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Dc motor drive for mechanical tach & speedometer

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CARTECH

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I have a vintage mechanical tach & speedometer. I need to come up with a circuit to drive them with dc motors, either open loop, or closed loop.the source can be chopper wheel opto, or hall effect, or mag. pickup. I also need calibration. IE. actual road speed for speedometer. I have spent hours on the net looking at PWM circuits. My knowledge limitations are trying to add to, or alter those circuits. There must be somebody who has done something similar. Thank you ahead for your generosity in this forum, ...J.R.
 
If you can get a pulsed signal from a speed sensor or ignition source you can use a LM2907 frequency to voltage converter IC to create a variable DC voltage source which can be amplified with a transistor or mosfet to drive the DC motor.
 
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Thank you for your prompt reply. Will a voltage controlled motors speed be accurate enough, as opposed to a PWM driven motor? If so, would it just be a matter of using a large enough motor to overcome the drag in the mechanical gauges?...J.R.
 
Sounds simple enough but it may not be. Does the car have points or do we need to find a signal for the tach. Sounds like we will need a magnetic pickup on the driveshaft or find a signal for anti-lock brakes? Driving the motors at higher voltages (150 mph or 6500 rpm) is probably ok, but may be more difficult (closed loop) at low speeds and rpms. How easy are the gages to turn, and how large a motor do you have in mind?
 
Its a 63 Buick 215 engine in a Lotus 23 replica. I converted the distributor from points to magnetic pickup with a toothed magnetised rotor. I would assume I could use the same ignition signal as a PWM source also. The source for the speedometer will be a chopper wheel mounted to the transaxle half shaft with opto,hall effect, or magnetic with a pickup coil. The input speed range of the Tach is, 250, to 4,000 rpm. The input speed range of the speedometer is as least far as I can determine roughly 4 times the rear wheel speed. I will calibrate the speedometer out on the highway by using GPS to determine my actual vehicle speed. The gauges are very easy to turn. Motor size will have to be determined by a seat of the pants approach. At least as far as I'm able to determine. Thanks for your reply.....J.R.
 
The LM2907 IC can read directly off of a points or electronic points conversion as well as low input voltages like what a common inductive speed sensor that most cruise control systems use as well.

As far as getting its output power high enough thats simple enough as well. Just use a common LM1875 power op-amp IC as a DC servo driver for the small motor.
 

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Do you think 1/2 volt will overcome friction at the low rpms, or does it have to be closed loop or at least IR compensated?
 
Thanks for your suggestions. I'll mock up a circuit of your approach and see if it works for my application. I,ll probably use a more powerful power amp IC to drive the motor though. Regards.....J.R.
 
Voltage motor control vs. closed loop PWM

Do you think 1/2 volt will overcome friction at the low rpms, or does it have to be closed loop or at least IR compensated?

I honestly don't know at this point, I'll have to build tcmtech's circuit and try it out.Your question is what's been on my mind all along. I do have the low friction drag of the mechanical tach and speedo on my side though. With a big motor it might work fine. I'll let you know whatever the outcome. Thanks....J.R.
 
I don't think you will need all that big of drive motors. I have a number of old tractor tachometers and mechanical speedometers from old cars and a small 12 volt DC motor can spin them easily. all you need is to know what RPM represents the top end of their input. Most tachometers ran off the distributor drive or camshaft at half the engines RPM so if 4000 RPM is the top engine speed the tachometer would only need 2000 RPM.

As far as bottom end response goes most mechanical speedometers had a dead zone at the bottom below 5 - 10 MPH or where very inaccurate in that range anyway and tachometers don't need to spin any slower than what the engines idle speed is.
 
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