I have caught a lot of mice and a few rats using a simple figure-4 deadfall. One thing I do, which I haven’t seen anywhere else, is to split the end of the bait stick. I force the bait well into the split so that the rodents need to push hard to get at it.
This trigger could be used to make a bigger deadfall for bigger animals. I’ve never seriously tried to do this. There is a lot less work in setting a snare, and snares have been very effective for me. Also, if a domestic animal is caught in a snare it can often be released without harm, but a big deadfall is kinda final. There is also a chance that the deadfall could drop on the trapper’s hand when it is being set, so great care is needed with big deadfalls.
In the diagram above, the trigger is somewhat 'shortened' compared to some diagrams you may see. You will note that the length of the portion of the diagonal between the vertical prop and the weight is particularly short. Making changes like this can help to keep the trigger sensitive even when using a heavy weight. It is all about leverage and friction. The critical friction, I think, is at the mating surfaces of the bait stick and the vertical prop. I could theorize at length about the trigger mechanism theory, but it is probably best for the beginner to just whittle a few triggers and try them out.
I was once building a shed and I became aware of a lot of mice hanging around. I had an old-fashioned wooden nail box. I'm not sure what these boxes weighed when full (25kg maybe?). Anyway, this box was part-filled with nails and was fairly heavy. I propped it up with a hastily-whittled trigger and soon I heard it 'thump'. The poor mouse could just about fit into a bank-card slot!
Occasionally I've set rat traps when I suspected that rats were interfering with my possum traps.
I've also caught rats around buildings using the figure-4.
I urge any reader to regularly check their traps (the law requires this in many cases).... and to be as humane as possible. I also urge all hunters and trappers to be excellent ambassadors when interacting with the public. We need to be seen in a good light.