Esp-01 and 3V3 relay module: unexpected resets (cause 4)

Question

I am working on a simple circuit (for debugging purpose) which switches a 3V3 relay every second through the Esp-01 GPIO2 pin. The power supply comes from the USB to ESP-01 adapter (but the same happens with another power supply)

This circuit is the following:

**ESP8266-01:** 
VCC <--> 3V3 power supply
GND <--> GND (power supply)
CH_PD <--> 3V3 power supply
GPIO2 <--> IN (relay)

**3V relay:**
VCC <--> 3V3 power supply
GND <--> GND (power supply)
IN <--> GPIO2 (ESP-01)

The relay is mounted on a power extension cord, on which I can connect various devices.

Here is my circuit diagram (the breadboard is not represented):

My sketch is the following:

#include <ESP8266WiFi.h>

const int switch_pin = 2;
int state = 0;


void setup() {
  Serial.begin(115200);

  WiFi.forceSleepBegin();

  pinMode(switch_pin, OUTPUT);
}



void loop() {

  if (state == 0){
    digitalWrite(switch_pin, HIGH);
    state = 1; 
    Serial.println("1");
  }

  else{
    digitalWrite(switch_pin, LOW);
    state = 0;
    Serial.println("0");
  }
  delay(1000);
}

which produces, on the serial:

0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1

 ets Jan  8 2013,rst cause:4, boot mode:(3,7)

wdt reset
load 0x4010f000, len 3584, room 16 
tail 0
chksum 0xb0
csum 0xb0
v2843a5ac
~ld
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1

 ets Jan  8 2013,rst cause:4, boot mode:(3,7)

wdt reset
load 0x4010f000, len 3584, room 16 
tail 0
chksum 0xb0
csum 0xb0
v2843a5ac
~ld

Thus, sometimes, the Esp-01 resets and, sometimes it cannot "recover" from such resets.

However, when I disconnect the extension cord (from the electrical outlet in the wall), the relay switches indefinitely as expected (I do not know if it's a clue to solve my problem).

I tried with 2 relays and 2 Esp-01 and always got the same behaviour.

Would you have any idea about the origin of such a problem?

Answer 1

It is likely that the power peak from when the relay coil is powered on, is causing a dip in the voltage and thereby causing the ESP to reboot.

This is a common topic on many forums like here in EE, https://arduino.stackexchange.com/ and others.

By adding a large electrolyte capacitor close to the VCC and GND on the ESP is a common way to solve this problem.

Another root cause is the usage of inferior cables (USB) with very high AWG (very low area) on the copper in the cable, see https://www.youtube.com/watch?v=n70N_sBYepQ

Answer 2

The relay contacts used to switch mains voltage arc and spark when the contacts are closed or opened. The radiated electromagnetic interference from the spark is enough to cause a malfunction in the microcontroller board. This arcing and sparking does not happen when mains is disconnected.

Answer 3

I added 104 and 106 capacitors to the reset pin and added a 3v3 pull-up resistor and the problem was almost completely solved. No more ESP crashes.