Reproducing a propeller RMP measuring tool

I have an airplane and a while back I had a purchased an expensive tool (about $200 US) that measured how fast my propeller was spinning (in RPM) from inside the cockpit. TRUTACH II OPTICAL DIGITAL TACHOMETER | Aircraft Spruce. I think it worked with a laser, but I am not sure.

I've since lost it and would like to create a new one and I am sure that there is a sensor that would help and its not important enough to invest that much money in again. The only other things I remember about it was that it didnt work at night but could measure the flicker of lights inside a building which provided a good cross check to make sure it was working.

Does anyone know what type of sensor / setup this might be using?

Thanks

A photodiode can be used to measure light flicker and convert that to RPM.

Such a sensor would need to know the number of blades on the propeller. Did your commercial sensor have an input for that?

Yes, there was a cheap plastic switch to change between 2, 3 or 4 blades

As I remember the back side of the propellers in the planes i flew were painted in black. Using a light sensor that somehow is well focused within the propeller surface it could work. Then regard the rules, FAA, etc.. Drilling and bolting stuff to a plane might have consequences like loss if ensurance. Likely You know that already. The dame applies th the electrical aspect.

Cool. The original sensor probably did not have a laser, otherwise I expect it would work at night.

Making such a sensor using a photodiode would require a fair bit of electronics know-how (an amplifier is probably required), and probably several cycles of design and trial.

I also imagine that the photodiode would need a lens to reduce the field of view and interference from background.

Removed

Wait a second. All planes I've seen were equipped with an RPM indicator as it it's needed for ignition pre take off check, power setting at high altitude.
What "stuff" is it You're flying?

I do have an RPM gauge however over years they tend to loose reliability yet still remain within maintenance specs.

I learned of this tool from my mechanic who uses one to ensure it is within spec.

Thank you @Jremington it pointed me in the right direction. I will purchase a photodiode and recess it a bit into a black hole like the commercially available one then take it out to see if it is sensitive enough to pick up the loss of light as the blades cross the path from the cockpit and if so I will work on the mathematics of it all.

A more easy way is that You borrow a reliable RPM meter and simply calibrate the existing RPM indicator and creates a little correction table.
Your old indicator is likely just old but still reliable with its little error.

True, technically it is reliably incorrect and the savings potential of creating a new one will hardly make up the cost of all of my time spent but there is no fun nor adventure.

A long time ago the speed indicator, in my car, got rellay bad. it needed a mile of driving before leaving 0. After parking in the garage it said 40..... The car mechanics said it was very expencive to repair. Then I built a card computer based on a Z80, an LED, senced the ignition and the gear shifter.... It served med well for 10 years.
Finally I managed to get the faulty indicator loose and sent for repair, and it didn't cost much to adjust the indicator. Loosing 4 screws, not the entire panel was the expencive part...

I would first check what modifications you are allowed to legally make on your aircraft

hammy:
I would first check what modifications you are allowed to legally make on your aircraft

Something like #3 I think?

Just re -enforcing it ,with a few more words - most important aspect of the project

This has to be a model aircraft, right...?? :o

2 souls, 1 thought, as we say here.

bluejets:
This has to be a model aircraft, right...?? :o

Eeh? What's the importance of the reading in that case?

I fly rc, and I don't see any reason to have rpm other than just the satisfaction of knowing what the rpm is.

I am not intending to make any modifications to my plane; it’s illegal, dangerous and would negate my insurance. The tool that I purchased and lost before and would like to replicate is a 3x2x.75" black box with an lcd front and what I imagine now is a “lens” hole with a photocell at the back. You simply turn the unit on and point the back at the propeller and it reads the RPM (providing there is enough light). It works well enough that I can leave it on top of the panel during flight.