Post by thread:[EE]: how to extract rpm signal from car's dc outl

Hi all, I want to build an rpm meter for my car with an acoustical rev limit signal. I have heard that you could quite easily extract the engine's rpm from the AC component riding on the DC of the cigarette lighter plug/outlet. As a matter of fact, there are some commercially available aftermarket in-car display units which can do the same from the DC outlet. However, these units are too extensive for my application. Has anybody have any experience with this kind of application? Is it just a matter of AC coupling the car outlet's ripple to some kind of comparator before feeding it to the PIC for further processing? Or is there a more advanced technique necessary like an FFT? Thanks! Daniel... -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email spam_OUTlistservTakeThisOuTmitvma.mit.edu with SET PICList DIGEST in the body

{Quote hidden}> From: D. Schouten[SMTP:.....d.schoutenKILLspam@spam@QUICKNET.NL] > Sent: Wednesday, May 19, 2004 3:59 PM > To: PICLISTKILLspamMITVMA.MIT.EDU > Subject: [EE]: how to extract rpm signal from car's dc outlet > Hi all, > I want to build an rpm meter for my car with an acoustical rev limit signal. > I have heard that you could quite easily extract the engine's rpm from the > AC component riding on the DC of the cigarette lighter plug/outlet. As a > matter of fact, there are some commercially available aftermarket in-car > display units which can do the same from the DC outlet. However, these units > are too extensive for my application. > Has anybody have any experience with this kind of application? > Is it just a matter of AC coupling the car outlet's ripple to some kind of > comparator before feeding it to the PIC for further processing? Or is there > more advanced technique necessary like an FFT? > Thanks! > Daniel... I've never heard of this, but I suspect that the problem will be getting any kind of sizable signal. The presumption is that the AC output of the alternator will put pulsations on the battery bus, but the amplitude is unpredictable. You will have to determine empirically the relation between the pulsation frequency and the RPM. Also, ignition noise will have to be dealt with, since it may occur at a different mutiple of the RPM. If there is a signal, the processing sequence would be filtering, amplification, and extraction. For extraction, I would use a phase locked loop. No FFT should be necessary. John Power -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email .....listservKILLspam.....mitvma.mit.edu with SET PICList DIGEST in the body

Sorry, forgot to quote the message i was responding to...Like I said the way to determine the rpm is by reading the ignition noise off of the 12v line, not the alternator noise. Mike Hillpot {Quote hidden}> > I've never heard of this, but I suspect that the problem will be getting any > kind of sizable signal. The presumption is that the AC output of the > alternator will put pulsations on the battery bus, but the amplitude > is unpredictable. You will have to determine empirically the relation > between the pulsation frequency and the RPM. Also, ignition noise will > have to be dealt with, since it may occur at a different mutiple of the > RPM. If there is a signal, the processing sequence would > be filtering, amplification, and extraction. For extraction, I would use > a phase locked loop. No FFT should be necessary. > > John Power > > -- > -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listservspam_OUTmitvma.mit.edu with SET PICList DIGEST in the body

My car has a diagnostics port under the bonnet that outputs a beautifully clean square wave with pulse width inversely proportional to RPM... The same signal is also available at some handy screw terminals on the back of the instrument panel. It also has outputs which represent the sine and cosine of the speedometer and tachometer needles. Very handy. It's a reasonably new car though, yor car may or may not have something like it. If it doesn't I've had moderate success picking up an RPM signal from the coil on my old (1975) car. Ber careful though as the voltages are in the thousands... {Original Message removed}

There is indeed a relation between the ignition frequency and the actual engine RPMs. The commercial devices I have seen, are either equipped with dip switches or an advanced setup menu to enter the number of cylinders of the engine. I'm sure this is done so select a certain division factor in software. My biggest concern is indeed the reliability of having significant ignition ripple/noise on the DC outlet under all conditions (high/low RPM, state of charge etc.). The second thing which is important is the spectrum of the noise, in other words, does the ignition noise have the biggest share in this signal. Other wise it might be too difficult to extract and waveshape the ignition pulses for further processing by the PIC. I have found a schematic on the net of an RPM counter using a kind of small pickup antenna which is placed near one of the ignition coils. However, since I have seen that it must be possible to extract this information from the DC outlet, I do not want to hassle with additional sensors or connection to an ODBII port. Daniel... {Original Message removed}

On Thu, 20 May 2004 11:10:36 +0200, D. Schouten wrote: :: I do not want to hassle with additional sensors or connection :: to an ODBII port. If by chance your car happens to be a Nissan, then you may find the CPU is in the passenger footwell of the car and the tachometer output is easily tapped in to. Well I know that's true for 200Sx and Silvias. Colin -- cdb, @spam@bodgy1KILLspamoptusnet.com.au on Thursday,20 May,2004 I have always been a few Dendrites short of an Axon and believe me it shows. Light travels faster than sound. That's why some people appear bright until they speak! -- http://www.piclist.com hint: To leave the PICList KILLspampiclist-unsubscribe-requestKILLspammitvma.mit.edu

> On Thu, 20 May 2004 11:10:36 +0200, D. Schouten wrote: > :: I do not want to hassle with additional sensors or connection > :: to an ODBII port. > > If by chance your car happens to be a Nissan, then you may find the > CPU is in the passenger footwell of the car and the tachometer output > is easily tapped in to. Well I know that's true for 200Sx and Silvias. Same with some older GMs, in my car the tach signal is available on a connector right above the passengers feet, MUCH easier to deal with then some fancy filtering of the DC level. TTYL ---------------------------------- Herbert's PIC Stuff: http://repatch.dyndns.org:8383/pic_stuff/ -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestTakeThisOuTmitvma.mit.edu

Actually, the primary coil terminals, on older cars with coils, are a 12Vdc level and that is precisely where the tach signal is "tapped" into when installing even an aftermarket tach. Of course, those aftermarket models are designed for that. The "ground" side of the primary wiring {marked - on the coil) is "chopped" either by the ignition points or a solid state switching system. By reading the pulses generated by this, one can measure the rpm of the motor. Keep in mind that you will need to divide these pulses by the number of cylinders in the engine. Randy Abernathy 4626 Old Stilesboro Road NW Acworth, GA 30101-4066 Phone / Fax: 770-974-5295 Cell: 678-772-4113 E-mail: spamBeGoneCnc002spamBeGoneaol.com I furnish technical support, repair, and other related services for your industrial woodworking machinery. My background as Senior Service Engineer for the SCMI Group for nearly fifteen years with factory training, combines with my extensive background in electronics, mechanics, pneumatics, electrical and CNC machinery to offer you needed support for your machinery. -- http://www.piclist.com hint: To leave the PICList TakeThisOuTpiclist-unsubscribe-requestEraseMEspam_OUTmitvma.mit.edu

A simple way to get rpm pulses from an ignition system is to wrap a few turns of wire around the HT lead between the coil & distributor (or on a single spark plug lead). This will form a capacitior between the HT conductor and the wire. Another cap to ground and you have a capacitive divider to drop the volts. Then a simple diode protection circuit & some filtering as required. e.g if the capacitor coupler is 10pF, the peak voltage 10kV (?) and the divider cap is 1nF, the output voltage prior to clipping would be ~ 10e3*10e-12/1e-9 = 10V. You can also use this method to connect a scope up to view the ignition pulses. This can help detect various ignition problems. Richard P Actually, the primary coil terminals, on older cars with coils, are a 12Vdc level and that is precisely where the tach signal is "tapped" into when installing even an aftermarket tach. Of course, those aftermarket models are designed for that. The "ground" side of the primary wiring {marked - on the coil) is "chopped" either by the ignition points or a solid state switching system. By reading the pulses generated by this, one can measure the rpm of the motor. Keep in mind that you will need to divide these pulses by the number of cylinders in the engine. Randy Abernathy 4626 Old Stilesboro Road NW Acworth, GA 30101-4066 Phone / Fax: 770-974-5295 Cell: 678-772-4113 E-mail: RemoveMECnc002TakeThisOuTaol.com I furnish technical support, repair, and other related services for your industrial woodworking machinery. My background as Senior Service Engineer for the SCMI Group for nearly fifteen years with factory training, combines with my extensive background in electronics, mechanics, pneumatics, electrical and CNC machinery to offer you needed support for your machinery. -- http://www.piclist.com hint: To leave the PICList piclist-unsubscribe-requestEraseME.....mitvma.mit.edu -- http://www.piclist.com hint: To leave the PICList EraseMEpiclist-unsubscribe-requestmitvma.mit.edu

> A simple way to get rpm pulses from an ignition system is to wrap a few > turns of wire around the HT lead between the coil & distributor (or on a > single spark plug lead). This will form a capacitior between the HT > conductor and the wire. Another cap to ground and you have a capacitive > divider to drop the volts. Then a simple diode protection circuit & some > filtering as required. > e.g if the capacitor coupler is 10pF, the peak voltage 10kV (?) and the > divider cap is 1nF, the output voltage prior to clipping would be ~ > 10e3*10e-12/1e-9 = 10V. > You can also use this method to connect a scope up to view the ignition > pulses. This can help detect various ignition problems. IMHO that method is not the best way to go anymore in this world of distributor less ignition systems. In those systems there is often a second spark that goes at the end of the exhaust stroke IIRC. This spark is called the "waste spark" and is used to try and burn away a few more hydrocarbons before they hit the cat. The problem is the size of this waste spark on an inductively coupled device is VERY variable, I've seen it as large as the main spark and after hitting the gas disappear completely for a short while. Therefore, on a car that does use a distributor, I'd check first if it issues a waste spark, if it does I'd go with another method to get RPM. TTYL ---------------------------------- Herbert's PIC Stuff: http://repatch.dyndns.org:8383/pic_stuff/ -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestEraseMEEraseMEmitvma.mit.edu

Yes, but the collapsing magnetic field raises even the primary side of the coil WELL above +12V. Dave ----- Original Message ----- > Actually, the primary coil terminals, on older cars with coils, are a 12Vdc > level and that is precisely where the tach signal is "tapped" into when > installing even an aftermarket tach. -- http://www.piclist.com hint: To leave the PICList RemoveMEpiclist-unsubscribe-requestspam_OUTKILLspammitvma.mit.edu