different multiplier?

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different multiplier?

Postby f_devocht » Sat Jan 14, 2012 8:44 pm

I have a 5V VSS signal (square wave) that I need to multiply by about 35 to 40. I figured the pwm board would be ideal for this, but the nearest multiplier is 32. Would it be possible to make me a custom one, that has different multipliers like 33, 34, 35, ... , 40?
This is for a speedometer that needs a signal that is 36ish times faster than what the tranny gives me.

Frank

edit: just noticed the pwm board works up to 300 Hz, but my input signal is up to 7kHz :(
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Re: different multiplier?

Postby jbelanger » Sun Jan 15, 2012 12:02 am

It would still give an output at 7kHz but it would have a terrible precision. So that would likely not work for your purpose. Moreover, the multipliers are all a power of 2 so the next possible multiplier would be 64 but it would be even worse for accuracy.

So you'll likely need to find another solution.

Jean
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Re: different multiplier?

Postby f_devocht » Sun Jan 29, 2012 3:36 pm

I did some measurements and turns out that I have 41.5Hz at 50km/h and I need to multiply by 33. I guess 32 will (have to) do. That's about 3% slow, but most speedo's are reading a bit fast anyway.
As it works from 8Hz, this would mean from about 10km/h, which is acceptable.
Guess I'll give it a try and order one.

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Re: different multiplier?

Postby f_devocht » Sat Feb 11, 2012 3:35 pm

I built the PWM circuit but have some trouble with the output.
(Luckely I bought 2 of them because I managed to blow up the first one. Probably shorted pin 8 to ground with the oscilloscope probe)

When I use a generic MS3X PWM output and feed that into the PWM converter, I get a correct output (on the Jimstim):
PWM output from MS3X is set at 78Hz, 50% duty cycle
Output of PWM converter is indeed 32*78Hz (multiplier set at 32).
I replaced the TIP120 with a PN2222 and use a 330R pullup to 5V on the output (collector).
Blue is input, red is output.

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However, in the car I get nothing. I have a VR circuit that converts the sinus speed signal from the transmission into a square wave (see screenshot below). I then send the VR output into the PWM converter but when I do that, I get no output at all?!?
I don't understand why. Anyone have an idea?
PWM output from my VR circuit is 50.1Hz, 35% duty cycle (guesstimate), 32 multiplier
Output of PWM converter is zero!
Again, with the PN2222 and the 33OR pullup to 5V. Nothing else is connected to the output.
I intend to feed that signal into my speedo, but it is not connected yet.
Red is input, blue is output

Image
Last edited by f_devocht on Sat Feb 11, 2012 4:06 pm, edited 2 times in total.
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Re: different multiplier?

Postby jbelanger » Sat Feb 11, 2012 3:43 pm

Are you actually sending a 9V signal to the PWM CPU? And does it actually go below 0V or is it just that your scope is set to AC and not DC? The CPU on the PWM converter board can't be used with a signal that goes outside the 0-5V range. I don't know if that's the issue here but I suspect it's not helping and might lead to failure.

Jean
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Re: different multiplier?

Postby f_devocht » Sat Feb 11, 2012 3:57 pm

(edited my previous post while you were replying)

I wondered about the same thing. I was expecting 5V because I'm using this VR circuit and my Vref is 5V.
Then again, the first chip gave up after a short while, so maybe I was indeed sending 9V into it (I thought I shorted it).
I'm not sure if it was set to AC or DC, I'll do some more testing in the morning.
Judging from the non-highlighted AC button, I think it's set to DC.
I connected my 2-wire speed sensor with one wire to ground on the instrument cluster and sent to other wire to the VR circuit. Thus the grounds of the sensor and the VR circuit are not the same. Would that matter?
The output of the circuit below goes into the PWM converter.
The output of the PWM converter has a pullup to 5V so I can drive my speedo with it.

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Re: different multiplier?

Postby f_devocht » Sun Feb 19, 2012 2:31 pm

Jean,
I managed to blow up the 2nd chip as well. I sent 5.6V into it thinking it would be ok. Guess not.
It worked for about 2 minutes. Are they that sensitive that even 5.6V is too much or could there be a different problem?
Here's a screenshot from the laptop, with the circuit functioning. 5.6V in and 5V out with x32 freq.
The circuit is working fine now. I used a resistor and 2 1N4148's to 5V and GND to get the pulse down from 12V.
Speed indication was working great for a short while. Of course, this is the chip that got 9V for a couple of minutes previously ...
I'll paypal you for a couple of new chips.

Image

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Re: different multiplier?

Postby jbelanger » Sun Feb 19, 2012 2:51 pm

The power supply can be up to 5.8V (absolute maximum). However, the absolute maximum for an input is Vdd + 0.3V. So if you power the chip with a 5V supply anything over 5.3V is potentially bad. If you have a choice, you're much better off having the lower voltage: anything from 0.7 x Vdd is seen as a '1' value. So with a 5V supply anything from 3.5V to 5.3V is good.

Jean
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Re: different multiplier?

Postby f_devocht » Sun Feb 19, 2012 3:45 pm

ah, that explains a lot. I'll put a zener over the input then.
Last edited by f_devocht on Wed Feb 22, 2012 7:39 am, edited 1 time in total.
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Re: different multiplier?

Postby f_devocht » Wed Feb 22, 2012 7:38 am

It's working perfect now. Thx for your input Jean.
Speed is spot on :mrgreen: .
As expected, it only reads from 10km/h up, but that's fine by me.

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