LED Machine Tach For Tired Eyes

[Sam]

Joe, thanks for all the ideas. I chose to go with the coil and magnet. I did this for several reasons, mainly I didn't want to wait for a new hall effect module to come in.

I used the coil from a 24v relay I had, I left the bar in the center of course. I got a small but quite strong round magnet from the earpiece of an old cell phone. I connected the coil to a NPN transistor which I forward biased to increase the sensitivity and then ran the output of that NPN into a PNP to give a good positive pulse to the 16F628A.

As a test while everything was still breadboarded, I put a cardboard disc about 6" in dia. on a variable speed drill, taped the magnet to the edge of the disc a gave it a spin. The readout appears to work perfectly, varying with the speed of the drill very reliably ! And the magnet will still work when it's about 1" away. I will have it setup so that the coil and magnet are in close proximity though.

Now I'm going to do the board design in Eagle, then use PCBGcode and mill several boards out on my CNC mill and I should be set.

I'm real happy about this and again, I really appreciate the code and all the help !

Best regards,

Sam

EDIT: I forgot to mention that it displays only in multiples of 60. ie: 60,120,180,240, etc. RPM. Not "linear", I left the formula as displays=(displays*60) as that's all I need for the way I'll have it attached to the first machine I use it on.

[Archangel]

EDIT: I forgot to mention that it displays only in multiples of 60. ie: 60,120,180,240, etc. RPM. Not "linear", I left the formula as displays=(displays*60) as that's all I need for the way I'll have it attached to the first machine I use it on.

I never noticed that, I will look into it.
EDIT: Ok I think it is Displays = Displays * 60 If you use 6 magnets and make this statement Displays = (displays * 10) you will get 10 RPM resolution. I plan to use a multiple pulse per revolution encoder scheme which will significantly increase resolution. I think I will build it using parts from an automotive distributor (the guts) and fabricate all of the rest.

[Sam]

Hi Joe,

I must be doing something wrong. I mounted the magnet to the spindle of my mill which on high should be spinning about 2400 rpm. The display is reading anywhere from 250 to 1010 and constantly fluctuating. This is with displays=displays*10. When programmed with displays=displays*60 it's varying from 3000 to over 5000 and again, constantly fluctuating.

I'm using a 10mhz xtal and have the code set at 10mhz OSC too. Could this be causing a problem ? I'm trying to figure it all out on my own but I'm not getting it, "obviously".

[Archangel]

Try putting a resistor from the input to ground, it sounds like it is picking up stray signals (rf?) and twist the input wires or shield them. I would try say 10K first. The displays=displays*10 would only be valid with 6 magnets.

[Sam]

Hey Joe,

I've been real busy with other things and meant to update this sooner.

The tach is working just fine, I have milled the PCB and installed one in my SC machine and it's great. I did switch to shielded wire and had to use a 1K resistor (I already had a 10K in place and that didn't work) and a .01 uf to ground.

When I get some more time I'm going to order two more of the 4 digit displays from Sure Electronics and add them to my lathe and CNC mill.

So again, I really appreciate you providing the code (I've learned from it too) and your help.

I'll post a pic of the tach in operation later. Also, if anyone would like the schematic and PCB layout from Eagle I'll provide that.

Best regards

[Archangel]

Hey Joe,

I've been real busy with other things and meant to update this sooner.

The tach is working just fine, I have milled the PCB and installed one in my SC machine and it's great. I did switch to shielded wire and had to use a 1K resistor (I already had a 10K in place and that didn't work) and a .01 uf to ground.

When I get some more time I'm going to order two more of the 4 digit displays from Sure Electronics and add them to my lathe and CNC mill.

So again, I really appreciate you providing the code (I've learned from it too) and your help.

I'll post a pic of the tach in operation later. Also, if anyone would like the schematic and PCB layout from Eagle I'll provide that.

Best regards

Hi Sam,
I appreciate the feedback! Thanks!

[Sam]

Here it is in operation ! It's doing a fine job, very happy with it.

Also a pic of the pickup coil embedded in epoxy inside a plastic tube, the magnet is epoxied to the shaft.