Tachometer

Hello all,

I need to design a tachometer for a Formula racing car. We have an FM
output from an ECL going into (most likely) an LM3914 Freq-Voltage
Converter (www.national.com). What most concerns me, however, is the
output of the LM3914. I want it to drive an LED bargraph, but have
been hard-pressed to find any decent schematics of the bargraph with a
good explanation so I can tweak it.

Any help with either a link or the theory of how an LED bargraph works
would be greatly appreciated.

Thanks,
Mike

Hi

Try tsm39341 bar graph modules. 10 lights, electronics built in can be
cascaded.

Geoff

"miketinte" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) om...
> Hello all,
>
> I need to design a tachometer for a Formula racing car. We have an FM
> output from an ECL going into (most likely) an LM3914 Freq-Voltage
> Converter (www.national.com). What most concerns me, however, is the
> output of the LM3914. I want it to drive an LED bargraph, but have
> been hard-pressed to find any decent schematics of the bargraph with a
> good explanation so I can tweak it.
>
> Any help with either a link or the theory of how an LED bargraph works
> would be greatly appreciated.
>
> Thanks,
> Mike

Thanks Geoff. Who makes the TSM39341. I tried googling it and came up
with one two sites, one was advertising brokerage software, another
was unreachable.

-Mike

How about using a PIC to drive the LEDs? I'm looking for about 15 LEDs
to provide a high enough resolution. Can I daisy chain these bad boys
to provide for the higher number of LEDs, or should I just go with a
PIC that has enough outputs?

I'm shooting for an ETA of October 18th with this thing, so any quick
help would be appreciated.

Thanks,
Mike

"miketinte" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) m...
How about using a PIC to drive the LEDs? I'm looking for about 15 LEDs
to provide a high enough resolution. Can I daisy chain these bad boys
to provide for the higher number of LEDs, or should I just go with a
PIC that has enough outputs?

I'm shooting for an ETA of October 18th with this thing, so any quick
help would be appreciated.

Thanks,
Mike

Mike,
I don't know you're exact requirements, but to measure rpm, try this:

Use BCD ripple counters with bcd-seven segment drivers and seven segment led
displays.

Use a timer chip circuit to make a pulse whose width, (time) is adjustable.
If something is putting out pulses and you want that pulse rate to indicate
a certain value, then adjust the time window pulse width for that time and
the count will be what you want. If the shaft slows to one half that speed,
then you will count exactly one half (+-1) as many pulses in that same fixed
time window. Latch the count into display drivers and reset the counters at
the end of the time window.

Example: Engine goes 2400rpm which is 40 revolutions per second. If you
have two magnets on opposite sides of the shaft, you will get 80 counts per
second. If you count for one second, the counter will read 80. But if you
count for 24/80 of a second, (0.3sec), the counter will read 24. You can
read in 100s of rpm. Similarly, if the engine goes 1200rpm = 20 rps then
the counter will read 12. A zener regulated voltage operating a 555 timer
with good quality resistors and capacitors and a calibration pot can provide
the adjustable time width pulse good enough for normal use. Crystal
oscillators and dividers can give more accurate pulse widths. Here the
count updates or changes 3 times a second. A simple and accurate
"speedometer". Put 2 cabinet door magnets clamped with 2 radiator clamps
onto your rear-wheel car's drive line, mount a pickup coil and you have a
good digital speedometer.

The example used "two teeth" per revolution. A gear with many teeth can
count much faster. If you had 20 teeth and counted for 3 seconds you could
actually get a count of 2400 with a resolution or 1rpm. (10 times as many
teeth and 10 times as long to count). This is somewhat a waste of effort
because the time window oscillator isn't accurate enough to really show +-
1rpm out of 2400, (0.04+%). But use 20 teeth and 0.3sec count window to
readout in 10s of rpm and just show the units digit always 0. You get fast
update and the counter resolves 2400, 2390, 2380, etc, every 10 rpm.
Same story with 10 teeth and 0.6 sec window etc.
Ghost
--
"The difference between fiction and reality
is that fiction must make sense."
Tom Clancey

Good points.
Ghost
--
"The difference between fiction and reality
is that fiction must make sense."
Tom Clancey


"default" <R75/(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> (snip)
>
> In the initial post Mike is looking for a bargraph display. Digital
> tachs are the best choice for accuracy and resolution, and easier to
> implement, but it takes too long to read them when one is "busy."
>
> Analog LED displays shine in that app. Concentrate on the driving and
> let the peripheral vision watch the tach. Using LEDs buys a faster
> response time than a damped mechanical movement.
>
>
> -----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
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Yes. The bargraph is going to be arranged in an arc shaped patter so
as to mimic the look and feel of an analog tach. My school is known
for their design every year, so we're doing our best to make this
original and eye-catching.

Just to clarify...so I want to put a frequency modulated signal into a
PIC, and then have that control a voltage at one of the outputs? Why
not just use a F-V converter directly driving the bargraph driver? Why
bother with the PIC?

Thanks,
Mike

default <R75/(E-Mail Removed)> wrote in message news:<(E-Mail Removed)>. ..
> (snip)
>
> In the initial post Mike is looking for a bargraph display. Digital
> tachs are the best choice for accuracy and resolution, and easier to
> implement, but it takes too long to read them when one is "busy."
>
> Analog LED displays shine in that app. Concentrate on the driving and
> let the peripheral vision watch the tach. Using LEDs buys a faster
> response time than a damped mechanical movement.
>
>
> -----= Posted via Newsfeeds.Com, Uncensored Usenet News =-----
> http://www.newsfeeds.com - The #1 Newsgroup Service in the World!
> -----== Over 100,000 Newsgroups - 19 Different Servers! =-----