Want to make a DC Brushed Motor Tach

[mazur50]

First i would like to thanks you guys for such a nice board and i was woundering if you could help.

I do both RC and Slot racing. and i want to make a Tach for my Slot cars. this is what i want to do.

The way the cars work is there is are two contacts on the track one positive and one negative. "I am sure you know this"

the way the controller works is there are three wires. a Red a Black and a white.

The red wire is the brake wire and is the negative side. the white wire is the positive and the black is the postive side that go's to the car.

http://www.hoslotcarracing.com/image/Wiring.gif

What i would like to do is log RPM, Voltage at the track, Voltage to the car, and Current draw by the car, But the most important is RPM.

I found this Circuit. http://www.piclist.com/techref/au/net/ezy/www/http/~fastvid/encoder.htm

What i was hopping was i can hook it to the black or red wire and use a pic to count the pulses from the Brushes hitting the com. " the motors are Three pole motors"

and i wanted to input in to a serial port or somthing to display on a laptop to i can log this info over time.

let me know if you have any idea or ways to help

thanks Mike

[jeffs555]

If you are talking about something connected to the track or the controller it is questionable how reliable the rpm readings would be, or if you could get any reading at all. You are talking about very short pulses. The distributed capacitance in the track may disipate the pulses so much that you can't get any valid info, and the noise of the contacts from the car to the track might mask too much of the signal. You would also have to make sure there were no capacitors across the motor in the car. If you have a scope or have access to one, you could look on the track wires to see if you can recognize pulses that vary with the speed of the motor.

[mazur50]

If you are talking about something connected to the track or the controller it is questionable how reliable the rpm readings would be, or if you could get any reading at all. You are talking about very short pulses. The distributed capacitance in the track may disipate the pulses so much that you can't get any valid info, and the noise of the contacts from the car to the track might mask too much of the signal. You would also have to make sure there were no capacitors across the motor in the car. If you have a scope or have access to one, you could look on the track wires to see if you can recognize pulses that vary with the speed of the motor.

What if i was going to do this right at my power supply with no contacts. and everthing sodered. Do you know where i might be able to find some code for this. I am new to all of this.

[jeffs555]

I don't know of any code for this, but it should be rather simple to count pulses and time them.

I would suspect that you could get a much more reliable value for RPM just by measuring the voltage, current, and motor characteristics(ie KV and resistance), and then calculating the RPM.

[mazur50]

how do i measure motor characteristics

[jeffs555]

Many brand name motors have specs available. The important specs are Kv and resistance. RPM = Kv * (volts - (current * resistance))
Here is one brief page on how to measure motor characteristics http://www.slkelectronics.com/help/Eca00004.htm
You would probably want to measure the resistance while on the track, as the resistance of the track wiring and contact resistance might be significant.

[mazur50]

Ok thanks
becasue we build the motors so there are no specs like that