# 5v dc to 12v dc pwm design (automotive application)

#### Steve430

Joined Apr 27, 2020
6
Hi guys I hope you can help I am by no means an electrical designer but I have a problem and are looking for a solution.

I am trying to convert a variable 5v pulse into a rotation of around a max 2000rpm (12v motor)

I’ve done some researching and a pulse width modulator maybe the way to go

the phsical application is a classic car that’s had a new engine and gearbox that used to have a mechanical speedometer by a rotating wire, the new gearbox has an electrical pulse output that I want to use to turn the original wire to create a correct speed shown on the clocks/cluster.

Steve

#### crutschow

Joined Mar 14, 2008
27,737
The speed of a standard DC motor depends somewhat on load, so its speed can vary for a given PWM input.
It's thus not a good choice to drive the speedometer cable.

Likely better would be a stepper type motor whose speed is exactly proportional to the pulse input.

Do you know the relation between the pulse frequency and the motor RPM the speedometer needs?
I believe the speedometer standard is 1000 revs per mile.

#### Steve430

Joined Apr 27, 2020
6
Thanks for your reply, I don’t know how many pulses per mile but I was hoping to get something that’s adjustable so I could get the car next to another car at say 50mph and kind of dial it in to show the correct reading.

So how does a stepper motor work ? Does that use a controller too ?

#### Steve430

Joined Apr 27, 2020
6
I done some more research and found

1mph = 16,000 pulses per hour = 4.44hz

this is the output of the speed sensor

&

the speedometer wire needs to rotate 1000-1200 per mile

#### Dodgydave

Joined Jun 22, 2012
10,050
So your 5v sensor gives out 4.4Hz per Mph, so at 10Mph it will give 44.4Hz .?.

#### Steve430

Joined Apr 27, 2020
6
I would imagine so , I’m still trying to get my head round it bare with me haha

#### Steve430

Joined Apr 27, 2020
6
A NEMA17 says the max rpm is over 4000 maybe this is the motor to use but the question is how to control it correctly ?
Thanks again for your help guys

#### Janis59

Joined Aug 21, 2017
1,348
The aim is not defined well to understand clearly. 1) You need not very stable but adjustable rpm. So, must be potentiometer but noone cares if the same position is not kept abolutely exact for any motor load. Then the elementary pwm is OK, 555 or 34063 or any. 2) Your demands on the stability are higher - do the same but introduce the negative loop fro output to pwm input - what is not available in 555 but is well set ar 34063, 494 etc bridge drivers. 3) You need to be rpm stable depending on load, but stability have a rather narrow margine. Then the tape recorder casettes motor current stabilizer may be appropriate in the negative loop of the pwm circuit, thus the 555 must forget, but 34063 is still ok. 4) You need an extraordinary stability - then use the rpm reader or position reader - disc with holes, Hall sensor, mouse segment counter, inductive reader, electronic eternal-multiturn (digital) potentiometer or plethora of other methods to read the motor position, and apply the ARM like Atmel328 and digital control. Be careful, because if it as end station is connected to Win computer, each day-long exploit will produce the few seconds time loss. Thus, in astronomy where such mistake is killing, only Linux station is acceptable as it have task priority over inner intra-system serviceing tasks, just a contrary in the Win.

#### Dodgydave

Joined Jun 22, 2012
10,050
So you want to use a Stepper motor to work the Speedometer? If so you need a stepper driver chip but mostly a micro to convert the 4.4hz to a pulsing output for the stepper driver, like an Arduino or Pic, is it not possible to use the prop shaft or one of the wheels as a mechanical pick up for the speedo cable.?

#### Janis59

Joined Aug 21, 2017
1,348
Ahoi! Does this isnt a thingy to fool the automatic lorry driver work-time logging system??? If so, its illegal.

#### Janis59

Joined Aug 21, 2017
1,348
If the motor is Nema series, no meaning on aim, motor must be serviced by CNC controller plus (some cases) driver. Driver is thingy like amplifier - need ONLY to larger than 17`th motor. Controller have small driver inside, normally enough strong for NEMA17 and digital wonderworks to understand the computer signals. It may be bought ebay for 5 USD such with a LPT port or 50 USD with USB port. Vcc source is other thing needed, according to motor parameters and load between 5V and 48 V, most widespread are 12V. Computer must have an exotic LPT port if You bought a cheaper LPT controller. Computer system must be Win-X OR LOWER (!!!) at LPT version thus the old comps may be used without of shame. The soft be installed nakes 3dMax. If the highest accuracy is the goal, any Linux may work on the FreeCNC software. However, if ok if the system will cost like 100x or 1000x expensiver, may build a genuine Your demands specific Atmel-328 based solution winning a lot of physical space.

#### Steve430

Joined Apr 27, 2020
6
janis59
Like I said this is not my specialty so I do find your early post very hard to Understand.
Your later post to the lorry logging time, this is to be used on a personal car, we put a Nissan 200sx s13 engine (ca18det) into a ford Sierra body shell along with the gearbox too , the clocks have an analogue input by a rotating cable from the gearbox.

dodgeydave -
I don’t know of any other way to get a mechanical signal other than the gearbox, the existing cable from the gearbox is still there under the car so I want to attach a motor to that to mimic the gearbox.
the struggle im having is working out what motor and controller will convert the frequency into the correct amount of rotations the Arduino or Pic is something I haven’t heard of but I will have a look into !

so the NEMA 17 needs a specific signal from software rather than a controller ? Sorry I’m trying to understand I promise haha