Making a Wireless RC Power Switch, Part 1 of 3: the Analog Way

The most common control signal in radio control systems is Pulse Width Modulation or PWM for short. It lets you control things like a servomotor, a motor speed controller or even turning lights on or off in your RC plane, car or drone.

By definition, PWM is a direct current (DC) voltage switched on or off, the on ratio is called a Duty Cycle which is expressed by D = (Ton/Period) * 100 (in percent) , a PWM signal is a very efficient way to vary a voltage or power by varying the duty cycle: Vout = (D * Vpwm) / 100.

But how can we decode such signals? Well, we have plenty of choices whether you are an analog electronics person or a digital one.

For this first part, I'm going to explain how we can do it using analog electronics and guide you through, step by step, its building blocks.

• 2 x 15K resistors
• 2 x 2.2µF capacitors (polarized or not)
• 1 x LM324
• 1 x 10k resistor
• 4 x 1k resistor
• 2 x 2N7002 transistor (2 x 2N2222 would do the job as well)
• 1 x IRFZ44
• 10K (R4) and 910ohm (R3) resistors in my example
• 1 screw terminal

Multiply the quantities by 4 except for the LM324 if you want to build a 4-channel switch

In this video I am explaining the "analog way" of decoding an RC signal.

It is a PWM signal with a duty cycle between 5 and 10%: and with a 5V high signal we should get an average voltage between 0.25V and 0.5V respectively.

Of course, we cannot use the PWM signal directly, we need filtering. A second-order lowpass filter is required to get our smooth output voltage.

We use a buffer because there are brands of RC receiver that use 3.3V micro-controllers, and others use 5V. That would affect our filtered voltage since we are measuring the average voltage instead of pulse length, which is why we need a buffer to get a known maximum value.

Now that we have our PWM converted to smooth voltage, we can use a comparator to compare the PWM voltage to a fixed voltage set by R3 and R4, then we can use the output to turn an LED on or off, or why not drive more powerful loads using a Mosfet?

The schematics for a 4-channel switch can be found here.

For a switch that triggers at over 1.8ms, the value for R3 is 910Ohms and 10kOhms for R4 but you need to calculate the values of R3 and R4 depending on what you want to achieve.

Here is an excel sheet than can help you facilitate this process.