How to build a Tachometer for RC car ?

hello guys,
i am trying to build a tachometer for an RC car, what i used to do before on my last prototype is to take input from the Throttle Potentiometer and make the tachometer’s LED change according to that

well what i need to do is to make the LEDs change according to the RPM of the engine ! so help me please what options do i have and what can i do in that case ?

this is the engine i am using (attachment files and link below) :

It looks as if that engine uses magneto ignition so there is no easy electrical connection. I would go for painting a stripe on the output shaft / pulley /gear or whatever and use a reflective optical pickup.

Russell.

russellz:
It looks as if that engine uses magneto ignition so there is no easy electrical connection.

right it works that way

russellz:
I would go for painting a stripe on the output shaft / pulley /gear or whatever and use a reflective optical pickup.

Russell.

how to make a reflection ?
well check this out i found this tachometer Search Results

and this is the datasheet, this tacho has one wire wrapped around the spark plug as it is shown in the datasheet but i can't tell how this works ? :confused:

firashelou:
. . . this tacho has one wire wrapped around the spark plug as it is shown in the datasheet but i can't tell how this works ? :confused:
http://gallery.trailtech.net/media/instructions/computers/tto/tto-tach-hour-manual.pdf

The wire around the spark plug wire couples (mostly capacitive?) to the high voltage in the wire. I've done this sort of thing as a quick and dirty way to get spark pulses to an oscilloscope in the past. The challenge with that approach is that you would have to do extensive conditioning of the resultant pulse before presenting it to an Arduino input.

A magneto engine should still have some sort of rotation position sensor to time the spark, either breaker points in an old engine or a hall effect sensor (?) in an electronic ignition system. You may be able to tap into that. Alternately you could add your own hall effect sensor to detect the magnet in the flywheel if there is room to install such and you don't have to understand or mess with the existing circuitry at the cost of having to work out the mechanical installation.

I'm curious, what do you plan to do with RPM information once you manage to sense it?

MrMark:
The wire around the spark plug wire couples (mostly capacitive?) to the high voltage in the wire. I've done this sort of thing as a quick and dirty way to get spark pulses to an oscilloscope in the past. The challenge with that approach is that you would have to do extensive conditioning of the resultant pulse before presenting it to an Arduino input.

A magneto engine should still have some sort of rotation position sensor to time the spark, either breaker points in an old engine or a hall effect sensor (?) in an electronic ignition system. You may be able to tap into that. Alternately you could add your own hall effect sensor to detect the magnet in the flywheel if there is room to install such and you don't have to understand or mess with the existing circuitry at the cost of having to work out the mechanical installation.

I'm curious, what do you plan to do with RPM information once you manage to sense it?

i am planing of doing a tachometer with LEDs :smiley:
well i was searching so i found that this is the best way of doing it by sensing the spark plug, the tachometer i posted in the link is for this kind of engine i am working on for RC
well i checked that i must build circuitry for safety as you said so can you please help me out with that ? what circuitry exactly and how ?

You could try something like this.

Russell.

russellz:
You could try something like this.

Russell.

that is nice, this circuit has security for the microcontroller right ? but i would like to use something smaller than that pickup clamp , the inductor cable would be good but i would like to know where should i attach the free end of the cable ?

Rather than the clamp you could just wind a few turns of wire round the lead. No nede to connect the other end for capacitive pickup.

Russell.

russellz:
Rather than the clamp you could just wind a few turns of wire round the lead. No nede to connect the other end for capacitive pickup.

Russell.

yes that's what i need, so u mean i take the wire i turn it fewer round like 4 around the spark plug, keep 1 end free in the air and the other connected to the circuit ?! but isn't that an open circuit ?

firashelou:
yes that's what i need, so u mean i take the wire i turn it fewer round like 4 around the spark plug, keep 1 end free in the air and the other connected to the circuit ?! but isn't that an open circuit ?

Have you noticed that there is an insulator between the two plates of a capacitor? Yet it still passes AC current? :slight_smile:

firashelou:
yes that's what i need, so u mean i take the wire i turn it fewer round like 4 around the spark plug, keep 1 end free in the air and the other connected to the circuit ?! but isn't that an open circuit ?

Not the spark plug! The HT lead to the plug. As Aarg implies above, it creates a low value capacitor.

Russell.

aarg:
Have you noticed that there is an insulator between the two plates of a capacitor? Yet it still passes AC current? :slight_smile:

ok but there is 2 plates parallel, but here 1 end is free far from anything so i dont see how that is relevant the capacitor with this case ?

russellz:
Not the spark plug! The HT lead to the plug. As Aarg implies above, it creates a low value capacitor.

Russell.

lol yes of course not the spark plug by itself but the "big wire" of it

ok but the circuit is open ! ahh wait is it like the antenna ? the RF link antenna which is open from 1 end and from the other connected to the circuit :smiley: ?

firashelou:
ok but there is 2 plates parallel, but here 1 end is free far from anything so i dont see how that is relevant the capacitor with this case ?

There is capacitance between the wire in the spark plug lead and the wire you wrapped around it. There is capacitance between any two conductors but it is low value and generally negligible at low frequencies. In this case it is enough because one of the wires is intermittently at something on the order of 20,000 volts so it doesn’t take all that much coupling to result in a detectable voltage on the “detector wire”.

MrMark:
There is capacitance between the wire in the spark plug lead and the wire you wrapped around it. There is capacitance between any two conductors but it is low value and generally negligible at low frequencies. In this case it is enough because one of the wires is intermittently at something on the order of 20,000 volts so it doesn't take all that much coupling to result in a detectable voltage on the "detector wire".

https://courses.cit.cornell.edu/ece303/Lectures/lecture6.pdf

what do you mean at something on the order of 20 000 volts ?!!! really ?! the spark plug is as close as that !?!! :o

russellz:
You could try something like this.

Russell.

hello russellz,
i would like please to know if there is anyway of testing this circuit without having the engine running ? i mean some circuit or if i turned the engine myself with a gun or with my hand would that be useful ?

Difficult to test it without the engine running unless you have a high voltage pulse generator. You could partially test it by connecting an ordinary pulse generator to the base of the transistor via a resistor of about 10 kΩ.

Turning the engine with a gun will only work if the ignition is turned on so could be done with the spark plug removed from the cylinder head and earthed somewhere or with no fuel in the tank.

Russell.

Test it on your lawnmower,weedeater,blower or any other type of power equipment you have.

detown:
Test it on your lawnmower,weedeater,blower or any other type of power equipment you have.

lol unfortunately i don’t have any of that :confused:

russellz:
Difficult to test it without the engine running unless you have a high voltage pulse generator. You could partially test it by connecting an ordinary pulse generator to the base of the transistor via a resistor of about 10 kΩ.

Turning the engine with a gun will only work if the ignition is turned on so could be done with the spark plug removed from the cylinder head and earthed somewhere or with no fuel in the tank.

Russell.

i do not have a pulse generator :S
so can i do it while the spark plug is in its place and with no fuel connected to the engine so the spark will be generated in the air ?