I am building an optical tachometer for my little CNC machine. I was so impressed with these Vishay TCRT5000 sensors, I thought I would post this project in two parts. My followup will be the tachometer itself built around the PIC18f45K22. Part 1 will be the sensor only.

I purchased the sensor here from eBay. I was very satisfied with my purchase. These little sensors had the best response out of all the I/R emitter receiver pairs I tested.

I have included the schematic and artwork in PDF format with the artwork being both mirrored and normal.

The schematic shows a 100 ohm current limiting resistor for the emitter. This seems to be adequate though I might try lowering it come in my next version to try and increase the range. I chose a 100K ohm resistor as the receiver's bias resistor. This seems to be pretty much minimum value for a good response from the sensor. I plan to experiment further and raise the value to get a better response but as you will wee below, the response I am getting is very adequate.

The output of the sensor is run through a LM2903 comparator in the inverting comparator configuration. This gives me a nice square signal out with reduced noise.

This image shows the sensor's output on the top trace and the comparator's output on the bottom. The test measurements where taken with a PC system fan with odd shaped blades a a rather sloppy paint job of the white mark the sensor was reflecting. The fan was approximately 2 inches (51 mm) from the sensor.



If you would like the sensor's footprint in Diptrace format then please contact me and I can try and get it to you.



Note: The 4.7K shown in the schematic will be used for a Dallas temperature sensor that will be off board,

Please stay tuned for the second half of this project.

 

This is not related to your question but it may save you the effort of writing the code for a tachometer. Here is a tachometer that I designed a few years ago that may meet your needs. it uses a PIC16F628 with an LED display.

Les

 

Have you really got pin7 of the LM2903 shorted to ground (not recommended), or is that a typo?

 

This is not related to your question but it may save you the effort of writing the code for a tachometer. Here is a tachometer that I designed a few years ago that may meet your needs. it uses a PIC16F628 with an LED display.

Les

Thanks but I don't want to save the "effort" of writing the code. Doing it is the whole purpose of a project. If I wanted to do that then I would just buy something off the shelf.

 

Have you really got pin7 of the LM2903 shorted to ground (not recommended), or is that a typo?

It is actually recommended in an older version of the datasheet. Mine makes no mention at all.

See this thread
http://forum.allaboutcircuits.com/t...h-unused-inputs-and-output-on-a-lm393.131577/

 

Have you really got pin7 of the LM2903 shorted to ground (not recommended), or is that a typo?

What catastrophe do you expect would happen if you put emitter and collector of a transistor at the same potential?

 

Fair point. I'd forgotten the output was open collector.

 

The schematic shows a 100 ohm current limiting resistor for the emitter. This seems to be adequate though I might try lowering it come in my next version to try and increase the range.

The spec sheet shows 60ma max for the emitter with a 1.25v drop at 60ma, this would leave 3.75v across the resistor for a 5v supply, it looks like a 62.5Ω minimum.
Your schematic shows 1k.
Max.

 

The spec sheet shows 60ma max for the emitter with a 1.25v drop at 60ma, this would leave 3.75v across the resistor for a 5v supply, it looks like a 62.5Ω minimum.
Your schematic shows 1k.
Max.

Sorry I will correct that. It is actually 100 ohm. It is mentioned in the OP. Thanks for the heads up.

There were actually two errors that I know I corrected before. I copied the file to a new directory and bet I corrected the copy. I should have just moved it.

Uploading corrections now.

 

Personally I much prefer the slot Opto type of tach sensor if at all possible.
2 resistors all that is needed.
Max.

 

Personally I much prefer the slot Opto type of tach sensor if at all possible.
2 resistors all that is needed.
Max.

I agree but that is difficult on a compact/lightweight spindle without an extra inch of space.