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Hi all,
Here I am to announce my success in modifying, calibrating, packaging and functioning of a Bajaj bike's tacho in my Premier 118-NE. It has been a dream for me to own an 118-NE since my dad sold his in 2004. Now at Pune I had got one about 8 months back. Since then I did all the DIY job of fixing a central locking system (@ a meagre price of Rs 450/-) and a DVD player. Next project was to have a tacho. The market price for a tacho seems to start from Rs.2500/- which in my opinion was too much to my budget. As a DIY guy, the idea to use a Motor cycle tachometer for a car. One fine day I even bought a tacho from a spare part shop. He charged me Rs 450/- for this.

Now the study of principle started. I ripped it apart and separated the coil from the circuit board so that I could identify the components and trace the circuit layout. From the data sheet I could locate where exactly to hit, to make the required calibration. In the motorcyle, the tacho displays the rate at which spark occurs, and the same principle is going to be used here. But the problem here is that in a bike the tacho would sense one spark per crank revolution. As an example while it senses 50 sparks per second (meaning 50*60) the needle points 3000rpm. On the other hand my car would deliver 2 sparks per rev, which would mean that the needle would point to 4000 rpm when the engine actually revs at 2000. And thats the need for playing with the circuit.

The circuit board after replacement with the appropriate components (a blue coloured preset and a 220K resistor) are also shown in the picture below. Now comes teh question of how I do I set the preset to meet the required setting. As a example while the tacho senses pulses at a rate of 50Hz the needle should actually point 50*60/2 = 1500rpm. You can simply use a step down transformer of about 15 volts rating to have a 50Hz or 100Hz signal. Feed the 15V signal to the signal input point on the board and adjust the preset to point 1.5. Your job is done for a 4 cylinder application.

Then comes the problem of packaging it...

Um, if the display is based on frequency of the AC, i doubt how using a step down transformer would help. that would only reduce the voltage right, not the frequency. I think you would need a divide by 4 counter

Hi Green horn,
The first point is that the signalling voltage should be some where equivalent to the signalling frequency in the motor cycle. So I used a step down transformer.
Secondly we need a standard frequecy source to standardize the tachometer for a 4 cylinder application. I took advantage of the domestic supply frequency 50Hz. Though they are sine waves, teh circuit has got a Schmidt trigger to convert it to square pulses. I had also mentioned that I had used the unfiltered output, which means that the signal input to the tacho was sinusoidal, and at a voltage of 15V. Hope this makes you clear.
This 50Hz is equivalent to 50 sparks per second. Any 4 cylinder petrol engine makes 2 sparks per revolution. Hence the 50Hz corresponds tp 50*60/2 = 1500 rpm. Now adjust the preset to show 1500 rpm. Thats it.

Hi all,
The hunt for a suitable housing started as soon I saw the tacho needle pointing the right rom at the right frequency. The guy who sold me the tacho was generous enough to offer me a worn out speedometer from his bullet. The speedo was ripped to pieces so as to remove the mechanism from inside. That was not the end. The tacho's dial would'nt go in. So I had to remove about 1mm of the dial from all sides. First I thought of doing it with a nail cutter (quite a crude tool, nothing else striked then). After traversing the third cut, I ended up breaking the nail cutter. What other options?? The dial is made out of acrylic or some semi transparent plastic. So a hot pen knife did the job, but took half an hour to do it slowly. A little of filing and the job was fine. The dial would perfectly fit in and again it proved wrong with my assumption.

Now stupid Ilango did'nt check if there was enough height available before cutting the dial. Yes, the height was too less. See the first pic. What to do now. Thought of cutting the rear half and extended it with a PVC tube. Bought a piece of 90mm PCV pipe for Rs 10. HEated it and tried to push it around the speedo case. Alas, only the pipe deformed. No result. Since I did'nt like to have a huge container for the tacho, I was exhausting all possiblities with that bullet's speedo. Nothing happened. So the next idea!!!

Went to a scrap yard to procure a speedo (who cares whether working or not, need one with scratch free glass and good looking case) for Rs 100/-
Came home and ripped it only to find that the box is now too long to accomodate. But that did have its own advantage. The second picture shows the acrylic sheet which I had cut for using as an insulation. Used 3 plastic spacers between the circuit board and the sheet. You can see another 3 holes at the centre, for bolting the rest to the box. Again used 3 spacers here. Got a foot long (3" wide) black sticker strip from a number plate guy in our neighbourhood and wrapped it around the casing. Crimped the speedo lens back after painting it black.

Sorry guys, fogot the bullet's speedo at the end. I didnt take a pic. This is a schematic for a better unbderstanding

Fantastic and very interesting. Waiting to read the rest of the story.

I still don't understand this bit :confused:
so you used the preset pot in the tacho to adjust the ratio?

Wow! I must say I've never heard such a dream before! :)

But congrats on following your heart. The DIY here is much beyond anything I'll ever be able to DM.

Perhaps this thread would be better suited in the Mods & Accessories subforum?

Originally Posted by ilangop
Hi Green horn,
The first point is that the signalling voltage should be some where equivalent to the signalling frequency in the motor cycle. So I used a step down transformer.

Sorry Greenhorn, that was a typing mistake. It should be voltage in place of frequency.

Yes, you can refer to the data sheet of CS8190.

Thanks buddy.
Since this was more of a study and DIY kind of activity I found this as the right place. This tacho experiment was more for learning rather than an accessory.

oh ok, got it now :)

The tacho was glued with the dash board with a strip of double-sided sticker tape. The wirings were all made and finally cranked the engine and raced it. Wow!! the needle moved. The pictures have been attached here. This was the first time I did such a job, so poor finish in painting. Please bear with that.

Hi ilangop !

Congratulations on you success !! Could you please post the wiring diagram and the modifications done to the bajaj tachoplease:

Regards,
Adheesh Parelkar

wow, interesting DIY.

Keep it updated once done.