cat flap for one indoor/outdoor but keep others in??

I foster kittens for the humane society and ended up adopting one of my fosters.  After much arguing, my husband has agreed to let me keep Merlin if he is an indoor/outdoor.  We also have 2 indoor.  I know it's not ideal, but Merlin was not likely to be successfully adopted from the shelter.  Merlin will be neutered this week so it is time to get a at flap that will keep the two in, but give Merlin his freedom.  I have found microchip and magnetic ones that seem to only work to prevent other cats from coming in.  Has anyone found one that works two ways to keep the 2 indoors in but other cats out?  I'm looking to put it on our screen as we live in a warm climate and we keep everything open 24/7 (raccoons and squirrels do not exist where I live).

There are ones that are activated by a magnetic collar...

https://www.worldforpets.com.au/products/15172?cat_slug=/Pet-Doors

@betsygee  reviewed this door which only allows certain cats in or out, I believe:

http://www.thecatsite.com/products/sureflap-microchip-cat-flap-white

I hadn't seen this thread--thanks for the mention, Anne.  

@mokabean   the door I reviewed works as you said--it lets all of our registered cats in but neighborhood cats or raccoons or whatever can't get in. But any cat can get out of it.   

Now, however, sure flap has come up with a new door.  You can program it so that certain cats can go out but others can't.  It's here:  http://www.sureflap.com/products/details/23-dualscan-microchip-cat-door

That's next on my to-buy list!  We have three cats who are indoor/outdoor who come and go as they please.  But we took in four more cats who are indoor only.  So far they haven't shown any interest in trying to get out the cat door but I'm afraid that one of these days they'll want to try it!  The dual flap hopefully will take care of that problem.