Author Topic: Can you help me? (Read 5166 times)

gldgti · « **on:** February 17, 2005, 02:40:08 pm »

Dear all you guys,

I got a tacho the other day that works from the coil and has settings for 4, 6 and 8 cyl cars. I managed to get an ABS sensor, similar to the one you used on your inducive pickup tach setup, and i want to use it to get a signal from the v-belt pulley on the cracnksharft (the 4 bolt heads). lacking an ocsilloscope, ive tried testing the pickup with a voltmeter and as far as i can tell, it puts out a bout a 20-30 mV signal.

obviously, this signal is not enough to run the tach, which was designed for a 12V signal form the coil on a gasser. what i need to do ( i think) is make a circuit to amplify the pickup signal so it peaks at around the 12V range, right?

i tried using a little NPN transistor all by itself and managed to amplify the signal to around 0.5V - still not enough. but apart form knowing basic principles of electronics, I'm at the end of my knowledge. noticing on the inductive pickup tach thread that you said making a tach driver circuit didnt make you blink (or something similar) i was wandering if you could suggest what i need to make a circuit to suit my needs.

can i use a small audio amplifier circuit?

thanks very much, your help and expertise is HUGELY appreciated.

regards,

Aydan

81rabbit · « **Reply #1 on:** February 17, 2005, 05:19:57 pm »

Someone might know a better way but the first think I thought of was a simple op-amp. Something like a 751. You should be able to pick one up along with a couple resistors for under $2 or so. If you have a basic electronics store you should be able to pick them pretty easily. If your input signal is peaking at 30mV and you want it to reach 12V then some simple math... 12/.03 = 33.3

here's a link that explains it...

http://ourworld.compuserve.com/homepages/Bill_Bowden/opamp.htm

Look at the non-inverting op amp. you now need to find resistor values that will give you a gain of 33.3

by using the formula (1+Rb/Ra) = 33.3

this simplifies down to Rb/Ra = 32.3

So Rb = 32.3 * Ra

So just pick any value you want for Ra and then multiply it by 32.2 and you get the value you need for Rb.

To make it easy pick Ra = 1k ohm. So Rb = 32.3 * 1k
So Rb = 32.3 k ohms. you get these two values and then connect them to the op amp as shown and there you go...

when you get the op amp it'll probably have 8 pins on it. some of the pins will need to be connected to 12V (aka you battary) and the others will just be left alone, grounded, or connected to the resistors. look up a basic op-amp circuit online and you should be able to figure that out...

If anyone has a better way then go with it but this is pretty cheap..

-J

gldgti · « **Reply #2 on:** February 17, 2005, 07:50:45 pm »

cheers, i'll have to look into that....
Aydan

fatmobile · « **Reply #3 on:** February 17, 2005, 11:55:11 pm »

I was thinking op amp too.
They are pretty easy to figure out once you understand that there are basically two referance voltages connected to it ... and there are two inputs.
Hold one input (it does matter which one) to the threshold voltage.
The other input comes from your device. When the voltage from your device is below the threshold, the output is connected to the lower of the referrance voltages. When the voltage from your device rises above the threshold voltage, the output slams to the other referance voltage.
As an amp it would operate in the middle of the 2 referance voltages.
As a comparitor it acts more like a switch.
30mv seems pretty low, you'd probably have to use a voltage regulator to supply power to the threshold voltage just to keep it from changing with rpm. Like they do with your fuel gauge power supply.

gldgti · « **Reply #4 on:** February 18, 2005, 12:53:15 pm »

thanks guys,

i had a feeling that a kind of 'square wave' would be the go. just tried downloading the pdf but on 56k its a bit of a hassle at the moment....i will read it all though.

IF its not a really big deal, it would be a huge help if i had a circuit diagram. I would be forever in your debt. thankyou very, very much.

the only thing i know anything much about is biodiesel, so if your ever stuck with that......

cheers,

Aydan

rackley · « **Reply #5 on:** February 20, 2005, 07:12:37 am »

I'd do it, but the NA engine in my car has coolant in the oil and oil in the coolant and won't run, and the 1.6TD engine I'm swapping in is on the stand awaiting misc parts :-(

gldgti · « **Reply #6 on:** February 20, 2005, 03:10:58 pm »

I'll put that together and give it a try. thanks a lot for that. I can do the buildng bit - its the real techy stuff i have trouble with.

news on how it goes when im done...

gldgti · « **Reply #7 on:** February 24, 2005, 12:30:50 pm »

well, made the circiut up yesterday....

lacking any kind of measurement equipment until i go home to my parents place, i'm not entirely sure everything is right. but, as a test, if i connect everything up to a 12V source and use a test lamp in place of the tach, i ought to get a flash when i pass some ferrous material past the pickup, right?

steve · « **Reply #8 on:** February 25, 2005, 12:21:30 pm »

I'm not a curcuts guy but I suspect that the light may not light up due to the current draw required to get a filament bulb glowing and overcoming the thermal mass of the filament... Would it hurt the opamp to short it across such a low load?

gldgti · « **Reply #9 on:** February 28, 2005, 03:23:02 am »

thankyou once again.....

you'l be pleased to know that ive gotta be an electrical experimenter now, as one of my uni subjects this year, "sensors and actuators" deals with this stuff all the time.....

thanks, still workin on it, will update you all when theres more news

aydan

gldgti · « **Reply #10 on:** August 31, 2005, 07:34:16 pm »

well, after a long break, i'm back working on the tach thing again...

i've constructed this circuit and put it in a little box. I have the sensor mounted on the car and it points at the water pump pulley bolts. I did this just because it was fr easier to mount the sensor here and after dricing around for 3 months with it in place i'm satisfied that it is in a safe spot - i havnt needed to tighten any bolts or adjust its position since i mounted it.

I've run wires from the sensor through the firewall and i have a place to mount the box..... all i need now is to get it working.

I connected everything up a couple of months ago and tried it with no success unforunatley. tacho didnt move :-(

from memory i get an output voltage around 0.6V from between ground and the tach output line. I still have about a 40-60mv signal from the sensor (better now its mounted and stays a good distance from the bolt heads.

should i try playing with resistor values and stuff? i rpobably cant hurt the tach, but i just wanted to update and see if you guys have any ideas.

aydan

Author Topic: Can you help me? (Read 5166 times)

81rabbit