A relais is the ubiquous low cost solution to switching large currents or high voltages, and separating the control circuit from the load circuit.
Basic Principle
All electromechanical relais work the same:
- Control Circuit: A control circuit is connected to a coil. When electrical power is applied to the coil, it produces a magnetic field.
- Load Circuit: A (separate) load circuit is placed in close proximity of the coil and uses an iron (or elsehow magnetic) latch. The magnetic field produced by the coil can move the latch physically, thus opening or closing the load circuit.
Advantages
The relais design is cheap, well-proven for many decades, and versatile:
Depending on construction and materials, a relais can be small, or it can switch huge loads. It can be normally open or normally closed, or both.
Disadvantages
In electronic projects, a mechanical relais is problematic primarily because of this:
- High Power Consumption: The current required for the coil to produce a magnetic field strong enough to operate the latch is comparably high. In car electric, it does not matter whether a relais requires 100-200mA of current just to stay in the on state. In electronics, this is a huge current that cannot be handled by microcontroller GPIO. In addition, a battery-operated device would soon deplete its batteries just by operating the relais.
- Dangerous Voltage Spikes: When a relais switches from on to off, the magnetic field produced by the coil collapses and induces a momentary spike of high voltage. In simple car electric, this spike does not harm. It can easily destroy sensitive electronics, though.
When using electromechanical relais, motors, or anything else with coils that can produce magnetic fields in sensitive electronics projects, always make sure to use a freewheeling or flyback diode: this is a regular diode that is connected anti-parallel to safely take care of the energy of the collapsing magnetic field: the diode enables the energy to circulate inside the coil, thus prolonging the time it takes for the magnetic field to collapse and protecting the rest of the circuit from unwanted energy spikes.
Controlling Relais
Discrete Solutions
Relais can control powerful loads by using tiny switches or buttons:
Since the control circuit requires only relatively low voltage and current, and since the load circuit is separated from the control circuit, it is sufficient for a switch to be capable of handling only low voltage and current of a few hundred milliamperes.
This is how relais are used in cars: when you turn on the (powerful) head lights with a tiny switch on your dashboard, the switch really only operates a relais. The relais in turn controls the large currents required by the head lamps.
Microcontrollers and Electronics
In microcontroller-based solutions or when using sensitive ICs, even a relais often requires too much current to be operated directly by a GPIO.
This is why transistors are used to amplify the current. Breakout boards with relais, flyback diode and amplifying transistor are readily available for low money.
Remote Control
Relais are often used to remotely control devices over the air. In this scenario, the relais is combined with a receiver:
- Radio Frequency (RF): Most cost-effective classic remote control solutions use license free radio bands, i.e. 315MHz** (US) or *433MHz (Europe). When the RF signal is picked up by the receiver, it turns the relais on or off.
- WiFi: With the increasing popularity of home WiFi networks, a relais can also be controlled by WiFi signals. In this case, the relais is connected to a WiFi-enabled microcontroller (such as a cheap ESP8266). This microcontroller can connect to the home WiFi. Instead of using dedicated senders or remote controls, the control signal comes from a smartphone app (and often involves intransparent cloud solutions operated by the vendor).
- Light: Less common is the use of infrared light (IR) that can also be used to transmit a control signal. Since light needs a free line of sight between sender and receiver, this only works for limited use cases.
WiFi-Controlled Devices
WiFi solutions work very similar to RF solutions and in fact both use radio frequencies.
Whereas RF solutions typically use 315/433MHz, WiFi uses 2.4GHz and can benefit from the potentially wide coverage of your home WiFi network.
The fundamental difference is how control signals are processed:
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RF: You Are In Control: With RF, there are distinct senders and receivers that directly communicate and that you control.
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WiFi: Vendor Is In Control: With WiFi (at least with prefabricated switches and remote control solutions), the digital control signals are beyond your control and often involve proprietary cloud processing and non-public encryption keys, controlled alone by the vendor architecture.
Security
WiFi solutions are convenient and flexible in normal times.
Once SHTF, they can quickly become insecure and fragile: should the vendor architecture go offline or fail, so does your remote control architecture. All of your remote switches can be operated by anyone in control of the vendor architecture.
As part of your planning, always consider what you want to remote control, and how critical this is for you and your safety: how would it affect you if someone else starts remote controlling? Turning on and off the lights in the living room probably has a low risk assessment, especially if you can still use regular switches, too. Opening your garage door or turning on the oven may have much more security implications.
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(content created Apr 29, 2024)