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.
Another Design Which Discusses Communication through Mains Power Line AC
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.
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.
How to Construct the Coupling Transformers T1, T2
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.
PLC Circuit using IC LM567
The above design uses a simple circuit which may perhaps get disturbed 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.
The idea was published in the datasheet of the IC LM567 as one of the application circuits, among the many other outstanding ones.
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:
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.
Appliance Remote Control Circuit using AC Mains Wiring
When there are no light switches in a room, it can be difficult to install lighting without damaging the walls to add wiring for a switch. The solution mentioned involves using the power lines already in the room to control the lights remotely, without needing to add new wiring.
The power is, of course, directly supplied by the power grid itself through the control push button BR. The capacitive coupling is ensured by the capacitor C1.
A first alternating current (which we will call positive by convention) goes through C1 and the limiting resistor R1 to charge the capacitor C2 via D2.
The zener diode DZ1 limits the potential available on the positive plate to 12V. During the next alternating current (still negative by convention), the diode D1 shunts the power supply located downstream of D2, thus allowing the capacitor C1 to discharge (and charge in the other direction) in order to be ready to face a new cycle.
The diode D2 prevents the discharge of C2 upstream. On the positive plate of C2, there is thus a quasi-continuous, slightly undulating potential of 12V, which constitutes the power supply of the transmitter.
Finally, the resistor R2 allows the discharge of C1 as soon as the solicitation is completed. This precaution can prevent many unpleasant shocks to the imprudent operator who would inadvertently touch the connection of a component.
The NAND gates III and IV form an astable oscillator. It delivers an asymmetric square wave signal at its output due to the action of the diode D3.
The period of the signal is about 0.8 ms, corresponding to a modulation frequency of 1250 Hz. The duration of the high states of this signal is about 0.15 ms, resulting in a duty cycle of about 20%.
The NAND gates I and II form a controlled oscillator. During the low states on input 2, the oscillator is blocked. However, as soon as this input is subjected to a high state, a square wave signal with a period of about 10 us, or 100 kHz, is generated at the output. This signal serves as the carrier wave.
Amplification and transmission The signal from the output of the NAND gate II is power-amplified by the transistor T, which has the limiting resistor R9 in its collector circuit.
Thanks to the coupling ensured by the capacitor C5, the amplified signal is injected into the 220V distribution network. The zener diode DZ2 protects the transistor T from any possible overvoltage from the power grid.
This has a completely classic power supply. The power comes from the mains via a transformer that delivers a potential of 12V on its secondary winding.
A bridge of diodes rectifies the two alternations and C1 performs a first filtering. On the output of the 7809 regulator, a stabilized DC potential of 9V is obtained. Capacitor C2 completes the filtering and C3 acts as a decoupling capacitor.
Remote Control Signal Detection
Capacitors C4 and C5 couple the low voltage part of the receiver to the mains. The signal is then taken into account by an RC filtering network consisting of the set R2, R5, R7, C6, C7.
The NPN transistor T2 is mounted in a common emitter configuration. It provides the necessary amplification.
Note that the emitter resistance is decoupled by C9 for better gain. In addition, it is essential to choose a class C transistor, characterized by a gain greater than 600. The thus amplified signal is available at the collector of T2.
Processing of the remote control signal
The PNP transistor T3, also mounted in common emitter configuration, is biased at its base in such a way that in the absence of an input signal, the potential at the collector level is zero.
On the other hand, as soon as the signal from the previous stage appears, a succession of positive pulses at the modulation frequency of 1.25 kHz generated by the transmitter can be observed on the collector of T3. The 100 kHz carrier has disappeared due to the filtering carried out by C11.
The NAND gates III and IV form a monostable circuit. It delivers on its output a series of high states characterized by a duration of about 0.5 ms, which are also taken into account by the integrating device constituted by D2, R16, R4, and C14.
During the high states, the capacitor C14 is charged through R16 and can only be discharged through R4, which has a much higher value.
Control of the Load relay
The IC2 reference integrated circuit contains two JK flip-flops, only one of which is used in the present application. For each high state from the trigger, the Q1 output changes state. When this output is in a high state, the transistor T1 is active.
It includes the coil of the load relay in its collector circuit. The relay, when closed, directly powers the receiver concerned by the remote control via the "common/working" contacts of the relay.
The diode D1 protects the transistor from the effects of self-induced overvoltage. When the relay is energized, the LED L signals that the receiver is powered by illuminating.
The relay coil is directly powered by the filtered 12V potential available on the positive armature of C1.
The AC wiring remote control thus operates like a remote switch: each press of the push button has the effect of alternately activating and deactivating the utilization receiver.
As a result, a permanent high state remains at the positive plate of C14 as long as the transmitter remote control push button is pressed.
This high state is then directed to a Schmitt trigger formed by the NAND gates I and II as well as by their peripheral resistances R17 and R18. This device gives the signal taken rising and falling edges with very steep slopes.
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