Hi all,
Trying to do what has been done a few times before, make a revision of a tachometer that is sensing the current from a spark plug wire. But i'm trying to simplify it and thinking of the best way to do so...

Instead of outputting the pulses to count RPM, I'd like to simply a design and just turn it into a inductance ammeter (like a fluke DC clamp meter). So when current is detected, it will output a logic level (0-5v) i can feed to a micocontroller. therefore i can ideally have an 0v when not detected, and 5v when detected. All i need is to identify when current is flowing, so it will act as an ON/OFF switch.

i know there are a lot of dc current sensors out there. but i'd like this to be a simple wire wrap or hall effect zip tied to the plug wire. so it is non intrusive and i can remove it.
I dont have any experience with hall effect sensors, but i think i need a bipolor or unipolor correct? I'm also unsure of the inductance put off by a spark plug wire and how to choose a hall effect sensor accordingly.

I also found this circuit, but dont know exactly what it is outputting since i dont have any SIM programs, so this may be of assistance.

My best guess is that i feed the signal from the hall effect into a circuit like this:

i've tested this before and it will take 0-30V input and switch the output from 1.2'ish V when low to 4.85V high.

Also, since a spark plug is pulsed signal, how would i go about smoothing it out so my microcontroller reads a steady output voltage from the circuit?
My electronics knowledge is at its beginning stages. so any drawings or schematics will help me tons.

Thanks for any help!
Taylor

 

This seems like a purely academic exercise rather than the most practical way to derive a 5v source only when an internal combustion engine is running?
One simpler way would be to have a charge pump or watch dog timer fed from the vehicle or engine supply such as 12vdc or alternator output, and use a proximity or gear detector as made by Honeywell etc on some rotating part.
This could then enable/disable a 12v-5v regulator circuit.
Max.

 

i agree, tapping into the electrical source would be easier. or using a missing tooth gear off the crankshaft to detect rotation. but the issue is i'd like to have a clamp on or easy to apply system, as it will be going between multiple engines, and they all have different powering supplies, ect, so it would be a pain.
yet they ALL have the same spark plug driven system, thats why i'm trying to use that design.

 

You need to get hold of an old timing light and salvage the inductive pickup?
Max.

 

every ammeter or timing light ive seen them talk about in detail says they use a hall effect sensor. i figured it would be easier to just buy one, and get the exact one i need for the application than to tear apart an existing clamp.
just didnt know which one would work the best, and the schematic behind it.

it seems this hall effect sensor has a schmitt trigger embedded.
http://www.digikey.com/product-detail/en/OH090U/365-1001-ND/374779

so does that mean this device would replace the schematic above? i'm worried about what the output signal would be since its next to a pulsed dc signal. would the voltage vary or would it be constant.

 

That is quite different to the ones I have used, they all refer to an inductive pick up, which is a clamp around the wire and a coil transmit to the Light unit.
I believe you will find the one you point out in the link requires a permanent magnet to operate?
Max.

 

i think that sensor i posted is the last one in this video (far right):

http://bildr.org/2011/04/various-hall-effect-sensors/

looks like it does a nice linear response to a source.

 

They all still appear to require a magnetic source?
Max.

 

i was under the impression that a magnet wasnt required? that the dc current gave enough radiation off for the hall effect to determine the current?

like this sensor:
http://www.dfrobot.com/index.php?route=product/product&product_id=580#.Ui5GTz-wUec

 

With a Tacho you are counting pulses not current. So ammeter circuits are not the way to go.

 

sorry, using the word 'tach' in my original post wasnt the best idea. Since a tach counts pulses to find rpm and i'm not trying to do that.

guess i'm just looking for a hall effect sensor that i can use on a spark plug wire, then the ability when current is detected to switch an output voltage from 0 to 5v or vice versa.

 

My little Briggs and Stratton engine hours timer does exactly that. With a wire wrapped around a spark plug wire, it only runs the clock when ignition pulses are detected. Not sure what's inside.

 

yep, just like an hour timer.
the first circuit i posted is a wire wrap method. im curious to if a hall effect sensor will do the same thing, as it will eliminate a ton of parts and size. then curious to what i have to do schematic wise for it to output or trigger an output of logic level steady voltage so I can read it as ON/OFF.

 

My little Briggs and Stratton engine hours timer does exactly that. With a wire wrapped around a spark plug wire, it only runs the clock when ignition pulses are detected. Not sure what's inside.

Agreed.

I thought about tearing into one to find out, but never did.

 

Great read here Buddy. Thanks a ton.!!

 

Agreed.

I thought about tearing into one to find out, but never did.

Now that you mention it, I tore apart the last one I had - the battery is supposedly not replaceable but I thought I would try. I'll see if I can find it and post a picture.

 

i hope this is ok to post, dont know why it wouldnt be, its public info. but heres patent info on one of the wired ones. componet values arent listed but may can figure out what style circuitry is being used.

http://patentimages.storage.googleapis.com/pages/US5644491-3.png
http://patentimages.storage.googleapis.com/pages/US5644491-4.png
http://patentimages.storage.googleapis.com/pages/US5644491-5.png

 

It's too bad that the components aren't identified, the pick up (42)? outputs a 300v p-p pulse by the look of it, which to me would be an inductive or capacitive pick-up WRT chassis?
Max.

 

Something like this might work for you. It has a little glitch when it first starts up - not sure if that will cause you a problem. It assumes the pulse is positive like the one you posted. The number of turns needed around the spark plug wire is another unknown.

 

thanks for the schematic, the design looks somewhat similar to this guys:

http://www.kokoras.com/rpmsparkplug/

his is up and running, even a video of it, i'm trying to pick apart the parts i dont need from his idea (LED and PSU) and just use the inductance idea.

what i DONT know is what the output of the schmitt trigger is. my guess is that its pulsed to still be able to read RPM by the arduino?
if so, i'm looking to maybe take that part out, and have it trigger another transisitor like the 2nd schematic in my first post to trigger HI/LO voltage output to act like a switch.

not sure if the output before the schmitt can be smoothed out somehow to give feed for the inverter circuit (2nd pic) so its not turning on-off-on-off constantly with the pulses.