Tachometer trick?

[Chris Fretz]

I have a Hobicco mini tach an I have a hard time getting a reading. Will these things work inside a garage? I pretty much have to have it skipping off the prop an in a perfect sweet spot to get a reading. What's the trick to getting these things working?

[Phil Krankowski]

Under artificial lighting you will have a miserable time getting a clean reading.  ALWAYS.  Take you tack up to a fluorescent light if you don't believe me.

In the US is should read 3600 rpm on a 2 blade prop.  It might read 360.  If it reads something else your tach is wonky... or your power is wonky.  Take it to an incandescent lamp if you have one (I have very few left in my house), it should read the same.  This is actually the calibration used in the Glowbee instructions.

You need natural lighting.  The great outdoors is best.  The tach should read from 18 inches away no problem.  It may even read from 3 or 4 feet out.  

A battery powered incandescent lamp (flashlight) might work too after turning off all the other lights...  LED lamps use PWM (Pulse Width Modulation) to change intensity causing all kinds of fun readings

Phil

[eric david conley]

     I have been using a flash light sense I've been building electric carrier planes. Building in winter and also testing different motor, ESCs, and battery combinations required a lot of testing to get it right. I found that my "large" flash light with a 3.5" lens worked the best, always get good readings in very gloomy conditions. Attached the plane very firmly to my bench leg then positioned the light to shine through the prop from an area behind the tail. Now that I think of it, I could have placed the light in front of the plane and done all my test from the back "much safer". Next time.  Eric

[Tim Wescott]

Yup.  Flashlight.  Turn the lights low.  Or turn the lights off and set up close to a window.

I didn't know about the LED problem -- I'll be there's some that work and some that don't.

[Motorman]

I use a Hobicco mini tach with great success outside on a sunny day. In low light situations forget it. The trick is to take the batteries out of it when not in use because it drains them slightly then it won't work right.

MM

[Steve Thompson]

I had a fancy calibrated optical tachometer for work that had a flashlight built in.

Optical tachs count the flashes they see per unit of time, that's all.  For the tach I had (no multi-blade options), pointing it at an incandescent would show 7200 RPM.  That's because AC sine wave has two peaks per cycle; one up, one down.  (2 peaks per cycle  x  60 cycles per second  x  60 seconds  =  7200 flashes per minute)   The tachs we use for airplanes have two, three, and four blade setting to compensate.

On a two blade tach setting, pointing at the light will show 3600; two flashes per cycle.  Pointing it at the prop will show 3600; two blade flashes per revolution.  In both cases, the two blade setting simply divides the total number of flashes (7200) by two.

If tach isn't working, it might not be seeing the flashes.  For the industrial tach I used, it came with reflective tape that you would put on the shaft.  For propellers, maybe try turning plane towards the available source of sunlight so there will be a detectable flash off of the propeller.

[Steve Helmick]

When you get tired of your Hobbico tacho, start a hurling event for them. Buy a Hangar 9 (brand) "Mini Tach" from Horizon Hobby. Not quite as good as a TNC tach, but a heck of a lot cheaper....about $28. Your LHS may stock them, if they sell Saito engines.  Steve

[Chris Fretz]

Well this is great information. My tach reads 3600 on an incandescent bulb. It reads 3600 on fluorescent an in my garage pretty much anywhere I point it it will read 3600.

[Chris Fretz]

When you get tired of your Hobbico tacho, start a hurling event for them. Buy a Hangar 9 (brand) "Mini Tach" from Horizon Hobby. Not quite as good as a TNC tach, but a heck of a lot cheaper....about $28. Your LHS may stock them, if they sell Saito engines.  Steve

I assume the Hangar 9 tach will still be difficult to read in my garage under fluorescent light?

[Steve Helmick]

Well this is great information. My tach reads 3600 on an incandescent bulb. It reads 3600 on fluorescent an in my garage pretty much anywhere I point it it will read 3600. I assume the Hangar 9 tach will still be difficult to read in my garage under fluorescent light?

I think some of the responders above meant that the artificial light was screwing with your tachometer, but didn't come out and say that.  Now, you've "Seen the light!"  

Yes, I would expect any optical tachometer would have the same problem when used under artificial light, unless yours is running on DC current! Note that in many parts of the world, their grid runs 50 cycles, unlike our 60 cycles. Your tachometer would read 3,000 rpm when held up to incandescent or florescent light source in those countries.  If my Common Core math works at all....

Many of these optical tachometers can be improved by various tricks, like making a tube that provides shade to the lens, painting the inside of the tube with flat black paint, making the tube restrictive, etc. Some experimenting may improve the function of your Hobbico tach.    Steve

[Tim Wescott]

Yes, the artificial light screws up an optical tach.  Optical tachs work by detecting the variations of incoming light as the prop blades pass by the photosensor.  Fluorescent lights pretty much blink on and off at 120Hz, which looks like an engine turning at 3600 RPM to a tach.  Incandescent lights don't go all the way out, but they still pulsate at 120Hz, and the tach detects it.

Hence -- flashlights.  Or the sun.

[Robert Dible]

I don't want to make enemies with Big Flashlight, but cars without the new LED lighting work pretty good when you have to get that rpm set.  Flashlights work pretty well when you just gotta knock some sense into somebody's head.