code to control relay module on a high/low dc input voltage

Projects Programming
1

I'm a new person to the Arduino world. I need help writing code to control a external relay module on a change in an input dc voltage. The normal input is 3 vdc. I need to activate the relay module (break contacts) if the voltage input goes high to 3.1 vdc or low to 2.9 vdc. This is being used as a safety interlock to control an external 120 volt amplifier. I know how to connect the Arduino board to the external relay board and I have the 3 vdc control wire from the amplifier but I don't know how to write the code to protect the amp as stated above.

Any help is greatly appreciated! I apologize for the beginner question.

Jim

2

In simplest terms you can try something like this:

#define UPPER_LIMIT     3.1
#define LOWER_LIMIT     2.9

//change to suit your circuit as needed
#define NORMAL_RELAY    HIGH    //normal operation
#define SAFE_RELAY      LOW     //when amp needs to be "safed"

const byte pinInput = A0;
const byte pinRelay = 2;

int currReading;
float volts;

void setup() 
{
    pinMode( pinInput, INPUT );     //not strictly needed, provided for clarity
    pinMode( pinRelay, OUTPUT );
    digitalWrite( pinRelay, NORMAL_RELAY );

}//setup

void loop() 
{
    currReading = analogRead( pinInput );    
    volts = (float)currReading * 5.0 / 1024.0;
    
    if( volts >= UPPER_LIMIT || volts <= LOWER_LIMIT )
        digitalWrite( pinRelay, SAFE_RELAY );
    else
        digitalWrite( pinRelay, NORMAL_RELAY );
    
}//loop

You can add some more functionality (and complexity) if you like; for example, if the control voltage goes high or low, the relay is safed and it stays that way until there's a user "reset" input. Perhaps you add a time element; if the relay is safed a minimum amount of time has to pass before it is re-enabled. Maybe you add some hysteresis to the system so the SAFE and NORMAL points are not one and the same.