Here is an affordable circuit of a tiny, lightweight, and easy to handle lost model alarm (LMA) for model RC planes. A simple LMA beeps so you can listen for it while you carry out your search and rescue mission. If you have a lost model alarm fitted in your RC model you will be able to walk straight to your model and retrieve it. In principle, Lost Model Alarms are often electronic beepers that monitor a servo channel and emit a loud alert signal when the transmitter is out of range or switched off. The standalone LMA presented here allows you to find the lost model almost anywhere, in any condition, even if the model battery is expelled in the crash!
The circuit built around the popular IC 4060 (IC1) employs its own power supply. The on-board battery (BAT) in the circuit activates the piezo-buzzer (BZ1) when radio link is missed and makes a short signal tone (bleep) intermittently for more than 24 Hrs. Besides, one twinkling LED (LED1) provides an obtrusive visual indication. All the time that the RC receiver output is giving valid servo pulses, the timing generator (IC1) is disabled by the components connected at its reset terminal (pin 12). Power switch (S1) in the circuit can be used to disarm the Lost Model Alarm. No doubt, this device will help you to locate your downed model RC planes. Similar to other traditional designs, this circuit plugs into a spare servo socket on the RC receiver. Since the alarm sounds only when pulses originating from the RC transmitter is no longer being received, you can turn off the RC transmitter to activate the alarm.
Eventhough, overall current consumption of the circuit is moderate, a 3.7V Lithium-Ion battery is preferred to ensure total reliablity of the system. The Lithium-Ion battery pack can be connected to the system through dc input terminals BAT-1 and BAT-2. Similarly, an external (dedicated) charger for re-charging the Lithium-Ion battery can be plugged, upon landing, to the system through charging input terminals CHG-1 and CHG-2.
The finished circuit must be protected and waterproofed. Heatshrinking is better because it renders enough protection and keep the electronics intact during a crash. If possible, try to embed the LMA into a wing and wire it in with the servo (wings often come off and largely survive even quite bad crashes). Make sure the servo connector of the model is rigidly connected to the LMA connector (J1) pins (labeled GND-VCC-PULSE, but VCC is not used here). Make sure the J1 is connected to the same port on the receiver that the servo would normally be connected to. The pin configuration of J1 is compatible with Futaba systems, and works with all others.
Lab Note: The servo pulse processing circuit (wired around T1 and T2) should suit most makes of receivers including those with low voltage servo pulse output. However, note that servo outputs from different makers vary in voltage from around 3.6 volts to the full rail voltage of 5.0 volts. So, in case of an error, do some experiments with the values of components CX and RX.
Pre-Flight Test: Insert J1 of LMA into one of the active channels of the receiver, making sure the connector is correctly polarized. Test LMA by switching off the transmitter, and verify that LMA should blink and beep after a short delay.