When relais are paired with a radio receiver, they can be controlled remotely over the air, i.e. with a remote control or in automation projects.
Most people use this setup every day without thinking about it: whenever you press the button on a garage door opener, switch channels on your TV set, or use home automation with a smartphone app, you are operating relais (or other type of switches) remotely - typically via RF (radio frequency).
Overview
With RF controlled relais, the control information is transmitted over the air using radio waves.
There are two types of radio waves used:
- Unlicensed Bands (RF): unlicensed bands (315MHz in the US, 433MHz in Europe) are legal to use for anyone. Dedicated senders (remote control) and receivers (relais) communicate directly and require no external setup.
- WiFi/Bluetooth: the 2.4GHz band is freely usable for anyone, too. This is where WiFi, Bluetooth, and related technologies operate. Control messages can be sent to relais, and since WiFi networks are typically connected to the Internet, this makes relais operable from anywhere in the world.
Examples
Ready-to-use RF relais are available for less than EUR 3.00 per piece and can add remote control capabilities to any device:
Look carefully at the device description to see which technology and radio frequency is used by a particular device. The picture shows (from left to right) a WiFi-only, a WiFi/RF, and a RF-only 433MHz device.
Which Technology?
There are advantages and disadvantages to both wireless technologies. Choose the one that matches your use case best.
315/433 MHz RF
In this band, your devices work almost like walky talkies: you operate a dedicated sender (in your remote control) and a dedicated receiver (attached to the relais).
The result is a direct one-way communication providing autonomy:
- Simple: Setup is simple and no special prerequisite is required, nor are there points of failure beyond your control.
- Robust: when your WiFi network fails or your phone company’s network is temporarily down, your RF-Controlled Solutions continue to work.
For the very same reasons, there are also *negative aspects:**
- Local Solution: The distance you can bridge is limited to <10m: you need to be close to the device to operate it. This may be ok for garage door openers but prohibitive for home automation (where you’d like to monitor and control devices from anywhere in the world).
- No Feedback: because of the one-way communication, the controlled device cannot return a feedback. Typically, the feedback you get is a visual feedback only (the garage door opens).
- Insecure: while RF-solutions are not inherently insecure, they often are: radio frequencies are public and can be picked up by anyone. Most commonly, data sent via RF is neither encrypted nor obscure. The guaranteed protection is the limited reach (an attacker would need to be in physical proximity to listen in). This protection may be sufficient with enough distance to publicly accessible areas. It can as well be risky in densly populated metropol areas where attackers easily get close enough to your sender to sniff the RF data packages (and replay them later, i.e., to open your garage door).
There are plenty of good solutions to secure RF transmissions, i.e. rolling codes or encryption. Most cheap out-of-the-box solutions do not come with these, though.
WiFi
Sending control messages via WiFi comes with a load of benefits:
- Anywhere: Commands issued by a smartphone app work anywhere and just need Internet Access. Your commands are automatically routed to your home WiFi, and then to the device that you want to control.
- Feedback: since WiFi uses two-way communications, the controlled device can return feedback, i.e. show its state or confirm the operation.
- Secure: WiFi data is encrypted by default. Attackers cannot listen in or use playback attacks.
For the very same reasons, there are important drawbacks as well:
- Infrastructure: This solution works on top of an existing WiFi infrastructure. It will not work with low or no WiFi coverage, i.e. outside your house or in the garden.
- App & Cloud: Most out-of-the-box solutions are very simple to set up because they base on specific vendor solutions and their smartphone apps. All of your actions are processed in the background by vendor-owned cloud datacenters. If their infrastructure is down, or when they decide to discontinue services, then you no longer can use your devices. Worse yet: anyone in control of the vendor backbone has the same control over your devices and could maliciously turn on or off your devices at will. In a nutshell, you are giving up a great amount of control.
Conclusions
There is no one best technology. You always need to make trade-offs.
When To Use RF
RF technology is best if you want to keep all control in your hands.
Security concerns can be leveraged by either using secure devices (i.e. garage door openers with rolling codes rather than transmitting predictable repetitive codes), or better yet, by using generic RF transceivers to build and program your own secure solution.
One disadvantage cannot be overcome, though: RF solutions only work when sender and receiver are within a 10-20m range.
When To Use WiFi
WiFi technology is best when your devices are inside your WiFi-covered home, and when you want to control them from anywhere in the world.
The primary disadvantage of depending on potentially insecure vendor cloud architecture can be overcome by replacing the device firmware with open source solutions, or purchase devices with such firmware in the first place.
When you build your project from scratch, you can use simple and cheap WiFi-enabled microprocessors (such as the ESP8266 and ESP32): set it up as a webserver, and connect a relais to one of its GPIO pins (using approproate techniques). This way, the microcontroller acts like a webservice and can control the relais.
When you go the microcontroller route, you maximize your control at the expense of maximizing your work: you are now in charge of everything yourself. It is up to you how you expose your microcontroller to the Internet to make it truly controllable from anywhere (i.e. using MQTT, or host the microcontroller REST APIs on publicly accessible servers).
Hardware
Whether you buy a pure ready-to-use remote-controllable relais, or whether it is embedded in a plug, or whether you choose to create the functionality from the required components yourself, here is an overview of the general setup.
Screw Terminals
If your device has screw terminals, here is what they do:
| Terminal | Description |
|---|---|
| Lin, Lout | The actual AC switch, connected to the relais |
| Nin, Nout | Short-circuited, the neutral AC line |
| S1, S2 | When connected, the relais can be closed manually |
Lin and Lout perform the switching while Nin and Nout are just passed through.
S1 and S2 typically enable you to optionally hook up a mechanical switch to also control the relais directly.
S1 and S2 are exposed to AC! Never ever hook up a low voltage push button to these terminals!
Beware: Hazardous Design
Whenever devices are connected to dangerous voltages, make sure you fully understand how these devices work, or else you (or users of your project) may get killed.
When looking at the traces of this particular device, it becomes evident that dangerous AC can be exposed at unexpected places:
The board interconnects Nin, Nout, S1, and the push button. AC mains voltage is present at all of these locations. N stands for neutral AC line, whereas L stands for live AC line.
If you connect the device accordingly, “only” the neutral AC line will be exposed. However, if you use a plug (that can be plugged in either way), or if you are just careless, then live AC is present at all of these locations.
While the push button is normally safely shielded by a plastic cover, the connections S1 and S2 are designed to hook up a manual switch.
In professional installations done by certified electricians, this would typically be a standard AC light switch.
DIY makers however may be intreagued to connect other type of switches (i.e. simple low voltage push buttons), possibly without proper insulation or housing.
This can turn out to be a deadly mistake with this particular board design.
Cheap Chinese devices often come in great quality. Just as often, though, they can have critical design issues and lack appropriate documentation. To be clear: the device discussed here was designed to be placed inside a normal AC light switch by a professional electrician. For this use case it can be considered reasonably safe. When used in hands of inexperienced DIY makers, though, it is extremely hazardous.
Let’s start the hardware review now, discussing the essential parts required by any RF-controlled relais:
RF Receiver
This is the commercial ready-to-use remote-controllable relais discussed above that can be controlled using RF:
Note the specs on the housing: this device is using 433MHz radio frequency (so it is designed for Europe). In the US, other frequencies (i.e. 315/915MHz are used).
Internally, it exposes all the components required by any RF-controlled relais.
Opening Housing
The two-part housing is pushed together by four pins and can easily be opened with a screw driver:
Relais
The relais is the predominant black part: its markings show the ratings: it is controlled by 5V and can switch loads of up to 16A at 220V:
RF Receiver and Antenna
This model uses a PCB antenna which can be seen in the picture below on the left side.
PCB antennas typically perform poorly: you either need a very strong sender, or you need to be very close to the device in order to control it.
[TIP] Occasionally, reception is so bad that the remote control needs to be within a 1m range to the device. By cutting the PCB trace to the antenna and replacing it with a 17.3cm wire, reception performace improves dramatically.
No Microcontroller
The circuit board has a rectangular elongated cutout, clearly seen on the above picture.
This is where WiFi-enabled devices plug in a separate microcontroller board that takes care of communicating with WiFi.
RF-controlled boards like this one do not need microcontrollers.
Instead, close to the antenna you find a chip that hosts the radio receiver circuit, and a 13.560MHz crystal that tunes it to 433,92MHz.
Power Supply
The discussed device can be directly connected to AC mains.
The majority of the remaining components on the board are resposible for supplying DC voltages to the electronic components, i.e. diodes for rectification, and the large electrolytic capacitors to smoothen voltage ripple.
Note the absence of a fuse, and missing proper physical separation of AC and DC parts on the PCB.
In the upper right corner of the board, a commonly used AMS1117 voltage regulator chip provides a stabilized 3.3V and can deliver up to 800mA.
Button
On the backside, a push button is located. This is used to pair the RF receiver with the remote control.
Pairing is required so that the receiver learns the unique hardware code sent by the remote control. After pairing, the device specifically listens to the remote control code(s) you paired it to.
Next to the button, you see an LED. It is used to provide feedback, i.e. to indicate whether a certain button press sequence has enabled pairing mode.
Pairing Process
Since most RF receivers use the same standard RF receiver chips, pairing is performed in a similar manner for most devices:
- Reset: most receivers require an initial reset that can be invoked by pressing the button eight times. The LED typically flashes on success.
- Enter Pairing: to enter pairing mode, either press the button once, twice, or three times and watch the LED. It lights up constantly when in pairing mode. If short button presses do not work, try holding the button for 2-3s.
- Pair Remote Control: While in pairing mode (the LED lights constantly), press a button on the remote control. On success, the LED starts blinking or turns off. The receiver is now paired to the remote control.
[!TIP:] Some devices allow pairing with multiple remote controls. Repeat the pairing process to pair the device with another remote control. Perform a reset to clear all paired remote controls from the device.
Conclusion
The examined device comes with the fundamentally required components (albeit in a potentially hazardous board design):
- Power Supply
- RF Receiver with Button and LED
- Relais
WiFi Receiver
A device designed to be controlled via WiFi (instead of RF) shares some components but requires also a few changes.
Here is a similar device which is WiFi-controlled:
Opening
The two housing parts are held together by four clips. You can pry them open with a screwdriver.
WiFi Module
A separate breakout board marked LN-02 is piggy-backed onto the main board.
On the top rim, the PCB WiFi antenna is seen:
The plug-in piggy-back board illustrates how these devices are manufactured: based on desired control type, the WiFi board can be plugged into the main board, or left out. Plug-in boards with different WiFi-enabled microcontrollers can be found, depending on microcontroller market prices and availability.
Microcontroller
The microcontroller is a LN882, produced by Lightning. This microcontroller is comparable in features to the well-known ESP8266 from Espressif.
For the longest time, WiFi-enabled devices used original ESP8266 microcontrollers that could easily be reprogrammed. Newer devices use less documented variants. Various Open Source Projects aim to document, access and reprogram them. There are detailed instructions for the LN882 (albeit not in German, use Google translate).
Power Supply
A fuse resistor is present, as well as three large electrolytic capacitors.
On the back, a MB10 bridge rectifier and a BP2525 transformerless AC-to-DC constant voltage converter turns 85-240V AC directly to 5V DC which is required to operate the relais.
A separate AMS1117 provides the 3.3V constant voltage required for the microcontroller.
Board Design
The board and its traces show a better (safer) design: Cut-outs separate AC from DC parts, and while Nin and Nout are still connected (the cheap relais switches just one line and not both), the screw terminals for the external switch are not connected to AC mains.
Conclusion
A WiFi-controlled relais shares the same fundamental components:
- Transformerless Power Supply
- Relais
- A separate WiFi-enabled microcontroller board is piggy-backed and digitally controls the relais via its GPIO
Datasheets
BP2525 AC-to-DC converter
MB10 Bridge Rectifier
AMS1117 3.3V Voltage Regulator
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(content created Apr 29, 2024 - last updated May 01, 2024)