func on_off( gpio: String, value: String ) {
        let todoEndpoint: String = "http://10.33.14.248:8000/GPIO/\(gpio)/value/\(value)"
        guard let url = URL(string: todoEndpoint) else {
            print("Error: cannot create URL")
            return
        }
        var urlRequest = URLRequest(url: url)
        urlRequest.httpMethod = "POST"

        // set up the session
        let username = "webiopi"
        let password = "raspberry"
        let loginString = String(format: "%@:%@", username, password)
        let loginData = loginString.data(using: String.Encoding.utf8)!
        let base64LoginString = loginData.base64EncodedString()
        let config = URLSessionConfiguration.default
        let authString = "Basic \(base64LoginString)"
        config.httpAdditionalHeaders = ["Authorization" : authString]
        let session = URLSession(configuration: config)

        // make the request
        let task = session.dataTask(with: urlRequest, completionHandler: { (data, response, error) in
            print(response)
        })
        task.resume()
    }