Downed Model Locator Circuit

Here’s a design circuit that available for your local Radio Shack store, and modified it to decrease false triggering from low voltage spikes in the on-board power system when full sized or higher torque servos are used. This circuit is called downed model locator. Your transmitter sends a set of pulses to your receiver every 20 milliseconds, and your receiver in turn sends an individual pulse to each of your servos at the same interval. This circuit is a pulse omission detector--an alarm sounds when the pulses, originating from your transmitter, are no longer present. By plugging this circuit into an unused servo socket on your receiver, you can turn on the alarm by turning off your transmitter. This figure is show about the circuit;

The first capacitor C1 filters out DC voltage, preventing an aggressive automatic gain control of some current receivers from shutting off the alarm even when your transmitter is off. The first transistor Q1 serves to flip the pulse to negative modulation that the 555 needs. The C2 capacitor and the R4 resistor establish the time interval--if no pulse is received in the time it takes to charge the capacitor through the resistor, the alarm sounds. The interval is the resistance multiplied by the capacitance: 1uF x 47k = 0.000001F x 47000 ohms = 0.047sec = 47msec which is a little over twice the standard 20msec R/C frame rate--this device uses a little longer interval than the frame rate to prevent false triggering. The other capacitor C3 smoothes the control voltage on the 555, preventing false triggering from spikes in the supply voltage. Unless a pulse opens the Q2 transistor to drain the C2 capacitor before the capacitor is fully charged, the pin 6 threshold senses a high voltage and triggers the output pin 3 to go low, sinking current across the buzzer and making noise. With the reset pin 4 high, the discharge pin 7 drains the capacitor, and the cycle starts again.