How to filter get_post with ACF by a checkbox field?

So, the scattered documentation on this is really less than clear.

For checking for unchecked posts added the following meta_query:

    'meta_query' => array(array(
    'key' =>  'external',
    'value'  => '0',
    'compare' =>  '='
    ))

For filtering by checked it was just this:

     meta_query => array(array(
    'key' =>  'external',
    'value'  => '0',
    'compare' =>  '='
    ))