Basics: Relays

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Basics: Relays - a never ending story


Relays are the most used actuators in home automation. Most people use breakout boards with a relay on it. Sadly there is no "standard" relay board. It seems every small manufacturing garage in china has it's own layout and schematic. Some use 5V, some use 12V, some do with opto couplers, some without.... the perfect chaos. Several problems result from this chaos. Relays keeping switched on no mater what your ESP says. Relays not switching at all. And last but not least - sometimes they work ;)

Let's get some order into that chaos.


The ESP8266 can't switch high currents or voltages and no AC at all. But it can switch a relay that can switch high load, isolating high voltage from the ESP circuit. There are several relay types in the wild.

Relay Types

There are mainly two relay types in the wild: The good old mechanical relay with the nice "click-clack" sound and the electronic relays, so called "Solid State Relays" (SSR) which are completely silent.

Mechanical Relay

Mechanical Relay for PCB.jpg
The mechanical relay is a simple construction.
It consist of a coil, one or more contacts and a spring. The spring keeps the contact in the "open" position. If the coil gets current, the magnetic field pulls the contact into the second position, the contact closes. As soon as the current is switched off the spring pulls the contact back.
Mechanical relays may just have one contact switching to closed if the coil works (Normally open or NO). The contact might be a changeover contact wich has a "common" connector and a NO (open without coil current, "normally open") and a NC (closed without coil current,"normally closed") connection. There might even be several contacts in one relay.

There are several special types of mechanical relay:
Latching Relay:
One current pulse switches on and keeps on when current goes off, a second pulse switches off.

Polarized Relays:
These relays have a defined + and - pin for the coil that must be followed.
The picture shows an example for a polarized micro relay:
Mechanical MicroRelay Polarized.jpg

Note: These MicroRelays can not switch mains voltage! Use them for low voltage door openers or similiar.

Advantages of mechanical relays:

 Several contacts in one relay availlable.
 NO/NC availlable.
 Limited resistance to overload and overvoltage.


 Noisy, might disturb in a livingroom or sleepingroom.
 Mechanical, moving parts that might wear out over time.
 Contact bumping when switching.
 High currrent for the coil, needs a transistor or FET for driving and generates peaks on power supply.
 The Coil induces high voltage when switching off.

Solid State Relays

SolidState Relay for PCB.jpg

The solid state relay, SSR for short, is a complex electronic circuit. It uses an opto coupler for input, this isolating the driving circuit from the load voltage. Isolation voltage is usually 2000V, 4000V or more. The switching element usually is a triac for AC switching types. For DC there are types with a MOSFET as switching element.
As with the mechanical relays there are specialized types of SSR's:
AC Types Can only (!!) switch AC.

AC with zero crossing circuit
This type switches on and off if the AC voltage is at the zero point. Expensive and necessary only for special applications.

DC Types
Can only (!!) switch DC!
And a lot more that are reserved for special cases.

Advantages of SSR's

 Completely noiseless
 Low driving current and voltage availlable. Some types can be driven directly from the ESP.
 no mechanical parts, no wearing out.
 no contact bumping.
 no voltage peaks on the low voltage side.
 Availlable for very high current for an affordable price.


 Very sensitive to overvoltage and overcurrent.
 No NC availlable usually.
 Usually with only one or sometimes three "normaly open" circuits.
 Needs cooling! Smaller relays up to 5A must be mounted with at least 1cm distance to other parts
 Bigger relays usually need a heat sink. Watch out: Heat sink may carry mains voltage with some types!
 Leak current! These relays leak some current even if switched off.

Leak Current from SSR's might lead into problems under certain circumstances.
The both types shown above ("Hoymk D3805KH" and "Mager GJ-6-L", 5A AC) were tested here.
The Mager GJ-5-L showed a low leaking current and is useable for most purposes.
The D3805HK showed a really high leaking current, enough for a 10W LED to glow and a small solenoid valve to keep open!
This is not recommended for normal use.

Meanwhile a third sample arrived: The SSR-D32A380/5 sold by Pollin.
Similar to the D3805HK it shows a leaking current, as the test LED is slightly glowing in the dark when off.
It seems to be a lower leaking then with the D3805HK, this is subject to some further investigation.
The advantage of the Pollin SSR: It contains a small LED on the top indicating if the relay is on.
This SSR is a bit bigger then the other two types tested, it just fits the case. So holes for the LEDs
can be drilled on the top side of the case.

 Anyways: Always remember that a switched-off SSR is not completely switched off!
 Even a small leaking current is enough to give a shock. Nice surprise if you're on the ladder changing bulbs....

Basic ESP Circuit - Mechanical Relays

The basic circuit is simple.
As the relay usually consumes more current as the ESP can handle it needs a transistor or FET to drive the relay.
To suppress the voltage peak when swiching off the coil a free-wheeling diode is needed. This can be a standard silicium diode, a 1N4148 for example. Thats all folks :) Relay-basic circuit.jpg
Well, there is another way to switch the relay. In the above example we used a NPN transistor or a N-channel FET. A PNP transistor or P-Channel FET can be used too. Some board manufacturers do this and now we run into problems promptly.

The relays usually need some higher voltage then the ESP, 5V or 12 V in most cases. The PNP transistor in the example is connected to +5V. It will switch the relay to "on" if the voltage at the input is at least 0.7V below the Vcc of 5V.
The ESP delivers a maximum of 3.3V if the GPIO is "high", around 0.5V if it is "low". Both voltages are far below the 5V-0.7V = 4.3V threshhold of the relay board. The relay is permanently on, no matter what your ESP says.

We can get this to work in a simple way with a 5V relay board. A standard diode has a loss of around 0.7V. We can use this to get a higher output level from the ESP. We simply use two diodes in the connection between relay board and ESP. This raises the voltage from the ESP up to 3.3v + 1.4V = 4.7V. This is above the threshhold of 4.3V. If the board uses a FET it might be necessary to use three diodes. That usually will work.

On top: This circuit switches the relay off if the GPIO is "high", this leading to irritations sometimes.
The picture shows the basic circuit with a PNP transistor, and the " 2 Diodes " solution:

Relay with PNP.jpg

More complex: Using an Opto Coupler

Many widespread relay boards use opto couplers. Honestly i don't know the reason besides it looks good in advertising.
In theory the opto coupler isolates the relay coil from the ESP circuit.
Well, that's nice theory. In real world we use the same power supply for relays and ESP. This bridges the opto coupler as the circuits are connected via power supply making the opto coupler completely useless. Anyways the relay isolates the load from the ESP anyways. It might be usefull only if relay switching interferes with the ESP. In this case you might try a separate power supply for relay coils.
This isolates the ESP completely from the coil circuits. Note that a jumper has to be set for that on some boards.
Again there are two possible ways to use this circuit. The opto coupler input has one pin to the ESP and one to ground - this works without any problem but it is not widespread. Relay Sample-Opto working.jpg
Most boards i've seen connect the opto coupler to 5V or 12V and the other input pin to the ESP.
Relay Basic-Opto Problem.jpg

This runs into same issues as with the PNP transistor above. The high voltage of 3.3V from ESP is far below 5V, this keeping the relay permanently active.
The solution is same as with the transistor board above. Use some diodes between ESP and relay board usually does the job with relay boards using 5V. Depending on the circuit you will have to experiment how many diodes are necessary.

Boards with 12V

If your relay board uses 12V and needs "Low" to switch the relay on it gets somewhat more complex. Basically it is possible to use a lot of diodes but it's not very handy. You may try a zener diode of around 8,5.. 8.7 V. Honestly i don't recommend it. In this case it is better to use an additional transistor or FET to get it to work. This has one advantage: It inverts the switching signal so a "low" output says the relay is off.
Relay 12V-solution.jpg

Special Board Types

There are some boards in the wild which can be configured if "low" or "high" should switch the relay on.
With this boards you will have to experiment a bit with your ESP and the board, there should be a combination that works.


Note the jumpers down left - that's the configuration for switching on high or low level.

Basic ESP Circuit - SSR

This is more simple. If you got a SSR that has an input of "3...12V" or "3...30V" it usually can be driven directly from the ESP. The only thing you should keep in mind: Do not ommit the varistor (RVx in the schematics). It protects the SSR from voltage peaks that may kill it within some milliseconds. Instead of the Varistor a TVS diode can be used, for example a P6K400CA.
Be carefull, you have to use a bidirectional type!

Basic-SSR Simple.jpg

It is possible to direct drive a SSR from the ESP but i won't recommend this. I'd prefer the same circuit as with the mechanical relay. It makes the relay circuit more universal and reduces load on the ESP.

Basic SSR - better.jpg

Instead of the 2N2222/BC547 you may use a small FET, a BSS138 or 2N7000 / 7002. Omit the 22K resistor if useing a FET.
R2 is needed if your relay can't take the Vcc directly. If you use a "3...30V" input SSR it's not necessary.

Any opto coupler is completely needless if you use a SSR. The SSR has an opto coupler built in to isolate the load from the ESP.
At this time there are not that much relay boards with SSR in the market, this will change over time as they get more popular. From what is on the market be carefull what you buy. Most boards i've seen nowadays on AliExpress and ebay have a DC relay installed - they can switch DC only! They can't switch mains voltage.

Choosing Your Relay

Which relay should be used?

Well, this depends on what you want to switch. If you control a heavy motor or pump, I'd always prefer a mechanical relay. The resistance against short overcurrent is a good point here.
For experimenting I prefer mechanical relays too - I love the "click" sound, I don't have to connect a load to see if it works.
If noise counts or if you place the relay in a dusty or humid environment a SSR is the better choice. It's completely encapsulated so dust and humidity can't damage it. As it has no mechanical parts it is completely noise-free. So it's the perfect choice to switch lighting in a living room or sleeping room.
Of course you have to chose a relay that fits your parameters.
Which voltage you want to switch? AC or DC? Which current?
If you want to use a SSR for high current you have to calculate the space for a heat sink and you have to use a vented case. All these parameters have to be figured into your project.

It is also possible to use a small SSR to drive a really heavy external relay for a heavy machine for example.

Some Words About DIY Relay Boards

All in all it's no problem to build your own relay board fitting your needs exactly. But watch out if you want to switch mains voltage. There is a reason why in some countries the policies specify a minimum distance between the parts with high voltage and the low voltage circuits. Do not use experimental PCB's with copper pads or stripes!
Good breakout boards are milled out, there is a gap in the PCB for isolating mains from low voltage. Prefer these boards over others without milling.

Be very carefull - mains voltage can burn your home or kill you.

In case of doubt ask an expert.


Now you have just got your brand new relay board and it does not work.
What to do?

At first you should check the switching level. Is it switching on with "high" or with "low"?

 Connect the board to power only (5V or 12V usually).
 Plug a jumper cable to one of the inputs.
 With the other end of the cable touch GND and +Vcc (5V or 12V).

On one of these touches the relay should switch. If it switches when connecting to GND the board uses "low" as active (relay on) signal.
In this case diodes or a transistor / FET as described above might help.

Connect the board to the ESP carefully. Check voltages, the power supply should be able to deliver enough current for the relays!
Check the connection cables. Jumper cables are well known for slack joints.