I'm building a driveway gate monitor, and am getting some readings on the A0 pin which I'm having a hard time understanding.
The gate is operated by a 12v battery, which I also want to use to power the monitor I am building. But while developing the code, I've been using a USB cable from my computer to power the NodeMCU.
I connected leads from a 12v battery to a voltage divider (100k and 10k resistors) and fed it into A0 on the NodeMCU. This is a 12E development board, so the A0 pin can accept up to 3.3v input. It then has an internal voltage divider using 100k and 220k resistors, to bring it down to the 1v limitation of the ESP8266 chip.
When using USB to power the board, this equation produces a good reading of the 12v battery's charge:
(((analogRead(A0) / 1024) / 0.3125) / (R2/(R1+R2)))
But when I switch to powering the NodeMCU through a 3v3 pin, from a 3.3v power supply connected to the same 12v battery I'm trying to monitor ..... the reading of A0 changes significantly.
Why would the A0 reading be different if powering on 5v USB vs. 3v3 pin? Is it due to the ADC's reference voltage maybe?
I've attached a picture of my breadboard setup, as well as a drawing of what I think is going on and how I came to the equation I am using.
Here is the code I'm using to try and figure this out.
#include <DNSServer.h>
#include <ESP8266WebServer.h>
#include <WiFiManager.h>
WiFiServer server(80); // Define web server name and port
// ********************************************************* DEFINITIONS *********************************************************
// PIN DEFINITIONS -----------------------------------------------
int Battery_PIN = A0; //Define pin to connect the output of the voltage divider from the gate controller's 12v battery (using a 100k and a 10k resistor to divide down to 1.x volts
// Variables for Monitoring Gate Controller's 12v Battery -----------------------
String buf = ""; // Define string to hold web response from client
int v = 0; //v counter
int t = 0; //t counter
float volt = 0.0; //Variable to add up the 1000 readings of the gate controller's 12v battery
float raw = 0.0; //Debug variable to display A0 raw value on web portal
float reading = 0.0; //Variable to store A0 reading
float vin = 0.0; //Varibale to store the result of averaging the reading's of the gate controller's 12v battery
float R1 = 96900.0; //Actual value of R1 used in the voltage divider, to bring the 12v gate controller battery down to a reasonable voltage
float R2 = 9970.0; //Actual value of R2 used in the voltage divider, to bring the 12v gate controller battery down to a reasonable voltage
// ********************************************************* INITIALIZE *********************************************************
void setup() {
// Pin Modes -----------------------------------------------
pinMode (Battery_PIN, INPUT); //Set the pin for commecnting to the voltage divider output from the gate controller's 12v battery, to an Input
// Initialize Serial and WiFi -------------------------------
Serial.begin(115200); // initialize serial for debugging
WiFiManager wifiManager; // Start WiFi Manager and connect to previous WiFi if found
// wifiManager.resetSettings(); // Uncomment and run it once, if you want to erase all the stored information
wifiManager.autoConnect("MySetupAP"); // If no previsous WiFi connection found, start configuration AP
Serial.println("<Controller is ready>"); // Success message printed to serial monitor
server.begin(); // Start the web server
}
// ********************************************************* MAIN LOOP *********************************************************
void loop() {
HandleClient(); //Handle any requests coming in over WiFi
ReadBatteryVoltage(); //Check the voltage of the gate controller's 12v battery
}
// ********************************************************* FUNCTION DEFINITIONS *********************************************************
// Check Gate Controller's Battery Voltage Level --------------------------------------------------------
void ReadBatteryVoltage() {
if (v == 100) { //If v has reached 100 cycles, meaning 100 voltage readings of the gate controller's 12v battery have been taken
vin = volt / v; //Set voltage variable (vin) to the average of the 100 readings
v = 0; //reset v counter
volt = 0; //reset volt to 0
}
if (t == 1000) { //Read the A0 value every 2000 cycles
raw = analogRead(Battery_PIN);
}
if (t == 2000) { //Read the A0 value every 2000 cycles
reading = analogRead(Battery_PIN);
volt += (((reading / 1024) / 0.3125) / (R2/(R1+R2))); //Convert reading to 12v battery voltage
v++; //Increment v counter
t = 0; //Reset t counter
}
t++; //Increment t counter
}
// Handle Client Connections ------------------------------------------------------------
void HandleClient() {
WiFiClient client = server.available(); // listen for incoming clients
if (!client) { // If no client ...
return; // ... Return
}
while (client.connected()) { // loop while the client's connected
if (client.available()) { // if there's data to read from the client
char c = client.read(); // read a byte
buf += c;
if (buf.endsWith("\r\n\r\n")) { // If two newline characters in a row
sendHttpResponse(client); // Call HTTP response function
break; // Exit function
}
}
}
buf = "";
}
// SEND HTTP REQUEST TO THE CLIENT ------------------------------------------------------------
void sendHttpResponse(WiFiClient client) {
client.println("HTTP/1.1 200 OK");
client.println("Content-type:text/html");
client.println("Connnection: close");
client.println();
client.println("<!DOCTYPE html>");
client.println("<html>");
client.println("<head>");
client.println("<title>Gate Monitor</title>");
client.println("<style type=\"text/css\">");
client.println(".tftable {width:950px;border-collapse: collapse; table-layout: fixed;}");
client.println(".tftable tr {background-color:#d4e3e5;}");
client.println(".tftable td {font-size:40px;}");
client.println("h1 {font-size:45px;}");
client.println("h2 {font-size: 45px; Line-Height:0px;}");
client.println("h4 {Line-Height:0px;}");
client.println("</style>");
client.println("</head>");
client.println("<body>");
client.print("<h4>Battery Voltage: ");
client.print(vin);
client.println("v</h4>");
client.print("<h4>Battery A0 Raw: ");
client.print(raw);
client.println("v</h4>");
client.println("</body>");
client.println("</html>");
client.println();
}
