ESP8266 Community Forum

- Fri Jun 29, 2018 9:18 am #76726 I recently created a circuit with a ESP-12 to connect to my doorbell ringer, so that I could perform actions whenever someone pressed the doorbell.



I now use it to send a MQTT message to home assistant, which in turn takes a snapshot with the camera next to the door, sends the picture via pushbullet to my phone and also sound the wireless door ringer. Great fun, and it works like a charm

Now I came up with another idea to improve the circuit: I would like to connect a relay to one of the gpio pins of the ESP, so that I can also switch the regular bell on and off, for instance to turn it off during the night.

Fot that purpose I bought a Songle SRD-03VDC-SL-C relay (like these: https://www.aliexpress.com/item/5PCS-lo ... autifyAB=0). Since they're meant to be used with 3V DC I thought I could just use them right away. I tested them with a breadbord adapter at 3v (like one of these: https://www.aliexpress.com/item/Breadbo ... autifyAB=0), and it worked, so I figured it would also work if i connected it to a ESP8366 GPIO and ground.

I tried it with this code:

Code: Select all 
void setup()   {
  Serial.begin(115200);
  pinMode(16, OUTPUT);       
 
  }
 
void loop() {
   Serial.println("off");
   digitalWrite(16,LOW);         

   delay(2000);                                   
   Serial.println("on");
   digitalWrite(16,HIGH);   
   delay(2000);     

}

but it seems that it doesn't work. Googling around, I found lots of similar issues, it seems the ESP pin cannot provide enough power to trigger the relay. I also saw lots of solutions that introduced a 5v source for the relay, but since I have a 3V relay, it seems to me that it would be unnecessary (I also don't have a 5v source in my circuit and I would like to keep the parts count as low as possible).

Any idea how I could solve this problem?

- Fri Jun 29, 2018 10:32 am #76727 Use a transistor or a FET to control the relay, it's really just basic electronics.

Just some random picture taken somewhere from the internet:


There are ready made shields for the ESP on Ali as well.

Assumption is the mother of all f*ckups. At least: that's what I'm assuming.

- Fri Jun 29, 2018 12:16 pm #76729 I would lower the base resistor values compared to the circuit shown above. For the same size relay the current to turn on the coil will be higher compared to a 5 volt circuit. So the drive to the transistor needs to be higher as well. I would change the 10K resistor between the port pin and the base of the transistor to 1K Ohms as a good starting point. It would be better to know the coil current, and the transistor's gain, to come up with a better base resistor, but, in general, the 1K should work.

Using mosfets will eliminate the concern about series resistors for base current (mosfets being a voltage rather than current controlled device) but you must take into account the minimum turn on voltage for the gate. (and be at minimum higher than that).

- Sat Jun 30, 2018 2:20 am #76738 I'll immediately admit that my knowledge of electronics is not that great.

I know there are ready made shields, but I would like to integrate the relay into my own pcb, so I rather use the bare relay.

On thing i don't understand is that all samples with a transistor I find include a 5v source, including the one posted by QuickFix and the one in this circuit:



Since my relay is meant to be used with 3V, I would think I don't need a 5V source. If I would take the above schematic and just connect 3.3v instead of 5V, would I be there? (Even though they do use a shield with a octocouper in this example)

The shield linked by QuickFix only seems to have a few resistors, a diode and a transistor