About connection class pdo

I'm developing a small project using php with standard mvc and the smarty template engine. I made a connection class without applying any pattern and it looks like this:

Class Conexao extends Config {

    private $bd_host;
    private $bd_user;
    private $bd_user_pass;
    private $bd_base;
    private $conexao;

    function __construct() {

        $this->bd_host = self::BD_HOST;
        $this->bd_user = self::BD_USER;
        $this->bd_user_pass = self::BD_USER_PASS;
        $this->bd_base = self::BD_BASE;

        try {

            if (is_null($this->conexao)) {
                $this->conectar();
            }
        } catch (PDOException $e) {

            echo "ERROR GERADO AO CONECTAR COM BD: " . $e->getMessage();
        }
    }

    private function conectar() {

        $options = [PDO::MYSQL_ATTR_INIT_COMMAND => "SET NAMES utf8"];

        $this->conexao = new PDO("mysql:host={$this->bd_host};dbname={$this->bd_base}", $this->bd_user, $this->bd_user_pass, $options);
    }

    function executeSQL($query, array $params = NULL) {
        $this->obj = $this->getConexao()->prepare($query);

        if (!empty($params)) {
            return $this->obj->execute($params);
        }

        return $this->obj->execute();
    }

    function getConexao() {
        return $this->conexao;
    }

}

In my MODEL classes, when I need to do some operation with the database, I'm instantiating it and using it more or less as follows:

    function registrarRemessaBd($time_insert){
    $query = "INSERT INTO caf_remessa (time_insert) VALUE (?)";
    $params = array($time_insert);

    $conexao = new Conexao();

    $conexao->executeSQL($query, $params);

    return $conexao->getConexao()->lastInsertId();

}

Is it wrong to use this way? or should I use some standard for the connection such as singleton? What problems could I have if I use it that way?