connected to the positive side of a DC circuit and the cathode to the negative side of the circuit. This causes current flow through the LED to produce light. The more current flow, the brighter the LED.

It is very important to understand that current through an LED must be limited, usually with a resistor, to the manufacturer’s specifications; 27 to 35 milliamps (ma) is a common range. If you decide to connect an LED directly across a power source, the fragile bond in the LED will fail.

The approximate resistor value should be between 300 ohms and 500 ohms for power values of 9V to 12V. Don’t worry if you connect the source voltage to an LED backwards. This is known as the reverse bias direction. Since it is a diode, it will only block current flow and not light. It will not be damaged. Since an LED does not have a diode band for the cathode, like regular diodes, there are a few tricks to identify their polarity.

You can hold the translucent body up to the light in which the profile will reveal the larger anvil-shaped electrode as the cathode. The cathode lead is often shorter and the casing on the cathode side might be flat inside a round LED casing. Through the years, I have enjoyed the versatility of LED lamps to annunciate the status of alarm systems. Manufacturers, such as Alarm Controls, carry a complete line of alarm remote LED plates and have often had just the right combination for my alarm application. Once a technician learns the simple basics, LEDs can be wired in many custom configurations to satisfy unique customer demands.

Multicolor LEDs Popular in Wireless
Jason Click, a service technician with Stanley Security Solutions in Charlotte, N.C., had a customer who wanted to see the status of some doors he controlled when using a wireless door control remote.

He comments, “The location of this door status annunciator was in an executive’s office and I had to make it as discreet as possible. That is why I selected the single multicolor LED.” (See diagram in magazine version of “Tech Talk”) The circuit also nicely illustrates the efficient use of both sides of a relay and the toggle options of the Linear receiver. Nice job Jason!

Check Color, Brightness, Angle
When applying LED-type annunciators, there are some application suggestions to keep in mind. Make sure if you are using a single multicolor LED, the people viewing it are not colorblind. About 5 percent of the total population has some sort of red/ green color deficiency. Just as with two-light stoplights, you might want to consider a dual LED annunciator in your system.

Additionally, the brilliance of normal LEDs can be washed out with direct sunlight. Try using new ultra-bright “daylight” LEDs. According to vision experts, brightness, not color, is the most important aspect in color deficiency.

The viewing angle of an LED can be as low as 10°. A diffused LED will have a wider viewing angle of around 60°. Both types have their own specific security applications. A very sophisticated, high security keypad made by a company called Hirsch uses the advantage of a narrow viewing angle so that others cannot view the keys being selected on the keypad. Each keypad key has a randomly generated numeric LED display to thwart onlookers.

How LEDs Can Aid Troubleshooting
LED indicators can help with servicing an alarm system. One of the best and simplest applications is the use of plug-in transformers, such as the ELK “TRG” series, that have built-in LED power indicators. This gives a positive indication of power to the system.

Some alarm dealers will also install a simple AC power indicator LED right in the cover of the alarm control. Simple annunciators like these can save you service calls when the customer swears his AC is good and the panel is not working. The LED will work nicely with low-voltage AC, just don’t forget that you still have to limit the LED current.

Another service and performance application is a highly dispersed LED annunciator display, such as in the new Altronix “R24” rack-mounted CCTV power supply. This can save considerable troubleshooting time. The unit has an LED indicator for every CCTV power channel.

There are times when you want to provide a highly visible switch annunciation, such as to shunt a motion sensor, but do not have extra power for an LED annunciator.

Try using a device called the “Schadow” switch. It is a pushbutton switch that has a mechanical shutter-type assembly that will change the color of the switch from white to red (other colors are available). It can easily be configured for either latch or momentary operation. The Schadow switch is a very handy device that I have used in many custom installations.

Seniors and Audible Annunciators
System annunciation can also be audible vs. visual. The most common type of audible annunciator is the piezo type often found in alarm system keypads. Dealers should be aware that both the amplitude and the frequency of an audible annunciator are important.

The obvious is that if I have enough amplitude, or loudness, in an annunciator tone, then it will be heard. I have seen dealers become frustrated with elderly customers that say they cannot easily hear the entry/exit delay beeps in their system only to have the technician come out and find the tones working perfectly fine.

As we get older, many of us have a hard time recognizing frequencies above 3kHz. Audio experts recommend alerting devices use two or more spectral components in the 500Hz to 4,500Hz range. The dealer may have to provide an auxiliary sounder for the system annunication. Basic standalone piezo sounders usually operate at around 3kHz.

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