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Everything you ever wanted to know about the EC2
tach output.
The tach output from the EC2 is an open
collector output with a 2K pullup to battery voltage (+12). It is a
completely independant driver so builders have the freedom to hook it up to any
random place they want to and blow it out without stoping the EC2 from
functioning. Several builders have exercised this freedom.
Unfortunately, some of them have used this same freedom on virtually
every other EC2 connection with less than good results.
Due to the very few tachs that use 12 pulses /rev,
I am dropping this feature from future versions of the controller. Even
Grand Rapids seems to be dropping this option. A new Tach output may take
its place with a more common output pulse rate.
The 24 tooth crank sensor (running at 1/2
crank speed) is the source of the signal timing so it is 12 pulses per
rev. This happened to be a pulse rate option on the Grand Rapids EIS I was
using so it worked out.
To answer the questions about EC2 software updates,
I will be posting details about when & how much (it's cheap) on the
website.
Tracy Crook, RWS
----- Original Message -----
Sent: Friday, August 22, 2003 12:56
AM
Subject: [FlyRotary] Re: tach
divider
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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