Found that I had this bookmarked from last fall... thought I'd share:
https://www.backyardchickens.com/t/720455/auto-pop-door-add-a-motor-d-20-repair-after-7-months
and this, from the same BYC'er:
https://www.backyardchickens.com/t/628008/automatic-pop-door
https://www.backyardchickens.com/t/720455/auto-pop-door-add-a-motor-d-20-repair-after-7-months
and this, from the same BYC'er:
https://www.backyardchickens.com/t/628008/automatic-pop-door