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KG4CGC
06-04-2011, 01:52 AM
How would I go about amplifying the AC current reading that I'm picking up from the outside of a spark plug wire. I'm building a tachometer.
I've considered a transformer then adding a rheostat for calibration.

KG4CGC
06-04-2011, 02:03 AM
I should add, it's 1 cylinder.

KK4AMI
06-04-2011, 05:44 AM
I should add, it's 1 cylinder.

Isn't that just a simple induction sensor clamped to the spark plug cable?

N8YX
06-04-2011, 06:07 AM
Isn't that just a simple induction sensor clamped to the spark plug cable?
Yes, and one would need to step up and condition the voltage - rather than the current - to get a usable reference. Something like a JFET would work nicely without excessively loading the pickup coil and reducing its output. For TTL gating purposes you're aiming for 4.5-5.2v on peaks.

N2CHX
06-04-2011, 06:16 AM
Add turns to your pickup inductor.

N8YX
06-04-2011, 11:27 AM
Add turns to your pickup inductor.
Assuming you're not using a sealed unit. Some of the setups built for the auto-repair community cannot be disassembled for modification. I have an old Sears diagnostics meter that includes a tach function and most of my motorcycle ignition systems don't generate enough high voltage to get an accurate reading. I had to go to a "Daytona" brand accessory digital tach (which triggers off the coil primary; switched 12v) in order to get a usable low-RPM reading for purposes of carb synching, idle-mixture settings and so forth. Frustrating, as the Sears unit's pickup merely clamps around the #1 plug wire...and I had to add a pigtail to the ignition circuits of each bike where the Daytona will be used.

KG4CGC
06-04-2011, 11:31 AM
Ah, I'm using coiled wire. Insulated probe wire.

K6BSO
06-04-2011, 11:31 AM
Just pull back the spark plug boot and connect a lead directly to the terminal of the plug wire. That should give you plenty of voltage...

KG4CGC
06-04-2011, 12:02 PM
Just pull back the spark plug boot and connect a lead directly to the terminal of the plug wire. That should give you plenty of voltage...
Next you'll tell me to touch my tongue to it.

N2CHX
06-04-2011, 12:35 PM
Ah, I'm using coiled wire. Insulated probe wire.

Toroid?

KG4CGC
06-04-2011, 01:54 PM
Toroid?
No, but there's an idea! Simple and I have a couple of rings in the parts bin.

KG4CGC
06-04-2011, 02:00 PM
Basically, I wrapped the spark plug wire and tried a few of the settings on my DMM to see what I would get. I can build this for zero dollars just from parts I have. I just have to figure out mounting the meter and weather proofing. I have some meters that were passed on to me from an amp builder who got outsourced from his regular job. After I get the electronics figured out, (with help from you guys:) ) I just have to wire up some lighting for night use.

KK4AMI
06-04-2011, 06:48 PM
Basically, I wrapped the spark plug wire and tried a few of the settings on my DMM to see what I would get. I can build this for zero dollars just from parts I have. I just have to figure out mounting the meter and weather proofing. I have some meters that were passed on to me from an amp builder who got outsourced from his regular job. After I get the electronics figured out, (with help from you guys:) ) I just have to wire up some lighting for night use.

OK, now you have my interest peaked. One cylinder, tachometer, night lights? Are you souping up a weed whacker for night time operations. I could see that, because I would hate cutting grass in the day down there :)

Another approach, some motorcycles use a magnet glued to the fly wheel and a magnetic sensor in the crank case. I guess its easier to count the revolutions of the fly wheel.

KG4CGC
06-04-2011, 07:33 PM
Scooter, or as some here like to say, moped. 50cc, so yeah, not much more than a weed whacker. Not going to crack open then engine. It's enough that I'm boring out a spare main jet by hand until the new one I ordered gets here. Already tore down and cleaned the carb including shimming the diaphragm needle. I opened up a baffle on the exhaust and now I want to match the main jet to the available extra top end. Working on a modified air filter to allow more air in for the slow jet which I also ordered. This all started with a leaky airbox that was making the carb act like the jets were wrong.

A tach is basically for my own curiosity but it should be an easy addition.

K6BSO
06-04-2011, 07:38 PM
Did you put the ignition pulse on your o-scope, Charles? I'd be curious to know what it looked like.

KG4CGC
06-04-2011, 11:08 PM
Did you put the ignition pulse on your o-scope, Charles? I'd be curious to know what it looked like.
Yes sir no I didn't. I don't have an O-scope. And we don't live in an alcove.
I've decided that a meter that measures from 0 to 10VAC is what I will use. I just have to amp the signal up enough to read 9.5VAC whens it's maxed out. That will save me the trouble of making a whole new meter face. I'll just have to cover where it says ''volts'' with RPM. :)

kc7jty
06-08-2011, 03:50 PM
Next you'll tell me to touch my tongue to it.

Only if it's a girl.

NA4BH
06-08-2011, 04:37 PM
If it's a lawnmower, save yourself some time. Just stick your hand under it and count how many times the blade hits it in one minute. It might take two people, one to hold the watch.

K6BSO
06-08-2011, 06:22 PM
If it's a lawnmower, save yourself some time. Just stick your hand under it and count how many times the blade hits it in one minute. It might take two people, one to hold the watch.

Don't forget to divide the resulting number by two.

NA4BH
06-08-2011, 06:29 PM
Don't forget to divide the resulting number by two.


Good catch.

KG4CGC
06-08-2011, 08:16 PM
That's how we counted RPMs on extruders. The end of the screw poked out a bit on the end of the outer case. I small piece of flat cardboard was taped on to it. You stuck your finger down, looked at your watch and started counting.

W1GUH
06-09-2011, 09:14 AM
Next you'll tell me to touch my tongue to it.

Don't knock it til you've tried it! :twisted:;)

w2amr
06-12-2011, 07:15 AM
When we were first married, she said the lawn mower wouldn't turn off. I told her to pull off the plug wire. She still hasn't forgiven me.