Rev counter/tachometer pulse reduction (divide by 2)

Hi. I'm trying to find the best way to quickly output one pulse after receiving two pulses with a 08m2.
I'm building a circuit for my car's tacho. 4 cylinder engine was replaced with an 8 cylinder engine and the tacho is reading double the RPM. The tacho signal is a square wave tied high, dropping low for each spark plug fire (measured it with a scope and it seems that the ''low'' part is the same width at any engine speed). Its a ~12V wave, so I'd probably need to use transistors or FETS both to receive the signal and to output it.
What do you people think would be the best way to tackle this (I'm quite new with this, so both hardware and software ideas are welcome)?

Sorry to go 'non micro' but a flip-flop with buffering before and after is an easy way to divide by 2.

samis_p said:

4 cylinder engine was replaced with an 8 cylinder engine and the tacho is reading double the RPM.

Click to expand...

Food for thought
http://www.physicsforums.com/showthread.php?t=550469
http://www.electronics-tutorials.ws/counter/count_1.html

samis_p said:

Its a ~12V wave, so I'd probably need to use transistors or FETS both to receive the signal and to output it.

Click to expand...

Receive it - no. Output it - probably.

If the logic is powered with less than 12V, then to receive the signal, you can just use the normal PICAXE download circuit on a normal input pin. If the pulse is push-pull, then you only need the 22k resistor and you don't need the 10k pull-down.

However if you use Dippy's suggestion of using a flip-flop and you get one that supports 12V (e.g. 4000 series) then there's no need for input resistors or output transistors as you could power the flip-flop from the 12V car battery and then the flip flop could safely receive a 12V signal and it could output that voltage too (or slightly less).

"measured it with a scope and it seems that the ''low'' part is the same width at any engine speed"

What width?

boriz said:

"measured it with a scope and it seems that the ''low'' part is the same width at any engine speed"

What width?

Click to expand...

From what I've seen it looks like the tacho signal comes from each ignition coil - it's tied high (at 13V) and goes to 0V each time the coil gets grounded to fire the spark-plug. The width of that pulse is the same no matter the engine speed. I will upload a screenshot later to explain it better. I was thinking of using a picaxe because I'm fairly new to electronics and I've only done a few things with them. Using a flip-flop seems to be a pretty easy solution, I'll have to read on about it.
Here's a screenshot of the wave at idle (~550rpm)

The part it goes to 0V is the same width (~5.5ms) at any engine speed (though I didn't go above 4k RPM with the scope attached). Anyway, I doubt there's anything difficult here, as it looks quite simple. I've ordered a couple of CD4013 D-Type flip-flops to try out, hopefully I'll receive them early next week. In the meantime I'll try to make a digital tacho to work with the AXE133Y screen. Thank you all for the input, but I'm always looking for any advice on this.

@samis_p:

What is the final goal? Use the analog tach, build a replacement digital with the 133Y, or run both tachs?

Most analog tachs are integrators, they use RC components to drive a meter - increase RPM means increased voltage on the capacitor and a larger meter deflection. More expensive tachs use some active circuitry to assist with linearity, etc.

The AXE133y is straight forward ... you could write all the code inside the existing 18M2 driver PICAXE on the daughter card! No RS232 needed.

My most significant concern about using the PICAXE as a divide-by-2 device is that to keep from starving the mechanical tach, you will need a tight loop which essentially makes the chip dedicated. You could use interrupts to monitor the ignition coil pulses, but the simple thing to do is to follow the previous consensus and use an external FF to divide.

With the mechanical tach handled by a flip-flop, the 133Y display is straight-forward using count (or pulsin if inclined). Calculate the pulses in a second, multiply by 60 for RPM, and feed the display.

-Ray

samis_p said:

From what I've seen it looks like the tacho signal comes from each ignition coil - it's tied high (at 13V) and goes to 0V each time the coil gets grounded to fire the spark-plug. The width of that pulse is the same no matter the engine speed. I will upload a screenshot later to explain it better. I was thinking of using a picaxe because I'm fairly new to electronics and I've only done a few things with them. Using a flip-flop seems to be a pretty easy solution, I'll have to read on about it.
Here's a screenshot of the wave at idle (~550rpm)
View attachment 11827
The part it goes to 0V is the same width (~5.5ms) at any engine speed (though I didn't go above 4k RPM with the scope attached). Anyway, I doubt there's anything difficult here, as it looks quite simple. I've ordered a couple of CD4013 D-Type flip-flops to try out, hopefully I'll receive them early next week. In the meantime I'll try to make a digital tacho to work with the AXE133Y screen. Thank you all for the input, but I'm always looking for any advice on this.

Click to expand...

Normally kettering ignition works where the coil is disconnected from ground to fire the plug. CDI works differently again.

Hi

from Dippy

Sorry to go 'non micro' but a flip-flop with buffering before and after is an easy way to divide by 2.

Click to expand...

I agree with Dippy that a flipflop would be the simplest way to divide the incomming pulses by 2




Bill

mrburnette said:

@samis_p:

What is the final goal? Use the analog tach, build a replacement digital with the 133Y, or run both tachs?

Most analog tachs are integrators, they use RC components to drive a meter - increase RPM means increased voltage on the capacitor and a larger meter deflection. More expensive tachs use some active circuitry to assist with linearity, etc.

The AXE133y is straight forward ... you could write all the code inside the existing 18M2 driver PICAXE on the daughter card! No RS232 needed.

My most significant concern about using the PICAXE as a divide-by-2 device is that to keep from starving the mechanical tach, you will need a tight loop which essentially makes the chip dedicated. You could use interrupts to monitor the ignition coil pulses, but the simple thing to do is to follow the previous consensus and use an external FF to divide.

With the mechanical tach handled by a flip-flop, the 133Y display is straight-forward using count (or pulsin if inclined). Calculate the pulses in a second, multiply by 60 for RPM, and feed the display.

-Ray

Click to expand...

My final goal is to run the stock tacho and use axe133y as a separate display for whatever else comes to my mind (coolant/oil temperatures etc). I've already made a tacho on my bench using one picaxe to generate pulses and a second one to count them and send to axe133y. Might not be the cleanest system, but I didn't want to touch anything on the 18m2.
This engine has a coil-on-plug system, so the signal might come from somewhere else. But it looks like it's an ECU control signal for the ignition and not from a hall-effect sensor or something like that.
As for the 133y - counting pulses in a second will give a very sluggish response, my last test was counting for 333ms at low engine speeds and for shorter periods at higher engine speeds to have quicker response on screen and better resolution at lower speeds (counting pulses for a very short time needs a very high multiplier to calculate the RPM).
I'm waiting for my flip-flops to arrive monday or tuesday - I think that it will be the simplest way AND will react faster to changes in engine speed.

Bill,
how will I know that I need to change those two 1n caps? Can I replace the BC337 with a 2N6488 (I have these at home)?

samis_p said:

From what I've seen it looks like the tacho signal comes from each ignition coil
-Here's a screenshot of the wave at idle (~550rpm) The part it goes to 0V is the same width (~5.5ms) at any engine speed (though I didn't go above 4k RPM with the scope attached).

Click to expand...

Hi,

That waveform looks too "clean" to be coming "directly" from the ignition coil(s).

The waveform appears to have a peroiod of about 23ms, which is indeed consistent with an 8-cylinder (4-stroke) engine running at about 650 rpm. But if the pulse width is a constant 5.5ms then what happens at about 23 / 5.5 * 650 rpm (say 2700 rpm?) since the duty cycle would top out at 100% ?

One of my first electronics projects, almost 50 years ago, was an electronic Rev Counter using a simple "monostable" flip-flop (no ics like 555s in those days). With a stabilised voltage rail, the pulses are integrated within a moving-coil meter to give a perfectly linear range.

If your Rev Counter works that way (i.e. the waveform shown is applied directly to a meter) then using a flip-flop to divide by two will NOT work, because the duty cycle will be a constant 50%. It might be "fixed" with an AND or OR gate (depending on the polarity of the logic) but something is likely to "go wrong" approaching 2700 rpm. Instead you probably need to narrow the pulse by 50% (which might be done with a 555 timer or PICaxe) providing that your drive waveform edges don't "vanish" at higher Revs.

So I think you need to make some more measurements, in particular what happens to the "square" waveform as the engine speed rises from 2500 through 3000 rpm?

Cheers, Alan.

AllyCat said:

Hi,

That waveform looks too "clean" to be coming "directly" from the ignition coil(s).

The waveform appears to have a peroiod of about 23ms, which is indeed consistent with an 8-cylinder (4-stroke) engine running at about 650 rpm. But if the pulse width is a constant 5.5ms then what happens at about 23 / 5.5 * 650 rpm (say 2700 rpm?) since the duty cycle would top out at 100% ?

One of my first electronics projects, almost 50 years ago, was an electronic Rev Counter using a simple "monostable" flip-flop (no ics like 555s in those days). With a stabilised voltage rail, the pulses are integrated within a moving-coil meter to give a perfectly linear range.

If your Rev Counter works that way (i.e. the waveform shown is applied directly to a meter) then using a flip-flop to divide by two will NOT work, because the duty cycle will be a constant 50%. It might be "fixed" with an AND or OR gate (depending on the polarity of the logic) but something is likely to "go wrong" approaching 2700 rpm. Instead you probably need to narrow the pulse by 50% (which might be done with a 555 timer or PICaxe) providing that your drive waveform edges don't "vanish" at higher Revs.

So I think you need to make some more measurements, in particular what happens to the "square" waveform as the engine speed rises from 2500 through 3000 rpm?

Cheers, Alan.

Click to expand...

What I meant is that this signal comes from the ECU, but is used to control the ignition coils.
I was thinking about that width too, the only logical explanation is that I didn't pay enough attention and didn't see the 0V pulse width change slightly with higher RPM.
I've read on this further (on various bmw forums) and most people recommend using a frequency divider to fix this. Since converters of this type cost between 50-100$, a DIY approach is not only beneficial for the mind, but also for the wallet.
Hopefully I can answer this tomorrow, as I've managed to get my hands on a CD4013 earlier than expected.

Hi

Bill,
how will I know that I need to change those two 1n caps? Can I replace the BC337 with a 2N6488 (I have these at home)?

Click to expand...

Any NPN transistor with suitable current handling will do.

The capacitors are for filtering any spikes. Testing with the CRO
is the easiest way to determine if the value is correct.
the 1nf should allow square wave frequency upto several kHz.

Bill

Well, I got everything set on a breadboard according to Bill's schematics and it's giving me ~14.4V DC on one output. I've checked everything and it looks okay, there's not much to mix up. Is it possible I've somehow damaged the IC?

Hi,

It might be damaged, but it's just as likely that the flip-flop is not being triggered. In that case there's a 50% chance that the output will be "high" (a power supply rail of 14.4 volts is quite typical for a "12 volt" battery being charged from an alternator) and the other output should be low.

That input circuit is really intended to filter the large (hundreds of volts) spike which appears on the "points" (and coil) of an "old" (non-electronic ignition) engine. Try disconnecting one or both of the 1nF capacitors, and preferably add a decoupling capacitor (perhaps 1uF - 10uF) onto the supply rail.

Cheers, Alan.

AllyCat said:

Hi,

It might be damaged, but it's just as likely that the flip-flop is not being triggered. In that case there's a 50% chance that the output will be "high" (a power supply rail of 14.4 volts is quite typical for a "12 volt" battery being charged from an alternator) and the other output should be low.

That input circuit is really intended to filter the large (hundreds of volts) spike which appears on the "points" (and coil) of an "old" (non-electronic ignition) engine. Try disconnecting one or both of the 1nF capacitors, and preferably add a decoupling capacitor (perhaps 1uF - 10uF) onto the supply rail.

Cheers, Alan.

Click to expand...

Thanks, that helped! I added a 100uF cap (the only ones I had here in garage) to the supply rail and removed those 1n caps. It looks like it's working well - RPM readouts are pretty close to what I would expect to see (idle is at around 500RPM, which is close to what it should be). The only thing that bugs me is that the pulse width is a bit too short:

you can see in the screenshot, that the pulse stays quite a long time "low", compared to the original pulse widths. I'd like it to be like I marked it with the red line, but it's probably impossible with just a flip flop? I'm not sure if this is causing problems, although the tacho seems a bit "lazy".
EDIT: It worked for a while and now I'm getting 14.4V and 0V on Q and Q- respectively. Checked for any loose components, but everything seems to be in order.
I'm using 24k resistors instead of 22k, could that cause a problem? Should I only leave 24k and 1M resistors on the input? I'll also try a new IC tomorrow, this one might have suffered a bit while I was trying to locate the problem.

Hi,

Yes, the FF is (or was) correctly dividing the number of edges by two (it triggers on the positive-going input edge, the negative-going is irrelevant). Generating a 50% duty cycle is ALL that a FF can do (in this type of circuit). The assumption (when using a FF) is that whatever circuit follows the divider is unaffected by the pulse width (so your red waveform would/should produce exactly the same indication). If this is not the case then you will need to use a "monostable" element (555 timer or PICaxe, etc.) to drive the tachometer, as I mentioned in #11 above.

No, those slightly different resistor values are very unlilely to be the issue.

Cheers, Alan.

Sorry.. a Little comedy relief when I saw the title of this thread.

Impulse Reduction :- Hiding the wife's credit card.

Dave

Marcwolf said:

Sorry.. a Little comedy relief when I saw the title of this thread.

Impulse Reduction :- Hiding the wife's credit card.

Dave

Click to expand...

Heh
It was late when I wrote the title - my head didn't work right. Now I can't find a way to fix it.

samis_p said:

Now I can't find a way to fix it.

Click to expand...

You can't, but I can and have done so. Shout if you don't approve

Hi,

IMHO an "ideal" title would include a reference to being for a "Tachometer", "Rev Counter" or perhaps "Car Ignition".

However, at the moment, I'm still doubtful whether the "division by two" needs to be of the number of pulses or the width of each pulse. These potenatially need rather different circuits, although of course a PICaxe can do either.

Cheers, Alan.

Last change made to title

AllyCat said:

Hi,

IMHO an "ideal" title would include a reference to being for a "Tachometer", "Rev Counter" or perhaps "Car Ignition".

However, at the moment, I'm still doubtful whether the "division by two" needs to be of the number of pulses or the width of each pulse. These potenatially need rather different circuits, although of course a PICaxe can do either.

Cheers, Alan.

Click to expand...

Thank you Martin for fixing the title - it's much better than what I had written before. I got new ICs, plopped one into the breadboard and still it looked that there's no trigger. After rechecking everything it worked, so I soldered everything on a stripboard and there's no trigger again. Q is low and Q- is high. Everything seems to be in order - what else could be causing this? Also, according to Bill's schematic, pin6 should be tied high, yet everywhere else I read that it should be left low, just like pin4. Currently it's tied low, and it worked with the pin tied low, although I've tried both positions... Which way is correct?
By the way - it seems that pulse width doesn't play a big role here , as I've counted the number of pulses vs the rpm indicated and it all falls together nicely. It may be a bit more lazy, but it might be just me - it used to show 2x the rpm, so of course the needle would move faster.
Edit: I can see, that the signal reaches pin3, yet for some reason just does not trigger anything. Could the voltage be too low? Supply voltage is 14.4V and signal voltage is ~12.4V.

According to the datasheet truth table, set and reset should both be tied low.
Not Q is tied directly to Data. Signal input to Clock.

If you had it working at one point then re-check your connections & solder joints.
Make sure you have good grounds where needed.

With a 14.4 volt supply, a 12.4v input signal is more than sufficient to toggle the flip flop.

Goeytex said:

According to the datasheet truth table, set and reset should both be tied low.
Not Q is tied directly to Data. Signal input to Clock.

If you had it working at one point then re-check your connections & solder joints make sure you have good grounds where needed.

With a 14.4 volt supply, a 12.4v input signal is more than sufficient to toggle the flip flop.

Click to expand...

I currently have a 24k resistor on the CLK input and a 1M as a pull-down. Could it be that 1M is too much resistance? I've rechecked the solder joints, fixed a few, but it looks to me that something is wrong with the CLK input - either the pulldown is too high, or something else. I only have a limited selection of resistors here, though. Some 10k, 24k, 500r and a couple of 1k...
What would be the correct way here? I've read on some other forum, that a 10k pull-down is okay, but there's a huge drop in voltage on CLK....

Could it be that 1M is too much resistance?

Click to expand...

Not likely, but 10K will not work because it forms part of a voltage divider with the 24K input resistor. The signal voltage will then be too low ..... unless you change the supply voltage to the Flip flop to be about 5v.

A 100K would work ok but, will probably not solve your problem, which I am guessing is a poor ground on either set or reset or on the gnd pin of the 4013 itself.
Unless ... the 4013 has been blown by transients on the 14.4 power source. Older CD4000 series device can work up to 15V and the Texas Instruments series is good up to 18v. All bets are off on cheap Asian Knockoffs.

Goeytex said:

Not likely, but 10K will not work because it forms part of a voltage divider with the 24K input resistor. The signal voltage will then be too low ..... unless you change the supply voltage to the Flip flop to be about 5v.

A 100K would work ok but, will probably not solve your problem, which I am guessing is a poor ground on either set or reset or on the gnd pin of the 4013 itself.
Unless ... the 4013 has been blown by transients on the 14.4 power source. Older CD4000 series device can work up to 15V and the Texas Instruments series is good up to 18v. All bets are off on cheap Asian Knockoffs.

Click to expand...

I checked grounds - all of them should be okay, my multimeter is showing 0 ohm between all of the pins that are grounded and the vehicle ground. I've lost all hope of making this work - everything is as it should be, it must be my rotten luck. I've tried 4 ICs, the fifth one I tried had a resistor on its supply rail just to see what that does - I could see some voltage spikes before the resistor, but everything after it looked much smoother and cleaner. That didn't work either. All of the FFs don't react to any voltage changes on CLK input, yet they do react if SET pin goes high. They also output some "random" looking square waves if I change the pull down to something smaller, like 10k and just connect the signal directly. If the pull-down is anything higher - no reaction. This didn't work from the beginning, and when it worked it was some sort of a miracle. I must say, though, that when it worked, I tried with that chip for several hours and it just started working. It stopped working randomly. The second time I managed to get this to work everything was fine on the breadboard, but then it didn't work when i soldered everything on the stripboard. It might be that these chips are very sensitive, or it might be something else.
Is there an easy way to check if the FF is still working? If pins 4,6 and 7 are grounded, pin 14 is connected to the supply, pin 5 to pin, I should see a change on both pin 1 and 2 if I apply voltage to pin3?
Checked the FF on a breadboard and it does switch states on both Q and Q- when I apply voltage to Pin3. It looks like there's something wrong with the CLK signal from the ECU - I've replaced the 1M pull-down with a 76k and now I'm getting a random looking output - upon looking closer with the scope, it seems to also pick up the noise and react to it. If I use 1M as a pull-down and 24k in series with the signal, then it just does not react to it. I might need to somehow find the right resistor values. Maybe use 1n caps?
EDIT: Success! Left the 76k as a pull-down, fed the signal without any resistors and used a 1nF cap. It's giving me the result I needed. Now I need to see if this will work on the stripboard prototype, as I still have 1M as a pull-down there and a 24k in series with the CLK input.

Hi,

Yes, the set/reset pins 4 and 6 must be low, I guess the designer got confused with the supply and ground pins (not shown). The "pull-down" resistor is not necessary, but must not be less than say 50k (with the 24k input resistor).

Risetime of the input signal might be an issue, but I think we're probably now looking for a damaged (or mis-connected) ic, or poor decoupling or "invisible" spikes on the trigger signal (or elsewhere).

The clock is edge-triggered so you must take the pin low then high for testing. Perhaps the most important part (for it to divide properly) is that the D input (5) must be connected to Qbar (2).

Cheers, Alan.

Attached is a diagram of how I would do it. The Bat 85 Diodes act as clamps for protection against both positive and negative voltage spikes. 1N4148 diodes could be used but will not be as effective.

With any Automotive circuit it is always a good practice to protect digital components with good regulation and protection against transients. Connecting a CMOS device ( 4013) directly to the battery is asking for trouble.

In my diagram I show a 12 V supply to the 4013. It is assumed that this is regulated and protected against transients.

Goey

I finally got it to work. What I have now is a 1M pull-down and 1nF on the CLK pin. Signal is connected directly to the CLK. Without the 1nF I get pretty much random output, it seems to pick up on noise. The thing is that if I use ANY resistor in series with the signal - I get no trigger or random triggering.
So, now it sort of works. What I can't figure out is why it's missing some pulses (screenshot).

The tacho works, it shows RPM correctly. But the needle gets jumpy when it misses that impulse. Also the fuel consumtion gauge and water temp gauge both "jump" when that pulse is missing. So now I need to clean it up a bit. Would another 1nF cap help? I'm pretty much out of ideas, as I'm used to working with 5V regulated power supplies and had no idea that so little noise could mess up things this much. I'm also "self-taught" by just trying things, so I'm missing a lot of knowledge, no matter how basic it might look.

UPDATE: After adding a 0.1uF decoupling capacitor things changed. A lot. Don't know if it's all good though. It seems to register 99.9999% of pulses, but now the wave is not so square.

I'm trying to work with what I have here now, so I can't get any diodes or anything else at the moment.

There should NEVER be a missing pulse. NEVER .

The thing is that if I use ANY resistor in series with the signal - I get no trigger or random triggering.

Click to expand...

1. Where EXACTLY are you getting the input signal from ? (A diagram is better than 1000 Words)

It sounds like this is a very weak (high impedance?) signal that is easily attenuated.

2. Will this signal directly drive the tachometer without a glitch ?

3. Do you have a 100nf decoupling capacitor across 4013 Pins 7 & 14. Yes or no ?

Isn't it amazing how a decoupling capacitor makes things work? ......
You might also add a 10uf Capacitor in parallel to the 100nf to act as a filter.

The Q output of the 4013 should be pretty square. The input does not need to be perfectly square and likely will not be. I cannot tell from your scope shot which is which.

If the Q output is not square then tie a 22k from Q to ground and read it again. Is it better ?

A CD4013 cannot sink or source much current. You may want/need to use the transistor shown in Bill.b's diagram. I would suggest a base resistor of 10K and a collector resistor of 1 K. Tie the collector to the tach input.

How about this.

Mount a magnet in the cars fly wheel and pick up pulses via a sensor. If you need more pulses fit more magnets. You could then run this to your taco.

Goeytex said:

There should NEVER be a missing pulse. NEVER .



1. Where EXACTLY are you getting the input signal from ? (A diagram is better than 1000 Words)

It sounds like this is a very weak (high impedance?) signal that is easily attenuated.

2. Will this signal directly drive the tachometer without a glitch ?

3. Do you have a 100nf decoupling capacitor across 4013 Pins 7 & 14. Yes or no ?

Absolutely try adding another capacitor or even 3 or 4

Click to expand...

Okay, first of all - I'm a complete idiot (or maybe lack of food today made me one) - that voltage drop was because I forgot to reconnect the power supply to Pin14 after adding a 100nF cap.
1. Can't really get a schematic, but this signal goes to the cluster and is used to drive the tacho (somehow). It's a tachometer signal cable and is referenced as one in the manual. If I connect it back the tacho works, showing 2x the RPM.

2. Yes.

3. I have the cap between pin14 and ground. Since pin7 is grounded anyway, it was closer for me to place that 100nF cap between pin14 and ground. Is that correct, or MUST it be between pin7 and pin14 (even though pin7 is essentialy ground)?

What I have done now is reconnected the power supply and tested - still a few missing pulses. I added a second 1nF cap between pin3 and gnd and now it looks like to be correct. I'll see how it all goes later on.

This is how things are (please excuse the MSPaint-modified schematic):

samis_p said:

3. I have the cap between pin14 and ground. Since pin7 is grounded anyway, it was closer for me to place that 100nF cap between pin14 and ground. Is that correct, or MUST it be between pin7 and pin14 (even though pin7 is essentialy ground)?

Click to expand...

It is MUCH better if the decoupling cap is directly across pins 7 & 14, ideally soldered directly to the pins.

As Goeytex says, a properly selected and fitted decoupling cap can make a HUGE difference.

The effects of noise, both external and from within the the chip, are very hard to trace, and a good decoupler solves a lot of headaches.

The 1nf capacitors are not doing much without at least some series resistance. The whole idea with the caps is to create an RC Filter. Try a 1K between the signal and the capacitors.

So is the "12v" on your schematic just the vehicle's 12v battery supply - the one that can have 4v dropouts/up to 60v or so transients on it? That could be bad news....both for instantaneous behaviour and long term survivability of your components

Goeytex said:

The 1nf capacitors are not doing much without at least some series resistance. The whole idea with the caps is to create an RC Filter. Try a 1K between the signal and the capacitors.

Click to expand...

Added a 1.5k between the signal and caps - everything still works.
I can't place the decoupling cap in any other position, because the one I'm using is salvaged from another project and the leads are too short
But the main thing is that everything is working, at least for now. I might replace the cap tomorrow, if I get the time to drive to the shops. Hopefully I won't have to resurrect this thread with any more problems.
Thank you guys for all the help - couldn't have done it without you. And it looked like such a simple task...

MartinM57 said:

So is the "12v" on your schematic just the vehicle's 12v battery supply - the one that can have 4v dropouts/up to 60v or so transients on it? That could be bad news....both for instantaneous behaviour and long term survivability of your components

Click to expand...

Yes, it is the battery, pretty much... I'll try to get my hands on a low-dropout 12V regulator and install the diodes shown on Goeytex's original schematic. I hope that will be enough to make the supply clean enough for all the components?

Goeytex said:

Isn't it amazing how a decoupling capacitor makes things work? ......
You might also add a 10uf Capacitor in parallel to the 100nf to act as a filter.

The Q output of the 4013 should be pretty square. The input does not need to be perfectly square and likely will not be. I cannot tell from your scope shot which is which.

If the Q output is not square then tie a 22k from Q to ground and read it again. Is it better ?

A CD4013 cannot sink or source much current. You may want/need to use the transistor shown in Bill.b's diagram. I would suggest a base resistor of 10K and a collector resistor of 1 K. Tie the collector to the tach input.

Click to expand...

The 10uf cap should be bipolar, correct?
I still can't pick up when I should use electrolytic caps. Probably NOT to filter things, but just as a local storage?

hey guys, just signed up as this is exactly what i need to sort out my tacho, i bought a cheap chinese digital dash and the tach and speedo are reading x2 what they should. i know very little about this sort of thing so brought the schematics down to maplins to get the components and the guy seemed to know less than me so i came away with no components, i already have the flip flop but the tach reads all over the shop, between 1000 and 4000rpm, im guessing this because of spikes or noise so i need the caps or diodes, but what would be best and where in the uk can i get these parts, thanks.

- how do you know its x2 if reading "between 1000 and 4000rpm"? ""im guessing this because of spikes or noise..." or the fact that it might be crap. you need to stop guessing and check/process the incoming signals or you could easily damage something. "uk can i get these parts" have you looked in the shop of the website you're looking at right now i.e. rev-ed shop "buy now" tab ansearch on diode etc.. or try rapid electronics, rs components, farnell and so many more if you look around.have you got a scope.where is the signal coming from, what is the signal level, digital or analogue, what is your skill level, what circuit have you used with your flip-flop. did you just cut'n'paste someones design. where does picaxe come into it.post a schematic of what you have done so far to give people a clue and a palce to start helping you. you need to tell us more and preferably put some spacing and punctuation in your posts to make them more readable so that people will read them and then help you.