Generally I try and machine a job with a single cutter to avoid tool changes. But recently I have had jobs that needed a number of different tools and so have started thinking about adopting a more streamlined process for tool changing by adding a tool length switch.( I have a touch off puck for setting Z and use the Z zeroing macro in UCCNC).

Proper Uk sourced ones seem to be about £75 (Link)
There are cheap Chinese copies on ebay for around £35-40 (Link)
Though not entirely clear what the compromise is: This guy seems unimpressed but mostly by the waterproofing (not an issue for a woodworker). Where as this guy demonstrates considerable accuracy is available though how 'low cost' his particular model was is hard to guess given the outsides all look alike.

At the more hobby focused end of the market, there are more options
Topcom.cz seem to have some nice products but around £80 (Link)
Stepcraft and BZT have a little one around £45 (Link)
and Carbide 3D do there own Bitsetter but that comes in at over £120! From the Shapeoko forum this appears a popular option even at this price

Then there are things like this on ebay for £18 (Link) which I believe is just a sprung loaded touch plate (but add a magnetic connector to replace the crocodile clip and that might be accurate and safe if switches are the source of cost and accuracy)

Interestingly when I looked at DIY options I came across this guy and he used a £6 metal push button! A high spec, metal 'Anti Vandal" non-latching switch. (Is that what Carbide 3D also use and mark up!)

It became clear the possible selling features of a tool length sensor were:
Coolant proof (Not interested as a woodworker)
Compressed air blow off to clear sensor surface (not an issue if you are manually changing tools)
Overtravel sensor - estops before breaking tool, (seems a reasonable backstop)
Sprung loaded - so it wont break tool immediately giving chance to manually press e-stop
No need to attach earth wire to tool (sensor uses switch rather than contact to ground like my touch off plate)
AND
Accuracy and repeatability........vital yet only looking for +/- 0.05mm, and most low cost products dont make promises

So at the moment I am rather tempted to start at the cheap end and mount a metal panel switch to the corner of my waste board. With the switch between the probe line and ground as normally open to act as the sensor. I don't get over travel protection but then I don't with the touch plate. A further enhancement could be to add an LED circuit to indicate the switch is triggered, givig a moment to react before it goes beyond full travel.

There is surprisingly little about fixed position tool length sensors on the forums (except Shapeoko one). Do most of you manage without? They seem strangely expensive for a switch but then perhaps I am missing something. I've shared what I've found, please enlighten me further please.