These DTs from J&H Aerospace work very well. Light and well-priced. Built on a commercial rotary damper. Rotary dampers are available in a wide range of sizes, even up to toilet- seat size. Used in a variety of consumer products, including automotive.

http://jhaerospace.com/product/dethermalizer-damper/

I really like the super light rotary damper style timers. But they do have the disadvantage that the damping rate changes quite radically with temperature.

This hit home recently when some of us were flying a small field casual semi contest with a 90 second max to avoid flying off the field. One flight of mine flew for a bit over two minutes and the model was on the ground for almost a minute before the DT popped just as I approached it. On the walk back I ran the timer again to get more of a feel for a 100 seconds or so of run time. I set it aside for about 20 minutes to fly something else then flew it again. It didn't FEEL like it had warmed up all that much but the setting for around 100 seconds found earlier tripped the DT at around 70 seconds on the next flight.

So we need to constantly check the running time just before winding for the flight to be sure we get the flight duration we want. As with so many things it's not all wine and honey.

Reviewing this thread, I see that I never got back about magnetic damping. Put a rare earth magnet on a slanted aluminum or copper plate, pot, etc. or inside a slanted aluminum or copper pipe. It will slide or roll slowly. Thick magnets may need thick plates. I know this is counter-intuitive, but it works. I'm sure a clever person could use this principle for a DT.

I made a Tomy style timer. I slowed it down by putting weight on the oscillating arm. Seems to work ok with the original spring.

I think I saw a video on YT or something like that which used a VERY strong and very heavy rare earth magnet dropping down through a copper tube which damped things well.

I've also take a stack of rare earth disc magnets that were a close fit in a section of copper water pipe and smiled at the slow fall through the pipe.

But when used directly like this none of them are light enough to be at all suitable on a small model.... at least not directly. I think it would need to be something along the lines of the air vane damped that were popular in FAI free flight (the name evades me at the moment). But instead of a little butterfly shaped high speed air vane we'd have a coil or magnet sped up and passing by the highly conductive damper that controls the time. Perhaps that way we could get away with a smaller and lighter magnet. But it would still need a clockwork style setup to run it. And so much for simplicity then.

I'm thinking something like a Tomy Toy clockwork rig with the escapement pawl having small rare earth magnets on the ends of a long'ish balanced arm setup which speeds past close fitting highly conductive copper plates.

And let's keep in mind that magnetic fields are extremely distance sensitive. So the plates would need to be VERY close to the magnets. And that means to avoid physical friction that the magnets would have to avoid wobbling around.

All in all I think there's a possible solution for a magnetic damped timer. But it would not be a simple home shop sort of deal.

Which leaves us back at viscous buttons, DT fuse and those of us with a collection of Tomy Toy guts....

If you're using a Tomy mechanism, the easy way is just to put a bit of weight on the part that oscillate.

As far as the copper pipe goes, the magnet doesn't have to be large, just strong for it's size. One of these days I should fiddle around with a DT mechanism using magnets to see if it's even remotely feasible.

Start out with some round disc magnets dropped down a close fitting but not grabby copper tube. It's impressive in how much it slows the fall of the magnets. But at the same time it's not something even close to being applicable for a DT.

The way it works is that a conductor cutting across a line of magnetic force will generate a voltage. Or if the conductor forms a short circuit a current flow. The more times it crosses the greater the flow and the higher the back EMF force. But it's a rather weak force and to work at all relies on close proximity.

To keep weight light we'd want small magnets and small conductors. But the EMF this generates would be hella tiny. So that's why I'm thinking it would require a sped up situation such as a Tomy clockwork setup so that one or the other can be spun quite fast. If we're thinking that the magnet and conductive copper, silver or gold disc would be simple and replace the viscous medium in a small button timer I'm afraid that's just not going to do it.
Instead it would need to be sort of like the timers I can't recall the name of just now that were popular in F2B some years back that had the high speed air vane instead of a clockwork escapement. But instead of an air vane it would be a copper or even silver conductor spinning past a small rare earth magnet very fast.

Yes, that's the way I see it, too.

My Tomy timer doesn't seem to have a wiggler. Just gears I can see from the access holes. Which type of toy uses a wiggler? Mine were just wind up walking toys.

The walking toys do have a little escapment pawl or wiggler. If you wind them up they run down at a steady slower speed with a strong buzzing sound to them. Adding extension weights to that little pawl is how we slow them down and make them act as a DT timer. The walking toys all have that little pawl. Or they would sprint instead of walking.

Tomy also made a lot of pull back and release toys that would shoot across the table or floor. And some transformer style toys that when stood on end had a lever that engaged the escapement pawl for walking and if set flat would act just as a spring motor. Those ones are harder to convert because the pawl is on a traveling lever that needs to be set and immobilized. And while not impossible it's more fussy work.

I took one apart once trying to figure it out and when I put back together it would just unwind instantly. So I either lost a part or who knows. It went in the garbage after too much time spent on it. I have two installed on water rockets for parachutes that work really well. But a free flight rubber plane isn't a rocket so just trying to figure out if I can use what timers I have left.

Ok, I know what the escapement pawl is after a searching in "clocks." Thank you BMatthews. I did put that back in on that one I threw away. That one had problems anyway. Sometimes it would start and stop at random. I may take a closer look again and see where it is so I can add the weights.

If it was an iffy thing then there's a good chance that the one you had came from a combination walker and zoomer style. It's been a few years since I looked at them but I vaguely recall that the little escapment rocker was set into a slot instead of holes for the pin so a lever could push it into the clocking position or let it slide out and run free. I looked at only one that one time and gave up right away. The toy got put pack together and since it was an F15 Eagle that transformed to a walking robot warrior it decorated my work place as a jet or robot for a bunch of years. It's still around here I believe.

The ones you want for DT work are the ones from the pure walking only toys.

I had it working for a little bit, but then the wire came off and I glued it back and then the glue ran down and seized it. Took it apart and cleaned glue etc........... I had 17 sec per revolution. I think I'll just buy one. It's now broke.

Never say die! It's alive! Over 4 minutes run time.

That's the spirit! And you got 4 minutes from the built in spring? I never managed that ! ! ! !