An inconveniently placed tall tree, a raging gale, a lapse of concentration combined with inexperience caused me to loose my model and all its gear one night. For the replacement I wanted to flash an LED just like the anti-collision strobes on real planes. Thinking about how to wire this into the new plane I came to the conclusion that I might as well combine the lost model alarm function as well. The circuit connects between the receiver and servo, just like an off the shelf lost model alarm. A single CD4049B hex inverter IC is used to provide lost model and LED flashing functions. IC1a and IC1b,c form an oscillator that provides a short on period of about 60ms on and a longer period 800ms off. This give the characteristic strobe appearance. The period of oscillation is set by C1 and R3. This period is caused to be asymmetric by the action R1 and D1. R2 reduces the effect of the input clamping diode on pin 5 allowing the period of oscillation to be independent of supply voltage. Inverters IC1b and IC1c are wired in parallel to increase the current available to LED. Servo pulses are AC coupled by C2. D4 clamps excursions above the 5v rail. R4 ensures that the input to IC1d relaxes to the 5V rail when servo pulses stop. Servo pulses are inverted by IC1d and charge C3 via D4. When servo pulses stop C3 discharges via R5. When servo pulses stop completely IC1e pin 6 goes high. R6 and D2 gate the signal at the oscillator rate. The gated signal passes via IC2f energising the self oscillating sounder at the same rate as the LED. Note that the very high component values used in this circuit mean that the assembly must be kept very dry for it to function reliably. C1 must be non-polarised as even a very small amount of leakage will prevent the oscillator from starting.

Article by Richard Jones and reproduced with his permission
for Bumpy Green Model Aerodrome
5th September 2002