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Might be an open collector output, in which case you'll need a pull-up resistor.
Just make sure you don't pull up to a higher voltage than what Tracy's output
chip is rated for.
In my design I use a chip with SchmittTrigger input (for example 74C14).
Still would recommend a series resistor before the input to the chip, and
the input to the chip protected with a Zener diode (voltage rated at less
than max input rating of the chip and supply voltage).
>Input - resistor - chip input ->
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Z
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>Ground--------------------------
For example, if you're supplying the chip with 12 Volts, use a 12V or less
Zener diode.
Size of resistor depends on type of chip, but for a 74C14, 10K or more would
be just fine.
A capacitor in parallel with zener to filter out noise would also be a good
idea.
10,000 RPM x 12 / 60 = 2 KHz would be the maximum you'd ever need to pass.
A .047 uF capacitor and 10K resistor would be fine (3 KHz).
Finn
Haywire wrote:
Hi Finn;
You got me thinking that I could come up with a
better solution, than a voltage divider. I built the circuit on a breadboard
again, but using a capacitor across VDD & VSS this time. I tried various
capacitors and was unable to improve on the stability at higher voltages
without a voltage divider. I was considering ordering a couple of regulators
(still the best idea) when I started considering that I really don't know
anything about the output pulse from the EC2 other than it is a 12pulse/rev
signal. So I borrowed a Fluke scope from our shop and brought it to the
airport to have a look at the wave. It is a square wave 768mV peak to peak,
base voltage 8mV (I guess I could have just asked Tracy :-). This surprised
me as I was expecting 5V. In experiments on the breadboard I found that
the counter performed well at voltages up to 20V as long as the clock input
was not greater than 5 volts below supply voltage. Now it's becoming a little
more clear (remember, I'm just an apprentice). When I showed the waveform
to a journeyman at work, he also was so surprised, he suspects it's not
correct. Tracy can you confirm this P/P voltage?
The voltage divider, while being a little crude,
is effective, so I may just stick with that, unless I can find a couple
of 5V regulators in the spare parts. Since I expect that once the EM2 becomes
available, most builders will chose to use that since it really is a great
tool, so there won't be any reason to get too carried away with this thing.
Besides I'm just giving the darned things away because I had 24 extra counter
chips, so I shouldn't start buying parts -- no wonder I can never save any
money :-)
John and Dave;
I'll try to get a couple sent off before next week.
John, I'll need an address.
S. Todd Bartrim
Turbo 13B RV-9Endurance
C-FSTB
http://www3.telus.net/haywire/RV-9/C-FSTB.htm
"Whatever you vividly imagine, Ardently desire, Sincerely believe in,
Enthusiastically act upon, Must inevitably come to pass".
A capacitor accross the supply pins should be fine there. I was thinking
about the input. Of course if your input comes from Tracy's controller,
that's probably not an issue. But from anywhere else, like coil pick up,
some input filtering/clamping would be wise.
Finn
Haywire wrote:
Hi Finn;
It is a CMOS device capable of handling 20vdc on
its VDD pin. So I didn't feel it was necessary. In reality I found it
was unstable when VDD was less than 5vdc or greater than 11vdc, so since
I had a bag of 470ohm resistors handy, I just put a voltage divider
on the VDD. With a regulated 13.8vdc it gives me just under 7 volts
and a stable reading.
a side note; I originally had an internally
regulated alternator but the regulator failed (have since converted
to an external reg as per "electric Bob"). When running with the alternator
tripped and less than 12vdc, I found the tach would become erratic at
high RPM's, due to a low VDD. When alt is charging, everything is
fine.
S. Todd Bartrim
Turbo 13B RV-9Endurance
C-FSTB
http://www3.telus.net/haywire/RV-9/C-FSTB.htm
"Whatever you vividly imagine, Ardently desire, Sincerely believe
in, Enthusiastically act upon, Must inevitably come to pass".
I don't see a Zener diode to clamp any spikes.
Finn
Haywire wrote:
Attached are a couple of pics of my tach
divider. It can be set to divide by any number necessary. I can't
guarantee it would work with any tach but it works very well with
my cheap off-the-shelf-at-any-autoparts-store, Hastings tach. I
have a few more if anybody wants one.
S. Todd Bartrim
Turbo 13B RV-9Endurance
C-FSTB
http://www3.telus.net/haywire/RV-9/C-FSTB.htm
"Whatever you vividly imagine, Ardently desire, Sincerely believe
in, Enthusiastically act upon, Must inevitably come to pass".
The EC-2 tach output has 12 pulses per revolution, and
won't directly work with any normal tach. Todd has made a divider
to bring the pulse rate down to normal, and it sounds like it's
working well. The TT that you have expects 1 pulse per rev, so
if you don't mind dividing the rpm reading by 12... :-)
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