divider circuit for a dummy!

[novicepaul]

Hi, can someone please help me, I've just brought a small tachometer for my car, but the tacho is for 4,6,3 cylinder cars and I have an 8 cylinder car. :roll:
What i need is a simple circuit to half the neg pluse from the coil so that I can run it in 4 cyl. mode, simple, yes I am , don't know where to start!

The tacho gets it's info from the neg side of the coil, which is grounded by the points in the distributer. My car's points open/close 4 times per revolution, the tacho is desigined to work with a ratio of 2 times per rev. How can I half the singal? I'm ok at following diagrams and making them work, but how they work is pure Magic to me! If any of you can steer me in the right direction I would be most greatful
Thanks Paul

 

[k7elp60]

How about just taking the RPM reading using the 4 cyclinder mode and dividing by 2

 

[stevez]

The problem isn't too complicated, I think. Of some importance is knowing what the tachometer expects to see for an input. It sounds like it's looking for contact closure from the points. What you'd need is something with an output that looks like contact closure to the tachometer that is triggered by contract closure as an input. The stuff in between needs to pass along only every other pulse - halve it as you've stated.

The input circuitry is likely going to see a lot of noise so you'll have to deal with that. At 8,000 rpm you'll see 4,000 cycles per minute - that's only 66 Hz - not too awful fast. I'd put some kind of transient suppression on the front end then possibly add some kind of trigger circuit with lots of hysteresis - maybe a comparator of some kind fashioned from an op amp - 741 comes to mind but there are plenty more. Comarators ICs exist as well - just configure with lots of hysteresis. Hysteresis is best described as the difference between the level it takes to trigger from off to on then from on to off. Schmitt triggers, among other things do this. The output of this could go to a flip flop or some other divide by two then drive an FET that hopefully looks like a low ohm closure to the tach.

Sorry for the babble and the less than complete details. It's just the thinking I'd go thru if faced with the task you have in front of you.

 

[JimB]

In principle the problem is simple, but as Stevez points out, the coil/contact breaker connection is not a friendly place for electronics.

However you said:

the tacho is for 4,6,3 cylinder cars and I have an 8 cylinder car. :roll:

Two things get me thinking here,
the order 4,6,3, I think most people would write 3,4,6 or 4,6,8, not 4,6,3.
and
the fact that I can't think of one car with a 3 cylinder engine, but lots with 8 cylinders, (even some with 5, but not 3).

Could it be that the 3 is a badly printed/scratched/damaged 8 ???
If so, end of problem.
Try it and see, it won't break it.

JimB

Edit: I just did a quick search on Google, and found that some Daihatsu cars have a 3 cyl engine.

 

[audioguru]

Suzuki had some 3 cylinder (motorcyle?) engines in some of their tiny cars.

 

[novicepaul]

4,6,3 cyl. cars

Hi the tacho is made in tiawan where 3 cylinder cars are more the norm than 8 cyl. the annoying thing is that the case was stamped 4,6,8 but in their wisdom they put a sticker on top stating 4,6,3! If the top wasn't crimped on I'd have popped it open to see how they'd got the 3 cyl option and if it could be returned to 8!!

Is there some way that I can construct a simple flip flop device, to count to 2 send the trigger pulse to the tacho and reset it's self? If so could you point me in the direction of possible drawings?

Oh and the bit about it getting to 8000 rpm/66Hzs is a bit optomistic, it's a '66 Tbird with a 390 ci, I think that 4000 rpm is about all I'd want to go to!! So as you can see 33 Hz should be fine :wink:

 

[Phasor]

Re: 4,6,3 cyl. cars

Oh and the bit about it getting to 8000 rpm/66Hzs is a bit optomistic, it's a '66 Tbird with a 390 ci, I think that 4000 rpm is about all I'd want to go to!! So as you can see 33 Hz should be fine :wink:

Don't forget that you've got 8 cylinders, so it's not 33Hz, it's 8 x 33, so it's actually 264Hz. Not that that's really significant.

And yes, 3 cylinder cars are around, though not very common, take for example, the Daihatsu Charade. EDIT: Sorry JimB, didn't see that you already mentioned that.

 

[ljcox]

My car's points open/close 4 times per revolution, the tacho is desigined to work with a ratio of 2 times per rev. How can I half the singal? I'm ok at following diagrams and making them work, but how they work is pure Magic to me! If any of you can steer me in the right direction I would be most greatful
Thanks Paul

The best way to divide a frequency by 2 is to use a Flip Flop such as the 4013.

However, if the pulses are coming from a contact, there will be contact bounce. So you will need a low pass filter to eliminate these.

I suggest youdo a search in this forum for "flip flop" and "contact bounce" so you can do some background reading. If you then need more help, just ask.