Had built an analog device some months back with a comparator
chip to convert a variable-frequency, variable-amplitude sine-ish wave
signal to a digital square-wave signal with just variable frequency.
The comparator just switched the output from Lo to Hi when the input
crossed a certain threshold. Worked well -- a most exhilarating
feeling since my analog-circuit knowledge is not too great.
The only thing I was not sure of was the max amplitude of the input,
but I've been thinking about it again since I heard recently that it
could go as high as 24V or more, and power supply is only 8V to the
comparator.
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?
>>
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.
How high an input impedance? In a recent project I did I had a similar
problem, I simply used a 1Mohm resitor and a zener diode. This might not be
high enough for your though, I don't know. TTYL
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.
See my other post a minute ago. I don't have a hard spec on
how high, but I'm tapping a signal between a magnetic sensor and
a vehicle ECU, so I want to impose as little load to the ECU as
possible.
If I use a zener/resistor, what happens if the input threshold is
exceeded? Would the diode "open up" and direct the input
signal to ground? Or would it only direct the excess to ground,
so that the comparator still receives a workable signal?
> How high an input impedance? In a recent project I did I had a similar
> problem, I simply used a 1Mohm resitor and a zener diode. This might not
be
> high enough for your though, I don't know. TTYL
>
> > {Original Message removed}
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
Please see my other post as well! :) You should be able to get a speed
signal coming OUT of the ECU going to the car's speedometer, that signal
should be much more useful to you.
The zener would dump the excess signal to ground, it efectivly clips the
input signal to the voltage of the zener. TTYL
I'm not aware of any, though the ECU has a signal wire going to the
cruise-control module. Can't tell what it is w/o an oscilloscope.
In the "mfgr-expected" system, the speedo is mechanical and the
speedo cable plugs into the back of the VSS.
The engine is a Ford 5.0 and I have it in a kitcar, hence the elec
speedo. Also, there is no room to piggyback one sensor to the
other, and I don't want to go thru the pain/nightmares of running
a piece of a speedo cable to a box, into which the electronic
speedo sender would go.
In any case, this device was built some months ago, and works.
I'm just re-thinking it a bit to ensure its protected.
> 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 removed}
> See my other post a minute ago. I don't have a hard spec on
> how high, but I'm tapping a signal between a magnetic sensor and
> a vehicle ECU, so I want to impose as little load to the ECU as
> possible.
I'd buffer it with an op-amp and clamp the little beggar with a Zener,
personally.
I guess the confusion here or the unknown is what the ECU puts out.
In the original vehicle, the car had a mechanical speedo. The cruise
control signal could be an output or an input, or perhaps a signal to
force the cruise control module to stop operating under some
condition.
If the zener's only dumping the excess, this may be the answer. I'll
have to go pick up a zener and experiment a bit.
> Please see my other post as well! :) You should be able to get a speed
> signal coming OUT of the ECU going to the car's speedometer, that signal
> should be much more useful to you.
>
> The zener would dump the excess signal to ground, it efectivly clips the
> input signal to the voltage of the zener. TTYL
>
> > {Original Message removed}
> 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.
>
> ...
>
> 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.
This could be separate sensor, or it could be right off the alternator.
Since the signal amplitude increases with frequency, a little low pass
filtering works to your advantage. 400Hz is also low enough that you can
put a reasonably large resistor in series with the comparator input. In
your case, you can even put a small capacitor to ground at the comparator
input. Try 1Mohm and 1mF, assuming the comparators has significantly higher
input impedance than 1Mohm. 1Mohm nd 1mF gives a rollover frequency of
about 160Hz, or about 64 miles/hour. Below that speed the capacitor won't
do much, and above that speed it will start to hold the AC voltage roughly
constant, which gives you better noise immunity working with the hysterisis.
You do have some hysterisis, right?
> If I use a zener/resistor, what happens if the input threshold is
> exceeded? Would the diode "open up" and direct the input
> signal to ground? Or would it only direct the excess to ground,
> so that the comparator still receives a workable signal?
The zener will clip the signal, not completely attenuate it.
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