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