Inverting Tacho output - now for the next electrical question

[Grega]

Folks

I've got a positive tacho output from a DFI module but have a negative pulse tacho (input)

How can i swap the pulses over??? ie: positive output to negative output?

I can't use a standard relay as it won't work fast enough ; i need a switching circuit to do this electronically for me.

Any ideas?

[madmont]

Someone who knows something about transistors should be able to do it easy.

[river]

Hi,

I assume it's a 0-12V waveform? Does it need to supply any current, or just a pulse?

If a pulse, and you don't need much current (ie only voltage) then you can use a CMOS 4069 inverting buffer.

seeyuzz
river

[Grega]

mont. ta
river. i think its just a pulse ; but ; don't have access to a CRO to check. WTF does a CMOS 4069 inverting buffer actually? by virtue of its name i take it ; it takes the signal and "inverts" it? remember the speed at which this thing needs to work is paramount (ie: for a tacho)

[Mos]

Something doesn't sound right - the output should still be a square wave... Whether it's positive or negative would probably only mean the length of the pulse at either voltage.

What tacho are you using? Toyota tachos trigger off a positive spike. I'd hazard a guess that aftermarket tachos will trigger off anything.
If it only gives you 12V that could be why your tacho isn't triggering (but I guess you've already explored that option?).

BTW, auto signals are very slow electronically - remember this is slower than most audio - so even a very slow transistor circuit is more than sufficient - a CMOS inverter is overkill for the application. Nothing paramount about it

Mos.

PS. please be a bit more reasonable with topic titles...

[Grega]

Mos. Thanks. I thought Inverting tacho output would have been pretty self explanatory, but, maybe not.

OK, its basically a VH Commodore tacho - which gets its signal from the - side of the coil by default factory. The DFI module I want to use is a positive output for tacho signal from what I can see.......

I've no real electronics expertise in CMOS and circuits ect ; so ; I'm kinda a noob in that regard.

[RA35GT]

The commodore would is expecting a voltage spike.

Have a look in here mate, there is a circuit you can build to emulate the coil- signal.

http://toymods.net/forums/showthread.php?t=3710

Rudi

[Mos]

Mos. Thanks. I thought Inverting tacho output would have been pretty self explanatory, but, maybe not.

An inverter is self explanatory, but as I have learnt over the years often one is in a better position to help when one understands the complete situation rather than the specific someone is asking, specifically, I don't think you need an inverter - see below...

OK, its basically a VH Commodore tacho - which gets its signal from the - side of the coil by default factory. The DFI module I want to use is a positive output for tacho signal from what I can see.......

Signals off the -ve side of the coil are generally positive spikes and not negative. The Tachos generally trigger off the flyback voltage generated on the primary of the coil which will be positive above 12V, rather than negative wrt 12V.
If this is the case here, you are likely to need (as pointed out by others) a booster (to raise the 12V spike to something like 40V) rather than an inverter.

Mos.

[Grega]

An inverter is self explanatory, but as I have learnt over the years often one is in a better position to help when one understands the complete situation rather than the specific someone is asking, specifically, I don't think you need an inverter - see below...

very very very good point. thanks mos
7mgte thanks mate for that diag.

[The Witzl]

I'd plus rep you Mos for the excellent response WRT getting people to explain the full situation rather than a specific question....

.... but apparently i've loved you too much.