5v vs 12v pull up option.

ronchinoy · Post by **ronchinoy** » Tue Feb 05, 2008 12:51 am

As the CKP signal output is typically under 5 volts at cranking thought I would try the 5 volt option on the stim.

So changed the jumpers from 12v to 5v. (Both)
THe mega does not detect tach.

Does this mean there is an issue with the mega and I need to install a pull up on the mega ?

Shouldn't the circuit automatically be configured to work with any VR circuit.
Or is it because we have moved from using JS10 to D14 and D16 that we need to install the pull up. ?
In which cause this should be covered in the conversion steps when converting from JS10 to D14, D16.

Suspect this is one of the reasons the Mega is not detecting the Tach on the car.

ronchinoy · Post by **ronchinoy** » Tue Feb 05, 2008 11:00 pm

Found the answer here. Along with some very good documentation.

Explains in detail if you need a pullup or pulldown.
Based on VR output.
Very well laid out and easy to understand documentation.
Need to compare to Mega input and see if it has implemented a pull or not.

http://www.vems.hu/wiki/index.php?page=AfreshTiny
(Scroll to bottom of page)

ronchinoy · Post by **ronchinoy** » Wed Feb 06, 2008 11:45 pm

Ok had a look at the Input circuit of the Mega.
It doesn't need a pull up on the VR side because we have U7 to do that job.

So then why does the stim not work with a 5volt output is something that I need to debug.

Will start by hooking up the scope to the Jimstim and seeing if indeed it is outputing a the right tach signal. In 5 volt mode.

I think its important to get the Mega to work with the Jim stim in 5 volt range. Cause that is the range where the VR is going to be outputting its signal at.

Some tweaking of the circuit leading up to U7 may be required if the Jimstim is outiputing the right signal in 5 Volt mode.

ronchinoy · Post by **ronchinoy** » Thu Feb 07, 2008 12:12 am

In 5volt pull up mode the output signal is around 1.5 volts.
This is probably why its not working.
You may want to increase this a bit.

Pics taken of the scope signal can mail it if interested or will attach it here.
Pcs taken in 2volt range 10ms/Div.

Is this normal or Ive made a mistake building the stim.
How can I increase this signal strength to better match what the car puts out.

ronchinoy · Post by **ronchinoy** » Thu Feb 07, 2008 2:42 am

racingmini_mtl · Post by **racingmini_mtl** » Thu Feb 07, 2008 7:34 am

What input circuit are you using? In any case, a VR sensor will output a voltage that will vary with RPM from possibly less than the 1.5V you see to more than 50V in some cases.

And the output voltage you will see out of the JimStim will depend on the impedance of the circuit you're using. With a relatively high impedance (like the MS V3.0 VR circuit), the voltage will be close to the 5V pull up. Also, if you're using another VR circuit, you may have problems with the fact that the JimStim's output (or other stim's) is a square wave output that never crosses zero. If you have a circuit that needs zero crossing , you will have to put a capacitor in series with the tach signal to generate a pseudo-VR signal that does go both positive and negative. This will not be exactly shaped like a real VR signal but will have real zero crossings and this does work with an LM1815. You can use a 0.1uF cap.

If you are using the MS VR circuit, just use the 12V pull up and you will be fine. If you must use the 5V pull up, you'll need to play with the trim pots to have a correct signal (also needed in some cases with the 12V pull up). And if you are using the MS VR circuit and you get only 1.5V out of the sitim's tach signal, there is something wrong with your MS board because that should not be the case.

By the way, the picture is not visible.

Jean

ronchinoy · Post by **ronchinoy** » Thu Feb 07, 2008 8:23 am

Q1 replaced. Nothing was wrong with the original.

Regulator outputting exactly 3.3 Volts. & 12 Volts.
If it helps with the RPM pots turned all the way down. I get a flat 5 volts signal. But as soon as the pot is turned to start generating RPM it drops to voltage shown 1.5 Volts.

Output is the same weather the mega is hooked up or not. (VR circuit loaded or unloaded) So VR part looks ok.

racingmini_mtl · Post by **racingmini_mtl** » Thu Feb 07, 2008 11:23 am

Where do you get your 5V from? With the Megasquirt not connected, there should not be any 5V power because it comes from the Megasquirt regulator.

Can you try with the 12V pull up to see what you get?

Jean

ronchinoy · Post by **ronchinoy** » Fri Feb 08, 2008 12:20 am

The 5 volts I got was with the mega hooked up RPM pots turned down to zero. I did not bother measuring the 0 RPM voltage with the Mega disconnected.

With a 12 Volt pickup I get a signal that is around 5 volts Im not 100% sure because it always worked in 12 Volt pull up mode.

Dont worry about it. The Stim has done its bit helping me test and build this unit. Im now moving to a bench setup with the actual Wheel and VR sensor for the last & final stage. I will try the cap idea though just to see how the signal looks.

Ill only be using the stim when I start work on the next car (Another untested setup but James has done the code again. For which Im thankful.)

The VR Circuit Im using is the default circuit with the default components as supplied by DIYAuto.

have added the cap I see the zero crossing on the scope now.
With the cap 12 volt pull up continues to work. 5 volt continues to not work.
But dont give it another thought. Will move to the wheel and tuning VR directly.

ronchinoy · Post by **ronchinoy** » Fri Feb 08, 2008 2:47 am

Ok got it to work on the 5 volt pullup on jimstim also.
Thanks for all the help.