As mentioned above, large feral cat populations can quickly become nuisances to the neighborhood, so it's wisest to have the cat spayed or neutered.
A slight force should be required to trip the trap.
This good how to shows a trap you can build yourself, and the necessary tequinique to catch those cats.
If possible, monitor the cats as they feed one or more nights.
Use hand protection as you handle the trap.
Or wrens that nest on my front porch being eaten!.