Usually, the radio transmitter provides a set of pulses every 20 milliseconds, which, in turn, sends the radio receiver a separate pulse to each of the servos at the same interval. In this circuit, an alarm will sound when the radio receiver no longer receives the set of pulses from the radio transmitter.
To locate the aircraft, all you have to do is switch your transmitter off and the system will start to beep. The circuit is very easy to build and does not require any special parts like costly microcontrollers. A typical lost plane finder is shown in Fig. 1.
Circuit and working
Circuit diagram of the lost plane finder is shown in Fig. 2. It is built around timer NE555 (IC1), one transistor BC547 (T1) and a few other components. The design is centred around timer chip NE555 (IC1), configured here as a missing-pulse sensor. The whole circuit can be powered with power supplies ranging from 4V up to 9V, so you can use any power source that your radio receiver/servo setup can handle.
The 4-pin connector (CON2) is the input connector for the lost plane finder system, while the 2-pin connector (CON3) is an optional output connector. In the circuit, when the pulse input is available at input pin 4 of CON2, BC547 transistor (T1) is turned on and a high-level (H) voltage (near 2.6V when Vcc=4V) is available through output pin 3 of NE555 (IC1). When the pulse input is absent (transmitter is off), T1 switches off and IC1 gives a low-level (L) output through its output pin 3.
As a result, the piezo buzzer (PZ1) and the indicator (LED1) wake up to raise a noticeable alert. Presence of the optional output connector (CON3) enables driving of external low-voltage blinker/beeper loads, in case of an extreme demand for such enhancements.