Terraform-AWS-Modules-using subnet-id of a VPC child module in another ec2 child module

Question

I will start by thanking you for your time and I have been googling and reading and maybe just overlooking something very simple. I have tried my best with some articles on medium and the terraform documentation.

So, my problem is.. I have a root module that looks like this

module "VPC" {
    source = "/home/jamie/Terraform_Project/modules/1_VPC/"
    subnet_id = "module.VPC.Public_Subnet_id" 
}

module "Key_Pair" {
    source = "/home/jamie/Terraform_Project/modules/2_Key_Pair/"
}

module "EC2_VPN" {
    source = "/home/jamie/Terraform_Project/modules/3_EC2_VPN/"
}

and three child modules as you can see. I cannot reference the "Public_Subnet_ID" from my VPC module in my EC2 module. I will show my main.tfs and my output.tfs below. I think its worth mentioning that I have tried various things I have found on google and don't seem to get anywhere below is my latest attempt. i have seen other answers on stackoverflow but they have not worked for me or i am still doing something wrong.

VPC - main.tf (will show subnet bit only)

/* Public Subnet */
resource "aws_subnet" "Public_Subnet" {
  vpc_id     = aws_vpc.main.id
  cidr_block = "10.0.1.0/24"
  map_public_ip_on_launch = true

  tags = {
    Name = "Public"
    Project = "${var.project}"
    Architect = "${var.architect}"
  }
}

VPC - output.tf (2 options i have tried)

**this**
output "Public_Subnet_id" {
    value = "${aws_subnet.Public_Subnet.id}"
}
**or this**
output "Public_Subnet_id" {
    value = aws_subnet.Public_Subnet.id
}

**EC2 - main.tf (problem bit)

resource "aws_instance" "web" {
  ami           = data.aws_ami.ubuntu.id
  instance_type = "t3.micro"
  subnet_id = "module.1_VPC.Public_Subnet_id"
  key_name = "${var.project}_Key"

  tags = {
    Name = "VPN_Server"
    Project = "${var.project}"
    Architect = "${var.architect}"
  }
}

i also tried the above with variable (maybe wrong from another thread/guide)

my first errors where the "module.1_VPC.Public_Subnet_id" wasnt referenced but managed to get that bit but now it just ends up with

Error: creating EC2 Instance: InvalidSubnetID.NotFound: The subnet ID 'module.1_VPC.Public_Subnet_id' does not exist │ status code: 400, request id: 00fa3944-4ea3-450b-9fd4-39645785269f │ │ with module.EC2_VPN.aws_instance.web, │ on .terraform/modules/EC2_VPN/main.tf line 17, in resource "aws_instance" "web": │ 17: resource "aws_instance" "web" {

Again thankyou for taking the time, I am learning and trying to build / learn as I go not just copy and paste other templates.

tried various guides / terraform docs (most ref modules but in same file not separated folders)

i just need to be able to export a resourse_id for use in another child modules. once i can do this i will be able to duplicate for security groups and anything else i need to ref.