Remote Control using Mains Power Line Communication

The proposed circuit will allow you to remotely control a mains AC operated appliance across rooms of your home through mains power line communication or PLC concept.

In PLC technology, an electronic circuit which works as a transmitter is plugged-in to the mains wiring (220 V or 120 V) injects a modulating high frequency data signal into the 50 Hz or 60 Hz mains AC frequency. Another circuit which acts like a receiver and coupled across the same AC mains wiring but in some other location detects this modulated signals via the mains wire and decodes or demodulates the data for the specified end results.

Imagine a device which can be plugged in your hall room's mains socket, and toggling its button controls another mains operated gadget in the adjoining room or in your kitchen. This sounds amazing right, yes this is an old concept which allows the user to communicate across rooms using the existing mains wiring of the house, through a coupled transmitter/receiver units.

In this article we discuss a couple of simple power line communication (PLC) based remote control circuits, which may be used for controlling devices across rooms through a plugged-in transmitter/receiver pair.

The first design below is built using ordinary electronic parts such transistors, resistors, capacitor, diodes etc. Let's first learn regarding the transmitter circuit and its operational details.

Power Line Communication Transmitter

The simple transmitter circuit can be witnessed in the following diagram.

The PLC transmitter circuit includes an oscillator stage using transistors T5/T6, tuned at at 150 kHz. This oscillator frequency is switched ON through a monostable multivibrator built around T4 transistor BC557.

This monostable can be triggered using the ON/OFF switch S1. This 150 kHz frequency is then injected into the mains wiring through the transformer T1 shown at the bottom right.

So now, the 150 kHz frequency rides over the mains 50 Hz or 60 z frequency, which can be picked up by a PLC receiver unit coupled with the same wiring at a distant location or in another room.

PLC Receiver

The following image depicts the power line communication receiver circuit

The receiver is configured around a two stage amplifier using transistors T7/T8, a rectifier circuit using two 1N4148 diodes, which has quite a long time constant.

The time delay helps to cancel out momentary interference pulses. The 150 kHz frequency is extracted through an attached transformer T2, and after suitable filtering stages, the amplifier detects and responds to the 150 kHz frequency and begins oscillating at the same rate.

The rectifier stage using the two 1N4148 and the subsequent 10 uF filter capacitor stabilizes the frequency into a stable DC for toggling ON the next relay driver transistor.

The relay driver stage switches ON the relay and the connected load, and remains ON as long as the transmitter switch S1 remains switched ON, and vice versa.

In case your neighbor might be also having a similar system installed in their house, then to avoid cross interference you may want to adjust the sensitivity of the receiver to a lowest possible setting, which may be just enough to work with your own system. This sensitivity may be tweaked with the 1 k preset.

The coupling transformers which are used for injecting and extracting the 150 kHz frequency across the mains wiring is built over 20 mm diameter pot core. The winding "b" which is towards the mains wiring has 20 turns using 31 SWG super enameled copper wire, and the side "a" which towards the circuit side has 40 turns using the same wire.

The above design uses a simple circuit which may perhaps get toggled with some nearby frequency such as 140 kHz or 155 kHz, which may not seem very desirable. For achieving a pin point accuracy with the frequency response, so that the unit responds precisely to the specific transmitter signals, a PLL based IC may be required as explained below.

PLC Circuit using IC LM567

The idea was published in the datasheet of the IC LM567 as one of the application circuits, among the many other outstanding ones.

Receiver Schematic

The IC LM 567 is actually a specialized tone decoder using PLL technology which enables the device to detect and respond only to a specific frequency as determined by an external RC network values, and reject all other irrelevant frequency in the spectrum.

The proposed remote control circuit using power line communication may be witnessed in the above diagram, the circuit functioning details may be learned from the following points:

How it Works

R1, and C1 are the external RC components which decide the sensing frequency of the device, and pin#3 becomes the sensing pinout of the IC.

Meaning, pin#3 will detect and acknowledge only that particular frequency which is set using the R1/C1 network. For example if the R1, C1 values are selected to assign a 100kHz frequency, pin#3 will pick only this frequency to activate its output and ignore all that may be different to this range.

The above feature enables the IC to single out the specific frequency from the superimposed AC 50 or 60 Hz frequency and trigger the output only in response to this predetermined set frequency.

In the figure we can see a small isolation transformer which is included in order to isolate the electronic circuit from the lethal mains current.

The mains low AC frequency acts like the carrier frequency, over which the triggering high frequency rides to reach the intended destination across the transmission line.

In the above receiver design, the IC is assigned to respond to a 100kHz frequency which is supposed to be injected into the mains line from a nearby location which could be an adjacent room or premise.

The 100kHz frequency could be injected through any oscillator circuit such as a IC 555, or IC 4047 circuit or another IC LM567 circuit installed as the transmitter unit.

In an event when a signal is injected into the mains from a relevant location, the receiver circuit shown above detects the specific frequency in the attached mains power line, and responds to it by producing a low logic across its pin#8.

The pin#8 being connected with the a 4017 flip flop circuit toggles the output relay and the load ON or OFF depending upon the previous situation of the relay.

The Transmitter Stage

The transmitter which is supposed to inject the 100kHz or the desired triggering frequency into the power line can be ideally built using a half bridge driver oscillator circuit as shown below:

Transmitter Schematic

The 12V input to the circuit must be switched through a push button arrangement so that the circuit is triggered only when required in order to switch on the intended appliance through the power line.

The RC component at pin2/3 of the IC are not calculated for generating 100kHz, The following formula can be used for determining the right oscillator frequency:

f  = 1/1.453× Rt x Ct

Ct is in Farads, Rt is in Ohms. and f in Hz
Alternatively the same can be evaluated using a frequency meter and with some experimentation.

This is an untested circuit designed as per the suggestions presented in the datasheet of the IC LM567.