Hmm, are you sure you can't access a form of that signal some other way? In most cars the tranny sends it's signal and the ECM (or ECU) sends out a clear stable version of that signal to the console (where the car's speedometer is located). Thanks, TTYL > -----Original Message----- > From: pic microcontroller discussion list > [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Pic Dude > Sent: Wednesday, March 06, 2002 12:08 > To: PICLIST@MITVMA.MIT.EDU > Subject: Re: [EE]: Limiting signal level > > > That sounds simple enough, but there are a lot of unknowns. Here's > more context on the situation.... > > Transmission has a VSS (speed sensor) which the ECU needs. It > apparently puts out a sine wave of increasing frequency AND amplitude > based on vehicle speed. I have an electronic speedometer so I needed > a sq.-wave signal (which it's own sensor puts out). Since I can't plug > both senders into the transmission, I tapped off the VSS signal, ran it > into an LM339 comparator, limited the o/p to ~5 V and sent that to the > speedo. [Works well, except under 4 or 5 mph, the speedo shows 0. > After that, the needle jumps to the correct speed. No problem though.] > > My fear is that one of these days when I get the car to a track, I'll hit > 160+ mph and damage the comparator circuit. > > W/o an oscilloscope, I could not tell how high the input voltage would > get to, but in a hand-drill, the VSS got up to about 10V, rising > proportionately with rpm. I could not calculate freq either, to arrive > at some formula/trend for amplitude vs freq (to project out to a higher > speed). Recently someone mentioned that they saw up to 24V on their > VSS, but not sure what rpm, etc. > > I also didn't have specs on the input source, such as how much > current it can put out. It's really just a magnetic position sensor type > device, so it may be low. > > The speedo spec (when calibrated correctly against a GPS), says > it's using approx 8000 pulses per mile. This translates to 400 Hz > at 160mph. > > How would I figure out the max resistance I can use then? I'm also not > sure how high is high, but I want to limit the load on the ECU so as > not to damage it. > > Thanks much. > > > > ----- Original Message ----- > From: "Olin Lathrop" > To: > Sent: Wednesday, March 06, 2002 6:59 AM > Subject: Re: [EE]: Limiting signal level > > > > >> > > Keeping the same 8V supply, is there an easy way to pre-"clip" or > > pre-limit the input to approx 8V before it gets to the comparator, > > but still maintaining a high input impedance? > > << > > > > Sortof. Maintaining high impedance within the comparator > supply range is > > easy. Most comparators have some sort of input protection, > usually diodes > > to the supply rails. The easiest answer is to put a large resistor in > > series with the comparator so that the current thru the > protection diodes > > will be limited. The impedance to the source will be high within the > common > > mode range, and will be the resistor value outside that range. > > > > This scheme is not practical if the input frequency is high, because the > > signal will get low pass filtered by the time it gets to the comparator. > > For example, if you are sensing 60Hz power signal, then you can stick a > > 1Mohm resistor in series and be done with it. However, a 1MHz signal > would > > probably be attenuated too much by this. > > > > > > ******************************************************************** > > Olin Lathrop, embedded systems consultant in Littleton Massachusetts > > (978) 742-9014, olin@embedinc.com, http://www.embedinc.com > > > > -- > > http://www.piclist.com#nomail Going offline? Don't AutoReply us! > > email listserv@mitvma.mit.edu with SET PICList DIGEST in the body > > > > > > > > -- > http://www.piclist.com#nomail Going offline? Don't AutoReply us! > email listserv@mitvma.mit.edu with SET PICList DIGEST in the body > > > -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body