I stand corrected.

 

I'd just use a capacitor to couple to a small speaker. Audible tone equals spinning. Done.

I like this even better!

Can you recommend a cap and the basic spec for a speaker that should work?

Sounds like a $1 solution!

Thank you kindly!

R

 

I like this even better!

Can you recommend a cap and the basic spec for a speaker that should work?

Sounds like a $1 solution!

Thank you kindly!

R

I'd start with a 0.1-1µF ceramic cap (which does not have a polarity). In general, a larger capacitance will give a higher volume with a given speaker. Try a small speaker out of a clock radio, a cheap computer speaker, an old TV, that sort of thing. These are all usually 3" or less. Impedance varies but I'd expect something in the range of 8-100Ω. A headphone speaker would also work but I wouldn't expect much volume from it.

A potential problem with this approach is the sensor output may be high impedance. In other words it may not be able to drive a speaker, which will just look like a short to it. In that case you could use the cheap - but powered - computer speaker. These have a built-in audio amp and can accept a line-level (high impedance) input.

 

I'd start with a 0.1-1µF ceramic cap (which does not have a polarity). In general, a larger capacitance will give a higher volume with a given speaker. Try a small speaker out of a clock radio, a cheap computer speaker, an old TV, that sort of thing. These are all usually 3" or less. Impedance varies but I'd expect something in the range of 8-100Ω. A headphone speaker would also work but I wouldn't expect much volume from it.

A potential problem with this approach is the sensor output may be high impedance. In other words it may not be able to drive a speaker, which will just look like a short to it. In that case you could use the cheap - but powered - computer speaker. These have a built-in audio amp and can accept a line-level (high impedance) input.

I think I could put that together... though I think I found a slightly "cooler" way to do this.

I researched minature/cheap oscopes and found the JYETech DSO112A... I think that will work perfectly... had no idea scopes were this small/cheap! ($60)

 

You could probably run oscilloscope software on a retired smartphone also. (Can you tell I'm a skinflint?)

 

If you truly want something off the shelf then you want a frequency counter. All of the test equipment manufactures make them.

Setup your gate time, trigger theshold, etc. Its also helpful for many other things as well. Note that hardware counters are designed to be more accurate than software based equivents. I had someone give me one with 6 digit accuracy that is about 20 years old now. Im sure there are some on fleabay for cheapish.

If you want a 1$ solution it looks like you have several options that have been described.