As noted in another reply, a diode may well not be necessary. However, if your circuit needs one, consider using a Schottky diode. Vfwd is small (similar to germanium). Beware their typically low reverse voltage (see relevant spec sheet) - excess reverse voltage mya destroy them depending on the circuit. Also, you say that >My goal is to produce an interrupt > at the PIC when a tooth aligns with the center of the pickup. It must also be > highly immune to emf noise.you wish the trigger point to occur Unless you need this point because it corresponds to some maximum energy condition (as might be the case with eg a magneto circuit (which is unlikely to be what you are using)) you should be able to have the trigger point occur at any position relative to the centre of the tooth, providing it did so reliably and consistently. This would free you up from having to force the circuit to work at a point which may not be the most obvious one. All distributors that I have seen (and I certainly haven't seen them all) give you near total control over the relative position of the firing point and the actual crankshaft position. Hopefully you will be able to alter the distributor position as required. I haven't followed all this thread - it has probably been pointed out that if you apply a voltage from a coil across a resistor across the inputs of a comparator (with suitable attention to device limits, a few resistors will be needed), then the comparator will switch VERY close to the zero crossing point. ---------- > From: Bruce Turrentine > To: PICLIST@MITVMA.MIT.EDU > Subject: Zero Crossing Detector Reposted > Date: Sunday, 28 June 1998 02:34 > > Thank you all for the valuable advise on my project. I think that there are a > few points that I should clarify. I am a hobbyist at electronics, a middle > school teacher by trade. I only took 2 semesters of electronics in college, so > when someone begins to argue about theorems and laws, I'm simply lost. > My goal is to develop an ignition system for my old truck that incorporates a > PIC controlling the timing curve and coil charging.I am aware that this > project has been done several times over, but the learning experience of > building from scratch is what I'm interested in. > > Clarifying a few points about the pickup and zero crossing. > The variable reluctance pickup produces a sine wave pulse every time a tooth > on the distributor shaft rotates in and out of the pickup's magnetic field. > There are four teeth spaced 90 deg apart. I get 2 pulses per rev of the crank > and 4 per rev on the distributor. The frequency and P to P voltage of the > pulse increases proportionally with an increase in crankshaft. The pulse goes > high as the tooth approaches, falls to zero at the center and goes negative > until the tooth leaves the magnetic field. The output V at cranking rpm is > about 2v P to P and about 12V at redline. My goal is to produce an interrupt > at the PIC when a tooth aligns with the center of the pickup. It must also be > highly immune to emf noise. > I'm considering simply running the wave through a germanium diode rectifier > and watch the trailing edge of the first pulse. When the wave falls, the slope > is nearly vertical, getting me close to the zero crossing. > > What does everybody think? > Bruce