The basic concept is simplicity itself. Two bits of wood set up to act like a kiddies train set signal. However, believe me, ye have to get the balance right and that can be surprising! Just make the long end sufficiantly long and ye'll be flying
Ok. Here's a sketch I knocked up and scanned for this Post. Self explanatory. Length of thin wire runs down from the short end of the Signal and is hitched just beneath the jaw of the Fenn trap. Set it from the outer side of the jaw, obviously. And make the 'hook' shallow so it lets go easily. When the trap fires, that wire comes clear and allows gravity to do the rest.
Point of note: I've since realised that 'Roof Straps', the pre multi hole drilled lenghs of galvanised steel (Think Meccano for big boys) would also lend themselves very well to this purpose. Cost more than scraps of timber, but be faster to make if ye have a small Angle Grinder handy
I've actually made these things and can assure ye they Work! The Tunnel I'll show below I had set in view of my kitchen window. One day I glanced out and did a double take. The Signal, in the 'Up' position ten minutes before, was Down. I got to the rat before even my Dogs winded it!
In the wider context; The bit of extra work involved in making and fitting these devices can make the Pro' Pester money! Quick march round the farm, checking these things even from a considerable distance. No more getting down and peering into every damn one of them. Save yeself some time and get off to that next call out I Know of what I speak!
So, have a go. See what ye think. Let us all know. And be aware; The book is chock full of devious and wonderfull variations on this basic theme too. Even one for telling ye when a trap set inside a building has been fired. Tells ye as ye march past the outside!
And you wonder why this man's my Hero?!
Signal ~ Basic Design
The Real Thing ~ Waiting
Go get 'em, lads!