RC Kill Switch – ElectroSchematics.com

There are countless RC Kill Switch devices available on the market. They are mostly simple and cheap devices, activated by signal loss or inactivity on RC channel which they are wired to.

However, recently I was contacted by a reader to design and fabricate a crude kill switch suitable for use with his gas-powered RC aircraft engine. Since I do not have an RC model aircraft to test my build, I worried about taking on the project. I later got enough courage to continue.

I developed my own little model of an RC Kill Switch using common electronics components lying around. My first dose of inspiration was the web image of an RC Kill Switch built on a perfboard (https://www.davesrce.com/). The circuit has three key components – a small microcontroller, an optocoupler, and a driver transistor. The “opto-coupled gas engine kill switch” will allow you to safely and remotely shutdown your electronic ignition equipped engine at any time (ground airborne) from your RC transmitter if a spare RC receiver channel is allotted for the switch. One remote mountable LED is included to indicate ignition arming status. The optical coupling is to isolate the ignition system from the radio system. Pretty great!

In other words, an RC Kill Switch installed between the ignition module and the radio receiver lets you control the ignition using the aux channel of your radio receiver. Remember, you can’t plug the ignition module directly into the receiver. An old ‘mechanical’ method is to use a small ignition on/off switch coupled to an RC servo on the aux channel and use that servo to flip the ignition switch.

The power cable from the ignition needs to plug into a fuselage mounted switch, and the ignition battery plugs into the switch. The ignition would be active when the fuselage switch is turned on. In order to control the ignition thru radio transmitter, you should install the kill switch between the fuselage switch and the ignition. The receiver plug of the kill switch must be plugged into a spare radio receiver channel.

Sidenote: An IBEC lets you to use a single battery pack for both ignition and radio. The IBEC also serves as a power switch from the aux channel to turn the ignition on or off as desired. There are several online resources available. If you do a Google search you will find many IEBC articles. Below you can see a popular IBEC (https://gator-rc.com/tech-aero-ultra-ibec-blue-light-led).

Let’s get back to my RC Kill Switch project. This is the proposed basic/default wiring diagram for my RC Kill Switch.

By default, the ignition circuit is powered from the ignition battery. That means the Kill Switch device is using power from receiver for system logic (and hence from an independent battery) to control the ignition module. Input cable of the Kill Switch is plugged into the RC receiver’s unused channel that is activated via a switch on the transmitter. It would be better to use a separate toggle switch to disconnect the ignition battery from the ignition system as such an addition will ensure a 2-layer protection (a very crucial consideration while dealing with powerful RC gas engines). Finally, mount the LED indicator (optional) somewhere on the model so that it can be seen clearly.

Below you can find the circuit diagram (v1) of my RC Kill Switch. It’s designed (hopefully) as a ‘universal’ kill switch to switch the ignition module/box on/off from the transmitter conveniently and safely. I think this design might work with RC gas engines that utilize a CDI (Capacitive Discharge Ignition), too. If you’ve any corrections/suggestions, don’t hesitate to leave your comments here ( I’ve no previous experience – Hope you understand the limits).

I observed the basic design of the common kill switch (killer switch) is usually centered on a small microcontroller (PIC/Attiny), and a logic-level power mosfet is used as the output switch (of course there’s an optocoupler in between). Some versions also have an onboard linear voltage regulator that makes the core electronics compatible even with high voltage type receivers running on 2S or 3S LiPo battery packs.

The task could be fulfilled by a variety of ways, however, my prime focus was to rig up a cheap, simple, and adaptable circuit. Therefore, I used one Digispark Attiny85 development board as the ‘brain’ of my design. Digispark is a known and cheap Arduino-like tiny microcontroller board that provides adequate peripherals for this particular application.

In the schematic, CN1 is the default input connector for radio receiver, and solder jumper SJ1 extends the 5VDC available from the receiver to 5VDC rail of the Digispark board. The same 5VDC is also used to drive the 5VDC sub miniature relay K1. Solder jumper SJ1 must be closed in this default mode.

CN2 is the optional “HVDC” input connector to feed 2S-4S LiPo power to operate the kill switch. This time, the onboard 5VDC regulator (78M05) of Digispark board regulates the high voltage dc input and caters requisite regulated 5VDC to the rest of the circuit. Solder jumper SJ1 must be opened and SJ2 must be closed to use CN2.

First port P0 of Digispark used to receive servo pulses (1ms-2ms) coming from the radio receiver. Next port P1 is used to drive the relay K1 thru T1 which is a low-power NPN BJT. Resistor R1 is the base bias resistor. Diode D1 eliminates the reverse inductor currents (coil of the relay) and LED1 indicates the relay activation. Resistor R2 limits the LED’s current. Capacitor C1 is used to reduce the power supply rail noise.

Do you see the absence of an optocoupler? An opto-isolator is necessary to galvanically isolate the ignition module from the radio receiver so that the high-frequency pulses (spark) created by the ignition module never affects the receiver’s electronics. I intentionally used the electromagnetic relay in lieu of the optocoupler + power mosfet combination to ensure better electrical isolation and sufficient load handling capacity. The relay is about the size of a sugar cube, but that is tolerable!

The flexible code was written in the style of an Arduino-Sketch. The code sets Digispark ready to read the radio signal coming from the radio receiver and energize the relay only when it can see a valid radio signal. The important part of the code which you might need to modify is the threshold value of the servo pulse input (and reverse the trigger state, if necessary). Needless to say, you can tailor this code for a ‘blank’ Attiny85 microcontroller or the like as well (see a sample below), it does need some skill and patience though. BTW, I already shared a few Attiny thoughts here, so you can read that old post through this link https://www.electroschematics.com/attiny85-pwm-primer-tutorial-using-arduino/

[code]

int drivePin = 1; // P1 with onboard LED

int pulsePin = 0; // P0


void setup() {

pinMode(drivePin, OUTPUT);

pinMode(pulsePin, INPUT_PULLUP);


}

void loop() {

int signal = pulseIn(pulsePin, HIGH, 25000);

if (signal < 1500 || signal == 0) { // SEE TEXT

digitalWrite(drivePin, LOW);  // Switch off the relay (C+NC linked). No valid signal!


} else {

digitalWrite(drivePin, HIGH); // Switch on the relay (C+NO linked). Valid signal!!

}

}

[/code]

On first run it might become necessary to tweak the radio channel in which the RC Kill Switch is plugged (like configuring the servomechanism end points). You need to do a thorough test at varying throttle positions. If the setup has been done properly, and the transmitter has the toggle switch that is controlling the kill switch applying decent throws (<50%> as coded), you will be good to go.

I built the circuit at first using jumper wires to be sure it’d work, and then moved to a rigid breadboard. I can get back to testing and refining the design with parts on hand quickly.

Random things to look at…

Confession: I didn’t understand how exactly I can wire the relay contacts to the ignition controller. According to what I read so far, the battery wires carry the primary (high current) dc supply and only a mighty toggle switch is needed there to cut the battery supply to the ignition system. Presumably the ignition wire is a part of a low current circuit that can turn the controller itself on/off to manage the rest of the ignition system (I’ll go over this once more before finalizing my kill switch project).

Finally, I hope this article is helpful and, perhaps encourages others to design their own remote kill switches and even install them on autonomous flying machines. Anyway, I’m not making any expressed or implied guarantees about the reliability or performance of my crude RC Kill Switch circuit. It’s your responsibility to determine the best method of implementing extra safety measures in your aircraft!

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