Droids Corporation

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A simple lost RC beacon (part 1)

With some friends, we play with RC planes, especially FPV (First Person Viewer): the RC model has a camera that transmits the video over radio, received and displayed in goggles, as if we were in the plane! But this hobby can be risky, we sometimes crash the plane, and unfortunately it can be crashed far, at an unknown location. So the question is: how can we find it easily?

To solve this issue, there are several solutions. The easiest one is to put an audio beeper on the plane (we already do that), but it doesn’t help if the plane is too far. Another common solution is to embed a GPS on the plane and transmit the position to the ground station. This is probably a good solution but it requires some quite complex electronics: the GPS board, a microcontroler to decode data, and a radio transmitter. Moreover, you need another GPS device that to get your current position and determine the relative position of the RC plane.

Although this solution is probably a good one, we wanted to design the simplest board with the following constraints:

  • cheap: no GPS, small uC
  • long autonomy: embeds its own battery
  • reliable: implies simple, board is protected by a box, embeds its own battery (again)
  • “long” range (500m)
  • fun ;) ...it is probably not the best design, but at least it is a not so bad one that works

Our system is composed of 2 boards: a transmitter and a receiver. The transmitter sends beeps over radio (433 Mhz), and the receiver module is connected to a buzzer that emits sound when the signal is received. To detect the direction of the transmitter, we use a directional antenna (yagi).

Here are the final version of the boards:

../../../_images/beacons.jpg

We decided to use the Dorji DRA886RX (receiver) and the Dorji DRA887TX (transmitter) as they are referenced by two (interesting) documents:

The datasheets of these modules are very poor, but they are easy to use. The radio power depends on the voltage. When using the maximum voltage, the range of these modules can be quite high, but we should take care to conform to legal constraints.

We were able to build a simple prototype in a couple of hours:

../../../_images/beacons-proto.jpg

Description of the transmitter

The transmitter board contains:

  • the radio TX (DRA887TX)
  • an audio buzzer to find the RC plane when distance is < 50m
  • a push button, for user interaction
  • a LED
  • a microcontroler (AVR ATtiny13) that will generate the radio and audio signals, and handle the button
  • an adjustable power for the TX, that can be switched off by the microcontroler

Here is the schematic:

../../../_images/beacon-tx-sch.png

Here is the PCB:

../../../_images/beacon-tx-pcb.png

Eagle files (including part list) are available on our git: http://git.droids-corp.org/?p=protos/xbee-elec.git;a=tree;f=beacon-tx;h=35ff6facfb31ee73fdde580f25aca327b1a5446d;hb=HEAD

The program in the ATtiny13 is very basic. It transmits a beep on the radio every second, and an audio beep every 2 seconds. If the button is pressed, the audio is mutted during one minute. A future evolution could be to add a servo input and monitor activity on it to mute the audio or radio, as it is done by this guy.

The program is available on our git: http://git.droids-corp.org/?p=beacon-tx-433.git;a=summary

Description of the receiver

The receiver board contains:

  • the radio RX (DRA886RX)
  • an audio buzzer to hear the output of the receiver and a trimmer to modify the volume
  • a push button, for user interaction
  • 2 LEDs (one for power, one for status)
  • a microcontroler (AVR ATtiny45) that will filter the received signal

Here is the schematic:

../../../_images/beacon-rx-sch.png

Here is the PCB:

../../../_images/beacon-rx-pcb.png

Eagle files (including part list) are available on our git:

http://git.droids-corp.org/?p=protos/xbee-elec.git;a=tree;f=beacon-rx;h=35ff6facfb31ee73fdde580f25aca327b1a5446d;hb=HEAD