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I bought a cheap drone to get into flying quadcopters and it seems it has only one big issue at that price, which is a very long failsafe time of 20 seconds, i.e. if the remote signal is lost, the drone keeps buzzing for 20 more seconds until it cuts off its engines. As an absolute beginner pilot living in a crowded city, that sounds terrifying. The package hasn't arrived yet, but I started thinking of ways to mitigate that, ideally without modifying the flight controller board. I've been dabbling with small electronics projects for a while, but still a beginner there as well.

What I came up with is using a cheap 433 MHz rx/tx pair (like this one https://randomnerdtutorials.com/rf-433mhz-transmitter-receiver-module-with-arduino/), with the rx + a reset switch controlling a FET latch circuit placed between the battery and the drone's VCC, and the tx on the ground emitting a constant signal while connected to a variable voltage source to adjust range. Therefore, when the 433 rx loses signal, the latch would shut down all battery power and the whole thing crashes on the spot.

A datasheet I found on the 433 pair said that the rx data output is maximum VCC/2, which is around 1.85 V for a 3.7 V drone battery. I think that's enough to drive an NFET. My main questions are:

  1. I imagine the "constant" data output from the 433 rx will in fact be flickering at 433 MHz, which could be fed down the line to the drone's board. How do I filter that out with as few (and light) components as possible?

  2. Are there any RF issues that I'm overlooking? I know the 433 MHz band is crowded, but I doubt I'm going to run into another always-on emitter very often. The drone itself is controlled via 2.4 GHz and I intend to add a 5.8 GHz FPV camera.

TL;DR: I want to control a drone battery with a 433 MHz receiver's data output by sending an always-on signal to a FET latch circuit. Is this feasible?

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  • \$\begingroup\$ Just curious: does it just drop from the sky if signal is lost? Would it not be wrecked? \$\endgroup\$ – Transistor Oct 21 '18 at 12:18
  • \$\begingroup\$ The small ones are surprisingly sturdy, because they're very light - kind of like ants. Either way, spare parts are very cheap. \$\endgroup\$ – Axl Vang Oct 21 '18 at 12:38
  • \$\begingroup\$ I'm not sure I'd feel comfortable knowing that my now-silent drone would come dropping out of the sky onto somebody's head (don't know how crowded your city is). Does the other cut-out at least land softly? Regardless, I've run into all sorts of things operating at 434MHz, location systems to amateur radio, so you'll need some kind of communications link that confirms it's -your- signal. The other issue is range, these systems to be legal (assuming USA) are very low power, so range is an issue- I'd check to see how far you can actually get with these devices and maintain a comm link. \$\endgroup\$ – isdi Oct 22 '18 at 4:48
  • \$\begingroup\$ The other failsafe is also an engine cutoff and drop. The range of the drone's remote is already low, about 30 meters, which is where I'm aiming with the 433 as well, maybe even lower. I'm hoping a signal lock in won't be necessary, but I guess I can always stick an Arduino Nano in there and make the whole thing digital. I'm trying to keep it all under 5-10 grams, though. \$\endgroup\$ – Axl Vang Oct 22 '18 at 6:34
  • \$\begingroup\$ The main issue is that some junk radio you got in a packet of corn flakes is likely illegal in most of the world. And unlikely to perform well when restricted to the 10dBm power that most countries set as restriction on the band. \$\endgroup\$ – Lundin Oct 31 '18 at 16:03
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I'd interface to your FET with this

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Is this a Schmitt trigger circuit? \$\endgroup\$ – Axl Vang Oct 22 '18 at 7:45
  • \$\begingroup\$ @ Axi Vang There is positive feedback. Yes, you are correct. \$\endgroup\$ – analogsystemsrf Oct 22 '18 at 15:21
  • \$\begingroup\$ Thanks for the answer, it seems to be just what I need! Is there any advantage to making my own circuit over using an IC like the SN74HC14N? \$\endgroup\$ – Axl Vang Oct 22 '18 at 20:24
  • \$\begingroup\$ Advantage? I think this will work well from 0.0 volts to VDD/2. A 7414 will require a large input voltage swing. This discrete circuit should not require a large swing. \$\endgroup\$ – analogsystemsrf Oct 23 '18 at 4:56

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