LM2907N F to V converter: input?

[stef]

This is yet another tachometer circuit :roll:

I'm using the LM2907N (14 pin; uncomitted tach inputs).
The signal is derived from a normal piece of panel wire cable-tied onto the spark plug lead on my kart engine. I get a very strong signal on the scope (I even tried it on my petrol trimmer, works like a charm).

The spikes measured as follows:
Noise = 4Vp-p (probe setting= x1) Pulses = 40Vp-p (probe setting= x10)

Obviously the signal wire should be hooked up to a voltage divider with say a 1:20 ratio to get the voltage down & to get rid of the noise.I will also ref the inverted input @ 0.5*Vcc, in other words, a open loop comparitor.

:?: Should I just wack a resistor in series with the non-inverted input to limit the current when the signal swings below ground (as per data sheet) or should I use a diode with the anode connected to Gnd to short out negative signal swing?

PS sorry for the lack of a schematic

 

[TillEulenspiegel]

I have seen this question many times ( with different devices) . I'm still quizzical as to why anyone would want to use the HT side of a coil. You start talking about direct coupling of a ignition coil, your asking for trouble. Your observed noise picture is deceptive because the discharge generates lots of harmonics and digital electronics dislike wildly varying signals at different freqs. Kind of an AF-RF white noise . Just take the input side of the coil/mag and use it to spec . 1 pull up resistor and your set.


Edit to add: If You insist on the HT path many amplifier repair parts suppliers make an opto isolator for old Fender amps in the tremolo circuit. Input side is a neon bulb, output side is a fast photo resistor. Or make one your self, a neon and a PR joined with black shrink wrap spaghetti

 

[stef]

The HT side just makes things easier. The signal wire is not electrically connected to the HT wire(I'm sure you gathered as much already).

The commercial karting tach's available use the same principle; it makes it easy to fit on any type engine without tapping into harnesses or knowing any specs on the various ignition sytems.

My Rotax has an electronically controlled ignition system with a sensor on the crankcase; I wanted to tap the signal there, but I'm not quite sure what sensor it is and how it interfaces with the rest of the circuit (which is is enclosed in a sealed unit with the coil). Don't really want to mess it up, it is quite expensive and hard to come by.

Tx for the isolator tip. The pulse spikes are very distinctive; the harmonics part of the spike is very low amplitude wise; if I make the ref Voltage larger than the harmonic it should solve the problem right?

cheers

 

[TillEulenspiegel]

Oh ya, your talking orders of magnitude of the different sigs. One is KV (signal)and V(noise) the other is mV and uV ( in terms of the Input side of the opto VS the output side ) the ole S/N ratio kicks in and the noise is negligible