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# modifying pulse counts

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#### douglas01

##### New Member
I have a problem that I hope someone may have a solution for. I am making a rev counter for my lathe and the easiest way to detect the spindle speed is to use a sensor to pick up the pulses generated by a drive gear in the headstock of the machine.Some lathes have a 60 tooth gear in this position which provides 60 pulses per revolution, so a frequency meter measuring these pulses gives a direct reading in revolutions per minute of the lathe spindle.
My problem is that my lathe has a 65 tooth gear wheel in this position and I need to produce 60 pulses per revolution. If I can eliminate every 13th pulse from the pulse train I get from the sensor ,this would solve my problem. My electronics knowledge is a bit limited and I need some help with this part of the project and would appreciate any help people can give.

Well the obvious fix is to just calibrate the display device, ie if it uses an analog meter there is normally a trimpot to adjust the calibration. You can make an analog freq meter with just a 555 timer monostable or even a 2 transistor monostable circuit.

that one will work fine with +5v or +12v level signals.

Since the op mentioned using a counter to display the RPM then a pulse rate modifier like the Yellow Box Speedo Recalibrator mentioned by MR RB would seem to be the easiest solution. Just set the correction factor to 60/65 = .923.

If you wanted to design you own reducer you could use a couple of rate multiplier chips such as the CD4089 Specialty Logic - Rate Multipliers/Frequency Dividers/Timers - CD4089B - TI.com. Two chips in cascade could be set for a pulse reduction factor of 236/256 = .922 which is within 0.1% of the desired ratio.

cheap method

Thanks for the replies to my query. These speedometer adjustors would be ideal for the task but too expensive for this project.Is there no simple circuit that will remove every 13th pulse from the pulse train?

Thanks for the replies to my query. These speedometer adjustors would be ideal for the task but too expensive for this project.Is there no simple circuit that will remove every 13th pulse from the pulse train?

hi,
Do you have the 'electrical' details of the pulses currently output from the '65' generator.?

Voltage, period etc,??

The output from the sensor is a 0-5V square wave with frequency up to about 2000Hz

hi,
Look at this option.
All it does is block every 13th pulse from passing thru to the output.

#### Attachments

• Block13a.gif
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Thank you for providing me with that circuit, it looks to be the answer to my problem. I will try it out once I get hold of the chips needed. Thanks again.

Thank you for providing me with that circuit, it looks to be the answer to my problem. I will try it out once I get hold of the chips needed. Thanks again.

hi,
Just a thought, where on the equipment is the 65 pulse/rev generated.?

Is it a slotted/holed disc or a toothed wheel which plays no part in the drive of the lathe.?

IF its possible could you block every 13th hole/tooth on the disc encoder.??

The ic's on the circuit are CMOS, so take care to avoid antistatic damage.

Since revs/min is a rate, why not just count the pulses but bugger the time base to be 65/60 of a minute, (or is it 60/65 of a minute?)

Thanks for the replies to my query. These speedometer adjustors would be ideal for the task but too expensive for this project.Is there no simple circuit that will remove every 13th pulse from the pulse train?
Don't think there's anything simpler than using the two rate multiplier chips. You could use a counter to count to 12 and inhibit the next pulse but that would require just as many parts.

Pulses come from a 65 tooth gear which cannot be modified, but I now have plenty of ideas to work with and should soon have a working system. Thanks for all the contributions, much appreciated.

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