RPM of small DC motors

I have an automotive DMM with an inductive clamp to clamp a spark plug lead to measure RPM of an internal combustion engine. I want to measure the RPM of some small DC motors with and without gearboxes. Range up to about 3000 rpm. I was wondering what would be the simplest way to do this. Perhaps a tiny strong magnet attached to a tooth of the sprocket I want to measure? I was thinking about a small coil near the spinning magnet and clamping the wire in this continuous loop, but then I wondered if I set the sprocket up to rotate between the open jaws of my DMM's induction clamp, would this likely give me a result? I realise that I should try this to see, but as I'm not able to get to an electronics shop to get magnets etc for several days, I want to try to find out more of what I should buy when there. Someone suggested a Hall Effect switch, but I have no idea how to set this up for my meter to read RPM. Thanks in advance for any help in this area.

Reply to
John Riley
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Hi, John. If you need to measure with an inductive clamp, you're probably going to need at least 100mA, and possibly twice that, going through that clamp in order to get a reading.

Also, a magnet will be an eccentric load, which might not be great for your motor bearings.

The best way to do this might be to make a small card stock disk with a good sized slot in it, mount it to the motor shaft, and use an opto-interrupter to drive a high current transistor switch. You can then put the inductive clamp around the high current wire, and you're good to go, without having to wait for the magnets. Of course, you might be able to get a fast hall effect switch to drive a high current transistor switch, too.

To make it simple, order an H21LOI opto-interrupter (Mouser 512-H21LOI, $1.89 in stock) and a TIP105 PNP Darlington transistor (Mouser

511-TIP105, $094 in stock) and use the 9VDC@0.5A or greater wall wart you've avoided throwing out. Put together something like this (view in fixed font or M$ Notepad):

| | VCC | + | | | .-. VCC |470| | + | | | | | '-' .-. VCC | | VCC 1K| | + | '-------. + | | | | 1| |3 '-' | | | | \\ ___ | |< | V ~ | H )--|___|-o--| TIP105 (PNP Darlington) | - ~ |__/ 4 1K |\\ | 2| |5 | | | === | | | GND | Inductive | === |

Reply to
Chris

If you have a scope (most new pc usb scopes have a freq button!) the opto technique cant be beat. Even with an old scope, the period is div x time/cm

Reply to
BobG

Hi John, My first inclination would be to use a stroboscope if you can get your hands on one. I see some on eBay.

Reply to
John Duffus

John wrote: "I have an automotive DMM with an inductive clamp to clamp a spark plug lead to measure RPM of an internal combustion engine. I want to measure the RPM of some small DC motors with and without gearboxes. Range up to about 3000 rpm." ______________________________________ Re; Just a reminder: if you do get the automotive "inductive-spark" tach to work, don't forget you'll need to divide the indicated RPM by two to get the actual motor RPM since 4 cycle engines only spark on every other revolution.

Good Luck, Dan Akers

Reply to
D. Akers

"D. Akers"

** Err - that would be *multiply* the indicated rpm by 2.

Plus beware - some 4 cycle engines spark every revolution.

....... Phil

Reply to
Phil Allison

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zwgearbox

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