Solenoid causing ESP32 to freeze/become unresponsive

Hi, I have an ESP32 (Adafruit HUZZAH32) and am connecting a FeatherWing power relay that I'm trying to use to switch on/off a solenoid through the web page. But for some reason, every switch of the relay or two, the ESP32 freezes and becomes unresponsive. I have to reset the ESP32 to get it working again and I'm not sure what's going on.

The solenoid I'm wanting to control is 24VAC and below is a picture of how I wired it up:

I also read on this forum that adding a flyback diode might help. So I went ahead and did that but that didn't seem to resolve the issue. (I'm unsure if I actually wired up the "flyback diode" correctly) Here's how I wired up the "flyback diode":

I probably have all of these wire up incorrectly, which might be my problem.

Here are some pictures I took that may help:

Your guys' help is much appreciated, thanks!

Here's my code as well. Thanks!

#include <WiFi.h>
#include <WebServer.h>
#include "SPIFFS.h"
#include "ESPAsyncWebServer.h"

/* SSID and PW */
const char* ssid = "";
const char* pw = "";

/* Network Details */
IPAddress local_ip(192,168,1,120);
IPAddress gateway(192,168,1,1);
IPAddress subnet(255,255,255,0);

//AsyncWebServer server(80);
WebServer server(80);

/* PIN usage */
uint8_t LED = 21;
uint8_t relayCtrl = 15;
bool LEDstatus = LOW;
bool relayCtrl_Sig = LOW;

String processor(const String& var) {
  return String();

void setup() {

  // Setting PINs
  digitalWrite(LED, LEDstatus);
  digitalWrite(relayCtrl, relayCtrl_Sig);
  // Setting up File System
  if(!SPIFFS.begin(true)) {
    Serial.println("An error has occurred while mounting SPIFFS\n");

  File file ="/index.html", FILE_WRITE);

  if (!file) {
    Serial.println("Error openin file");


  // Setting up ESP32 Station Mode
  WiFi.begin(ssid, pw);

  Serial.print("Connecting to WiFi...");

  while (WiFi.status() != WL_CONNECTED) {

  server.on("/", handle_OnConnect);
  server.on("/ledon", handle_ledon);
  server.on("/ledoff", handle_ledoff);

  Serial.println("HTTP server started");
uint16_t count = 0;
void loop() {

  //Serial.println("Count: ");
  //count = count + 1;
  digitalWrite(LED, LEDstatus);
  digitalWrite(relayCtrl, relayCtrl_Sig);

void handle_OnConnect() {
  LEDstatus = LOW;
  relayCtrl_Sig = LOW;
  Serial.println("LED OFF");
  server.send(200, "text/html", SendHTML(false, LEDstatus));

void handle_ledon() {
  LEDstatus = HIGH;
  relayCtrl_Sig = HIGH;
  Serial.println("LED ON");
  server.send(200, "text/html", SendHTML(true, LEDstatus));

void handle_ledoff() {
  LEDstatus = LOW;
  relayCtrl_Sig = LOW;
  count = 0;
  Serial.println("LED OFF");
  server.send(200, "text/html", SendHTML(false, LEDstatus));

void handle_NotFound() {
  server.send(404, "text/plain", "Not Found");

String SendHTML(uint8_t led1stat,uint8_t led2stat){
  String ptr = "<!DOCTYPE html> <html>\n";
  ptr +="<head><meta name=\"viewport\" content=\"width=device-width, initial-scale=1.0, user-scalable=no\">\n";
  ptr +="<title>LED Control</title>\n";
  ptr +="<style>html { font-family: Helvetica; display: inline-block; margin: 0px auto; text-align: center;}\n";
  ptr +="body{margin-top: 50px;} h1 {color: #444444;margin: 50px auto 30px;} h3 {color: #444444;margin-bottom: 50px;}\n";
  ptr +=".button {display: block;width: 80px;background-color: #3498db;border: none;color: white;padding: 13px 30px;text-decoration: none;font-size: 25px;margin: 0px auto 35px;cursor: pointer;border-radius: 4px;}\n";
  ptr +=".button-on {background-color: #3498db;}\n";
  ptr +=".button-on:active {background-color: #2980b9;}\n";
  ptr +=".button-off {background-color: #34495e;}\n";
  ptr +=".button-off:active {background-color: #2c3e50;}\n";
  ptr +="p {font-size: 14px;color: #888;margin-bottom: 10px;}\n";
  ptr +="</style>\n";
  ptr +="</head>\n";
  ptr +="<body>\n";
  ptr +="<h1>ESP32 Web Server</h1>\n";
  ptr +="<h3>Using Access Point(AP) Mode</h3>\n";
  {ptr +="<p>LED1 Status: ON</p><a class=\"button button-off\" href=\"/ledoff\">OFF</a>\n";}
  {ptr +="<p>LED1 Status: OFF</p><a class=\"button button-on\" href=\"/ledon\">ON</a>\n";}

  ptr +="</body>\n";
  ptr +="</html>\n";

  return ptr;

Thanks for the info.
A flyback diode will not work with AC, you need some form of "snubber" circuit across the contacts of the relay.

I assume if you disconnect the solenoid the fault does not occur?

If you Google

24vac solenoid snubber circuit

It will show you some solutions that may help.

Tom... :smiley: :+1: :coffee: :australia:

That diode is most likely blown. Throw it away.

You are driving a 5V relay with an ESP32 GPIO pin. Yea, no wonder you are having issues.

It is not a relay, it is a relay module.

And it is not a 5 V relay in any case, but a 3.3 V one.

I use the optically isolated relay modules for the ESP32, they work better, for the ESP32.

Hi Tom,

You're correct, if I disconnect the solenoid, the fault does not occur.

I will look into the snubber circuit.

Thanks for your help! I'll let you guys know how it goes.

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