tachometer

[V6'er]

There's a much smaller formfactor pot that you could have used instead that would have made it easier to make happen.

[Spektrewing386]

not according to what i saw at radioshack.


also i just used what i saw in sengk writeup

[V6'er]

Originally Posted by Spektrewing386

not according to what i saw at radioshack.


also i just used what i saw in sengk writeup

Stop looking a retail schlock shops and go look at 3/8" square pots that have been available for 40+ years.

[conor]

Radiocrap definitely has a poor selection of electrical goodies. I'd search around town for another shop (or someone online) with a better inventory

But hey.. if it works it works, I suppose!

[Spektrewing386]

anyway. My idea is working somewhat, but is very hard. I have ghetto rigged both tachometer to the same place on the instrument cluster. But keeping the RPM steady, turning the pot screw, and comparing both RPM readouts all while trying not to get dioxin poisioning is really hard.

Now im thinking I dont have the money for a full blown funciton generator. But there has got to be some program out there that turns my PC into a function generator and outputs waves either through USB or soundcard. And then theres some way I can hook that up to the tach and get an exact calibration.

I see a lot of tone generators on the internet for soundcards. I have some old computer speakers that I can cut the wires off of, plug the wires into the soundcard and the tone generator will send electrical pulses through the wires. good idea? of course with an amp.

[V6'er]

SourceForge.net: Audacity

[Spektrewing386]

yeah i got a program called NCH Tone Generator that can do square waves of any freq.

Now I can hook my computer up to a stereo I have and use the volume knob on the stereo to adjust voltage. Thing is speaker wire has a - and a +. I know theres a 12v and a ground on the gauge, I can send 12v to there fine, but the 3rd connection is the signal. So which part of speaker wire do i use for the signal? Positive? and then I connect the - side of the speaker wire to the - connection (thats also hooked up to the 12v battery)???

[V6'er]

Connect (-) to the tach's Ground lead.

[Spektrewing386]

k cool i got it to work. right now im testing the setup with an old 4cyl tach my dad had in the garage. Im using a program called SigJenny, making a square wave at a particular freq using a chart i found here: MGB Electrical Systems: Your Colour ... - Google Book Search

sending the square sound wave to an old Sony stereo I have. And sending the amplifed signal through speakerwire connected to the Sony stereo to the tachometer.

I also have the negative side of the speakerwire connected to the negative post of the tach, also the negative side of the 9v battery is connected to the same post.

And when I send it a 100Hz wave the tach reads exactly 3000RPM (4cyl tach here) and when i send it a 33.33Hz wave it reads exactly 1000rpm... cool!

[V6'er]

Make the V6 tach read 6000 RPM at 300 Hz and 1000 RPM at 50 Hz. You might have to remove the needle and reset it's position.

[Spektrewing386]

for some reason its working perfectly with the old tachometer i have, but not the toyota one. It the needle doesnt go above 1000rpm, but with the old tachometer im testing with (from like a lotus elan or something) the needle will shoot up to whatever RPM i tell it to.

[V6'er]

Don't forget that the input voltage is different on the newer tach, AND, that you are sending a signal that symetrical around Zero volts (ground). Ideally, you need a diode and a resistor to only send the positive going signal. (That's why I use a function generator that allows me to adjust the DC offset of the signal.)

[Spektrewing386]

ok i found a LED celica brake light in my garage thinking i can use that to rectify the signal. Well the LED lights up, but the needle doesnt move.


i only have an LED on the + signal line.

If i set the freqency low enough (10-20hz) I can actually see the LED pulsate/flash, its pretty bright too so that should be enough power.

I thought that would fix my problem, but now the needle isnt moving.

[Spektrewing386]

ook I have also hooked up a DMM to the signal portion of the gauge. I can now precise send signals at about 12 volts (DMM reads 11.1v) at any freqency i want. The toyota gauge still doesnt want to work correctly.


this is my exact setup, and whenever i put the diode in, it doesnt work at all.

But it works perfectly with the older tach.

[V6'er]

Unless you have a True RMS meter, it will only be accurate for a 50 Hz or 60 Hz sine wave.

Again, you need a diode and a resistor to modify the signal.

[Spektrewing386]

so if i send it a 50 - 60Hz signal I can calibrate the volume knob for 12 volts and have it somewhat accurate?

The diode and resistor would go on the + audio output line correct? but why does it stop working when I do that (yeah i had the diode pointing the right way). I used a celica brake light LED as my diode and I also smashed apart an old AC adapter i have and tried the diodes out of it... nothing.

I guess I could send it to you, but i came so far.

[V6'er]

You have to have the resistor AFTER the diode to load down the signal. Use 1k to 5k or so. The diode will be in the (+) lead with the band towards the tach. Use a 1N914 or 1N4148 diode.

Use a sinewave to measure BEFORE the diode at 60 Hz, and set the output of the amp so that the meter reads about 9VAC. This will give about a 12V peak, positive going signal.

[Spektrewing386]

now will that just half rectify it or will i need to fully rectify it?



*edit*
just realized a fulley rectified square wave creates a straight line.

[V6'er]

That will half rectify it. Then with a square wave input, you will have a square wave output with no below-zero transitions.

[Spektrewing386]

*edit*

nevermind i answered my own question