Introduction

This circuit provides an emergency locator alarm for radio-control airplanes. If the plane goes down in a dense area and you know the general vicinity, the alarm can aid in locating the plane.

Three features are provided:

1. When the device is powered on, it beeps three times in succession to indicate that it is initialized.
2. If the servo pulses are lost for more than ten seconds, such as when the transmitter is shut off or the receiver fails, the alarm starts beeping.
3. Grounding the "delay input" removes the ten-second delay and makes it beep immediately.

Instructions

This device is connected to a spare servo channel on the aircraft's receiver, or paralleled with a servo channel already in use. (Servos have three wires (power, ground, and signal). Although connection configurations vary a bit, generally the ground wire is black, the 4.8V power wire is red, and the signal wire is something else such as orange. If the connection is right, you should measure between 0.15 and 0.20 volts from the ground wire to the signal wire with a DC voltmeter.)

The device is powered by a 9 volt battery, separate from the airplanes battery pack.

The piezo buzzer is mounted on the outside of the plane.

It's important to mount the battery, circuitry, piezo buzzer, etc, so it will reasonably survive a crash.

Technical details

HARDWARE

• This circuit expects standard servo pulses (~1-2mS every ~25mS).
• The purpose of the 2N7000 FET is to provide a greater amplitude (9V) to the piezo-buzzer. If the microprocessor's 5V amplitude is sufficient for your purposes, connect the piezo-buzzer directly to pin 5 and omit the FET and 200 ohm resistor.
• The piezo alarms are really loud only when the input frequency is matched to their resonant frequency. It will probably take some experimentation to get the frequency matched well.
• The purpose of using a separate power source for this alarm is for reliability. If you can derive a reliable 5V supply from the planes R/C battery pack and believe it would survive a crash, then use it.
• The built-in pullup resistors are enabled.
• The external MCLR input is turned off; reset is handled internally.
• The 0.01uF decoupling capacitor is included to keep this device from radiating any RF energy that might have adverse effects on an R/C receiver when at the limits of its range.

SOFTWARE

• The microprocessor clock uses the internal R/C oscillator. The first instruction in the PIC 12C509 code must be a "MOVWF 05", which stores the factory-calibrated oscillator value into the OSCCAL register.