Post by thread:Car tacho / Lawn mower tach

Michael Hagberg wrote: > would this circuit work for a lawn mower tach, could the pick up be just a > piece of wire wrapped around the spark plug wire a few times? I apologize if I have posted this advice before. I know I have posted it to a couple of newgroups. If you use inductive or capacitive coupling to the spark plug wire you will meet a few problems. One, of course, is that you will be hooking your PIC up to a HV circuit. Things happen. Also, my experience is that the waveform seen on a spark plug wire is a dampened oscillation and you will need to deal with more than one "hit" per spark (and probably more than one spark per revolution, no?). For my money, the use of a hall effect detector is the best way to go. It is small, cheap, robust, can be used up to 35V (check the spec sheet for your brand!), can handle over 100K hits/sec, is solid-state, and the output signal's amplitude is not dependant on frequency. While not as cheap as a piece of wire, I think it will pay for itself in savings on asprins d8^) Just my opinion. YMMV. Michael REMOVE THE .NS (NO SPAM) FROM MY ADDRESS TO REPLY ***************************************** The strong do what they have to do and the weak accept what they have to accept. --Thucydides

I made a sensor for an ignition timing strobe by just using an NPN transistor with its base lead open circuit and about 25mm of wire soldered to it as a kind of antenna. The emitter was grounded and a 470R resistor connected to the 12V rail from the collector. Output is a negative going pulse at the collector which may need debouncing by the PIC. I didn't look at the waveform with a scope but it works fine for me when I put the transistor, which is at the end of a piece of twin screened cable, near the HT spark plug wire. It doesn't have to touch and can be well insulated. I don't know if the transistor will suffer in the long term from the high voltage fields, but it has worked well whenever I have got the strobe out to check my bike or cars timing. Transistor was a 2N3904 out of my junk box. May be worth a try? Regards, Gerry Cox ---------- From: Michael Ghormley To: .....PICLISTKILLspam@spam@MITVMA.MIT.EDU Subject: Re: Car tacho / Lawn mower tach Date: 03 April 1998 09:03 Michael Hagberg wrote: > would this circuit work for a lawn mower tach, could the pick up be just a > piece of wire wrapped around the spark plug wire a few times? I apologize if I have posted this advice before. I know I have posted it to a couple of newgroups. If you use inductive or capacitive coupling to the spark plug wire you will meet a few problems. One, of course, is that you will be hooking your PIC up to a HV circuit. Things happen. Also, my experience is that the waveform seen on a spark plug wire is a dampened oscillation and you will need to deal with more than one "hit" per spark (and probably more than one spark per revolution, no?). For my money, the use of a hall effect detector is the best way to go. It is small, cheap, robust, can be used up to 35V (check the spec sheet for your brand!), can handle over 100K hits/sec, is solid-state, and the output signal's amplitude is not dependant on frequency. While not as cheap as a piece of wire, I think it will pay for itself in savings on asprins d8^) Just my opinion. YMMV. Michael