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Alarm Tone Annunciator (AL-1A) KITSET

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0.05 KGS
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Product Description

The kit will require moderate to good experience with soldering of small components and is not suitable for first-time builders.

The AL-1A device is a tone annunciator module for aircraft applications such as stall or oil pressure warning systems. It senses either high-level or low level inputs from switches or sensors and generates a pulsed (beeped) tone in the aircraft audio system. All of the electronics are contained in a 15-pin D-subminiature connector backshell, making the device compact, light and easy to install. The tone is adjustable in both frequency and beep rate so that several distinctive annunciators may be provided in one installation. The tone output is approximately a 12 volt peak-to-peak waveform driving through a 5.6 Kohm resistor.  It may be connected directly to a high-level auxiliary input of an intercom or audio panel.

To attenuate the output signal, a 620 ohm resistor is provided internally in series with the normal output.  Grounding this resistor will provide a 20 dB reduction in audio tone level (to about 1.2 volts peak-to-peak).  Use this function if necessary in the application. The inputs have 2 kohm  pull-up or pull-down resistors to provide wetting current to external switches or sensors. Wetting current helps to keep switches and relay contacts clean by providing a small amount of current that burns off contamination. Usually, only one sense input (SN or SP) is used at a time. SN is used for sensing inputs that are grounded, while SP is used for sensing inputs that are switched to +12 volts.  It is also possible to use both inputs to sense two separate circuits—one low going (grounded) and the other high going (+12 volts).  The tone will sound if either input is activated.
See the project page in The Hangar Workshop for the open source documentation. This product is also available fully assembled and tested.
Technical Summary
  • Inputs:
    • One active low with 2 kohm pull-up
    • One active high with 2 kohm pull-down
  • Audio output:
    • 12 Vpk-pk, 5.6 Kohm series resistance
    • 1.2 Vpk-pk, with internal 620 ohm resistor grounded
  • Tone frequency typically adjustable from 400 Hz to 4 kHz
  • Beep rate typically adjustable from 1 Hz to 10 Hz
  • Less than 10 mA current draw

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