How to build your own tacho adaptor

[ViPeR_NiPPleX]

Howdy,

Tonight whilst preparing to make another adaptor I thought I might aswell document it and make an article. The adaptor is designed to take a signal (5v square wave) usually from a newer style ecu and spit out a signal like what is found on older ignition systems.

Tools list:
--------------
Side cutters
Soldering iron/solder
Electrical tape

Parts list:
--------------
1x 1N4007 Diode
1x BC546 (BC547 could be substituted as quite a few other small signal transistors)
1x 2.7kohm resistor 1/4w (i've tried this with 2.2k and 3.3k ohm resistors with no problem)
1x Gutted coil from a relay
4x lengths of coloured wire


I'll appologise in advance as to the quality of my photos. It's an old camera, and i've tried to enhance the photos... just doesn't do good close ups!

The schematic from where this all started


This is the type of relay I gutted, it doesn't say what voltage it is (i suspect 240v), don't think it matters in this case.


Using the side cutters, gut the relay. Be careful not to snip the tiny copper strands going to the coil. Here it is with the diode.


A resistor and transistor.


Here i've taken the gutted coil and insulated it with black tape. This will be the base of the adaptor.


My dodgy mspaint skills at work showing the pinout of the transistor.


Spread the legs of the transistor out so you can tin the collector "C" pin and solder it to the coil (doesn't matter which pin of the coil, it doesn't have polarity)


Again, sorry about the picture quality. Here i've tied the resistor and the diode together for soldering to the base "B" pin of the transistor. In the original schematic, take note of the stripe on the diode, it MUST be around the right way for this to work!


Now once you have arranged the legs of the components in a neat way you can now strip and tin the ends of your wires and attach them to where they need to go. I've done Red to unused pin of coil, Black to emitter "E" pin of transistor, Orange(Input) to diode leg, and Purple(Output) to collector "C" leg of transistor.


Phew! After thats done, you should be able to wrap another layer of insulating tape around the adaptor and it's literally ready to go. You may choose to put this in a box for extra protection. I've used a tube of plastic and potted it with hot glue in the past.

[ViPeR_NiPPleX]

Most of you wont have an oscilloscope at home so testing your new adaptor will have to be on the car. I've tested the design on an MA61 with a 7mgte and another MA61 with a 1jz-gte, both work as if you were looking at a normal tacho. I've got a diagram which is handy for those of you who had a 5m-e and have now swapped something else in... other toyotas might have this plug too.



Well that's enough from me. Hopefully this guide will be useful to someone out there. Any questions or problems feel free to ask.

Cheers,
James

[RObErT_RaTh]

Top guide mate, definately worth adding to Sam's MA61 part and info thread

[Cuzzo]

could this be used in reverse? Such as old motor new tacho?

[ViPeR_NiPPleX]

I don't believe so. There is a way to do it using opto-couplers, i'll have to look into it and draw up a diagram.

[Cuzzo]

Never mind it was just for an experiment.

[JetspeedCamry]

Simply awesome!

[The Witzl]

have you ever measured the inductance of the relay coil? Perhaps there might be something neater, cheaper and easier to use off the shelves in Jaycar.....

Typically most relay coils have a DC resistance of about 180ohms, and you are usually told the DC resistance of an inductor when reading its specs.

[ViPeR_NiPPleX]

I'll measure the inductance of the coil and post it up later... I have a plenty of these relays at work, as with all of the parts I used

[Hen]

Another packaging option is to use a normal 5pin automotive relay. I cut the top off it, and gutted the unused parts (all bar the coil). Then I added the resistor, transistor and diode inside the relay case and used the existing 5 pins in the bottom for inputs and outputs.

It came up quite neatly.

Hen

[ViPeR_NiPPleX]

Just measured these:

DC resistance - 280ohms
Impedence - 330mH

According to my dad (boss), they are a 12v relay. I like Hen's idea of using the body of a auto relay for the package. You could do the work then fill it up with hot glue/epoxy to seal it up when ur done.

[JustCallMeOrlando]

I'll try and get a pic of mine. I had some random 4 pin relay kicking around. I removed everything but the relay coil. I then purchased the transistor, I can't remember what I used, i'll check it up this arvo. I used a 1/4 watt 1k2 resistor, and I didn't bother with the diode.

I glued the transistor in the corner, then neatly packaged everything, and now it just looks like a relay, and because I used the relay plug, I can plug and unplug at will.

[JustCallMeOrlando]

Oh, and the resistance was 180 across the coil

[ed]

+ rep for the thread
-ve rep for the photos


*rubs headache away*

[ViPeR_NiPPleX]

appologizing twice in one post doesn't cut it.... I knew it wouldn't