Hi- I have experience with some very basics and have repaired things but dont know much about electronics. I'm trying to learn.
I'm trying to build a convertor to install a (12v - negative ground) isolated tachometer in a positive 12v +ground antique car for antiques.
I found a thread when it has been figured out. This is on the trigger input side beteen the "sensor wire on the tachometer' and the ignition coil.
https://forum.allaboutcircuits.com/...ative-signal-into-positive-signal.8030/page-2
Did I find the right circuit to build?

I have the simple diagram and kind of embarrassed to say I dont know how to wire it up.
I can order a transistor and think I can figure out what resistors to buy, but struggling with my limited knowlege and could use some help.
The diagram shows a 3 leg transistor and some resistors and a possible power feed? - but I don't know how to read a diagram. I feel like I'm doing my homework but just dont get it.

My experience is automotive. I'm reading up on resistor colors and stripes and ends, and looked at the data sheet for the required transistor-
and see the 1 2 3 legs are oriented in comparison to a flat side.
What I need is a translation of the diagram into a simple diagram that shows required resistor install direction (even a drawing that says "stripes this end'", Lines to show connections/wires for the transistor to confim I wire the legs correctly,
and identify if I need to connect a positive wire or a negative wire in the circuit. Like a automoptive simple analog diagram.
One more question is should I be figuring out how to heatsink the transistor or is it ok as is?

Can someone sketch me a diagram for a dummy?
And confirm parts in plain english what I need to buy?
One 2N3906 transistor, what 2 1000 ohm resistors and 2 10,000ohm resistors?
(is there a voltage or or power or tolerance rating)
I'd bet you could draw the diagram on a scrap of paper or cocktail napkin at the bar!

 

Many of those old (Smiths) tachos are driven by a current transformer in the wire to the coil. If that is the case it might not need any changes.

 

What is the tachometer model number?, as the Smiths type used a pulse transformer inside so you won't need to modify it, other types go across the points .

 

Vintage Datsun: planning to remove cable drive and installing new modern internals. Or using a new faceplate in modern neg ground tach, MGA/early mgb are cable drive also. I know some later ones are electric and an be modified by changing loops inside. I really prefer to use the Datsun and modify with a modern guts so everything looks/ is "stock".
and be able to have a shift light too if I want.
How would I know which smiths types could be used without any mod?
On the other hand ther is a celica neg ground that is a smaller case with a bezel that is similar that can fit better,
If the guage face was redone to look like Datsun it would be a good choice

So options can be:
Find out what models smiths types can be used without modification or are positive ground, and use the guts for a swap,.
Just need detailed info as I cant guess my way to sucecess.
Or Use neg ground tach or guts and decode the diagam so I can make up an inline signal convertor.

 

How would I know which smiths types could be used without any mod?

You'll spot the weird connection for the wire to the coil - it doesn't actually connect- it goes twice around a piece of steel which is held in place with a screw, and that steel forms the core of a transformer.

 

This is a smiths tachometer.

 

This is a smiths tachometer.

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I don't need go out the shed to photograph one now! The one I've got is a lot rustier than that.