Need to multiply rev counter signal ?

Hello all,

I'm after advice for a simple frequency multiplier, I need to increase the rev counter signal on my car by 1.5 i.e. if revs are 2000 I need a signal of

Its to give the correct rev signal to a LPG system I have fitted but for some reason it only reads 2/3 of the rev's. It may be something to do with the fact that its a V6 working on a lost spark system. I.e. the plugs fire every rev not just on the compression stroke.

TIA.

George.

Thanks for the quick reply,

Its digital, and possibly a very spiky pulse. It is a wide ranging input, in that it can get its signal from the HT coil primary or a wire wrapped around one HT lead or the TDC sensor or the feed to the taco gauge.

George.

I'd say you have at least 3 alternatives to solving your problem :

2)Easier way. There is probably a microcontroller that is counting the number of revs from the engine shaft. You'll have to check for a DAC chip in your tach that changes those engine pulses to an analog voltage. They normally have an output pin where you can connect a pot to adjust the output voltages. If the pot is already there everything is simpler, so you'll avoid all the hassle of building up a seperate circuit to get the job done, which would in any case be a simple circuit. You might need the DACs datasheet to get that done.

theJackal

--- I'm confused. If you have so many input types available it seems like there must be at least _one_ available which would fill the bill. Does the possibility exist that you could change from the signal source you're now using to another one which might work right off the bat?

For example, the output from the TDC sensor gives you just one pulse per revolution, which the interface might like a lot.

If you can't do it by selecting the signal source, then I can think of two methods that might work, the first being to use your current stream of pulses to fire a non-retriggerable one-shot with a small, fixed period, then integrate those pulses and use the resultant DC to drive a VCO which would, in turn, drive the interface.

The other way would be to use the input spikes to drive a non-retriggerable one shot with a period of about half the time between two input spikes at the highest speed at which the system is supposed to work. Then use an EXOR with a delay on one of its inputs to double the output frequency of the one shot, and a circuit to remove every fourth EXOR output pulse. That way, for every 2 spikes in you'll get 3 EXOR output pulses out, which is what you want, and if the interface is just counting pulses and time and isn't worried about symmetry, then it'll probably work.

If you can't make it work by using another input source, and you need a schematic, let me know, here, and I'll put something together for you.

-- John Fields Professional Circuit Designer

This sounds like a job for a CD4046.

You divide the frequency by 2 to feed one input of the phase detector-2 and the VCO by 3 to feed the other. Since an engine can't change RPM too fast, it would be ok, I think.

Better yet, times three then divide by 2.

Sounds good, will this chip need many discreet components? as you can guess electronic design is not my profession.

I'm a HVAC controls man.

George.

Go find at least a hobbiest electronics guy and suggest this to him:

----------- ! ! 4046 part 4046 part ! ----- ! ------- ----- -!D Q! ! in ---------! PD II !---/\\/\\/---+----! VCO !-------!> ! ! ------- ! ----- ! Q/!-+--+-- Out ! --- R1 R2 C1 ----- ! ! --- ! ! ! C2 ! ! GND ! ! ! ! ---------- ! --------------! Div by 3 !---------------- ----------

Both C1 and C2 will have to be film capacitors. The resistors can be done from the 4046's data sheet.

Oooops yes!