RF-controlled relais are available for less than €2 in a small and ready-to-use form factor. Unlike their WiFi-controlled counterparts, they do not contain a microcontroller and are never dependent on a cloud service.
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).
Overview
RF-controlled relais are controlled by radio signals:
- Frequency: make sure the device you purchase uses a radio frequency that is legal to use in your region. In Europe, such devices use 433MHz.
- Remote Control: the device is a RF receiver only. To use it, you also need a (separately available) compatible RF sender (the remote control).
Works Everywhere
Using radio signals to control makes this technique flexible: no WiFi infrastructure is needed, so this technique works also away from your home, in locations with poor WiFi coverage, or in cars, boats, or recreational vehicles.
You must be close to the device
Since RF control is a direct connection between your remote control (sender) and the device (receiver), you need to be in close proximity of the device to control it.
This is typically no problem with use cases like garage door openers (as you want to enter the garage which you can’t do when being far away).
You will be unable to remote control such devices from great distance. If for example you took off from home but aren’t sure whether the garage door was closed, you cannot close it from abroad.
It doesn’t take much to build a WiFi-RF gateway: take a WiFi-enabled microcontroller such as an ESP32, hook up a simple RF sender to it, and now can control RF-controlled devices from anywhere in the world, too. Before you consider this, make sure you understand the security implications (see below).
Safety Concerns
The proximity requirement can be a valuable security feature: no hacker from the other end of the world can ever have access to the device.
However, since RF isn’t confined to wires, and since RF-controlled relais do not use sophisticated wireless protocols with encryption and access protection, anyone who is also in close proximity can pick up the signal when you operate your remote control, record it, and play it back later.
There are more sophisticated devices that use rolling codes (automatically changing code sequences that can’t easily recorded, played back and reused by someone else). The true security risk depends on a number of factors. If you operate a security-critical entity like a garage door, and if you do this in a crowded rural area where other people can pick up your radio signal easily, there is a significant risk. If however you just control your bedside light, or your garage door is located on a large estate that keeps other people at distance, then it’s a different ballgame.
One-Way Communications
Unlike with WiFi-controlled devices, RF is a one-way communication: you can control the device, but the device cannot talk back to you.
That’s why RF-controlled devices are typically used for scenarios with visual feedback: when you open a garage door, you immediately see whether it is moving, and if it is currently open or closed.
RF-controlled devices cannot provide you with an electronic feedback though (that you might want use to operate a control lamp to visualize the current device state), so you cannot check whether the control signal was actually received by the device (other than by using your eyes and witness the effect).
Hardware
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.
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.
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.
Datasheets
BP2525 AC-to-DC converter
MB10 Bridge Rectifier
AMS1117 3.3V Voltage Regulator
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(content created Jul 31, 2024)