Made a video of the mod …
3v button cell > bye bye
AAA battery case ( 3v ) Hello
Open the tack , and cut or de solder the button slot cover …
This will expose the negative contact …
Cut a hole in the back cover
Solder wires ,
Put the case back together
I used double sided tape to hold the battery case to the back cover of the Tach
Switches came today , so I added a switch to isolate the batteries from the parasitic drain these Tach’s suffer from … ( Button cell is always depleted )
Good idea ! Fairly easy fix to that problem . 
Nice solution.
Though I must admit that at first I didn’t have the foggiest notion what you use it for.
So I *oogled and found on *anggood.com:
Function:To measure RPM of a spinning propeller,2-9 blades ones,memorize peak RPM datas.
Maybe you said that in your video, but on my FireFox that is a silent movie.
Nefarious ?
I guess I am taking money away from the button cell manufacturers ?
I dont know why , but they never last ( button cells ) for any length of time .
Except maybe in digital watches …
But every other device that uses one seem to always be flat , requiring a new battery .
Which is a little annoying !
And I do fly RC , so a Tachometer is standard kit .
Probably not something that Flashaholics might need , but I thought it an interesting mod . ( To share )
Micro Wing !
Buttons cells never last long before of extremely high internal resistance in anything remotely powerful.
I wonder if it could read the RPM of a spinning shaft if a line were painted on it?
Yeah, I found out the hard way that my calipers went dead just sitting around. Good thing I kept the original package (cardboard slides into formed killer-plastic) and could remove the (replacement) cell and stick it into the recess for storage.
Wonder if that doodad can measure pwm frequency…
Funny you should say !
It does read something …
I put a small light to it ( WITH PWM ) and got about 22000 rpm