# Please Help!!!! LM2907 Speed Switch

Discussion in 'Electronic Basics' started by Eddie, Oct 28, 2005.

1. ### EddieGuest

I am trying to make a speed switch with the LM2907. My power supply
will be 12v and I am wanting this to light up a group of lights that
will indicate to me when my wheel speed decreases below a certain
limit. I want to use a wheel speed sensor that produces a sine wave and
not a square wave. The schematic I have to go by is this.
http://img313.imageshack.us/img313/7242/speedswitch8xc.jpg

I built this with what components I had laying around. So for C I used
a 1 micro farad 50v cap. The other cap is used is a 10 micro farad 50v.
R is a 100K potentiometer. I have a resistor in series with the load
that isn't pictured and it is a 500K ohm. The other two resistor's are
both 10K. After having all this hooked up to an oscillator and a dc
power supply to provide all the inputs I need, the circuit was eratic
when I changed the oscillator output frequencies from 5 hz to 25 Khz.
For instance with the oscillator off the light would come on for 6 sec
and then go off. Turn the oscillator on and inject a signal and it
would never come on. After 2 min it would then come on and stay on.
Turn off oscillator and light would go off and stay off. Turn
oscillator on and it would still stay off even 10 min after the fact. I
am just a beginner when it comes to this stuff so please walk me
through this. The whole load being energized when Fin> 1/2C1R1. I don't
even know how to do these calculations as to when the load would
energize with the values I have already given above. Thanks for all the
help you plan on giving me.

Again.... my main goal is to have this bank of lights(4-12v lights)
illuminate when my wheel speed in my race car goes below a certain
speed. Again.......please help......

eddie

2. ### John PopelishGuest

Your description of the component values you are using is confusing to
me. Is it possible that you could post a schematic of what you are
working with, that includes the parts values on the components, so we
don't waste a lot of time on a misunderstanding?

3. ### Bob MonsenGuest

The sine wave makes no difference, since the input is just going to a
comparator anyway. However, the sine input must be centered on 0. If you
are giving it 0 to 5V, it won't work, since the comparator in the LM2907
compares against 0. So, put a cap in line with the sensor output, and a
big resistor (like 1MEG) from the chip to center the input on ground.

Also, the timing capacitor is on pin 2, I think, and the one on pin 3 is
used stabilize the output voltage. So, the resistor on pin 3 is R, the cap
on pin 2 is C, and the thing will go on when f >= 1/(2*R*C).

The resistors between 6&7 and 7&8 aren't critical, but the data sheet uses
5k, so I'd go 4.7k for these. They are supplying a switching voltage. I
suppose you could use a pot to change the voltage at which the output
switched. (see below)

If you want it to switch on someplace between 10 and 100Hz, use C = 1uF, R
= 10k, the stabilizing C (at pin 3) at 0.1uF, and put a 10k pot between
Vcc and ground, with its wiper connected to pin 7. (instead of the fixed
10k resistors). You should then be able to adjust the switching frequency
by changing that pot. I think that works.

---
Regards,
Bob Monsen

Let us grant that the pursuit of mathematics is a divine madness of the
human spirit.
- Alfred North Whitehead

4. ### ehsjrGuest

Change it to .01 uF
Change it to 1 uF
No, it isn't. If it was 500K ohms, the lights would never
glow.

The other two resistor's are
After you have made the component changes mentioned above and
tested things, post the schematic with ALL the components
shown and labeled with their values, as John requested. We
can't do more than make very general comments without correct
infromation.

By the way - I suspect that once you get it working, it will
work backwards, turning the lights on when you want them off,
and vice-versa, based on what you've said. That's a simple
matter to address - but we have to wait until you get the LM2907
circuit sorted out.

Ed

5. ### ahonda55Guest

oneday i tryed to make this circuit and i have failed , it i
energizing the load when the frequency is LOW not HIGH than Fin !!

so , there is a very good solution for you
make a simple frequency to voltage converter circuit with this I
and then use a comparator to compare the output of the LM2907 wit
thedesired voltag
and then take the output of the comparator to run a transistor to ru
a relay to run whatever you want

may be i could not explain it good but it is so easy to make

6. ### EddieGuest

Thanks for all the help. I will put all this to the test. First does
anyone know of a good company to buy all these parts from. Thanks for
all the inputs and i will surely keep everyone posted on my success.
Again Thanks

Eddie

7. ### ahonda55Guest

By the way , does the LM2907 output is perfect linear
i have tchometer circuit with 33000 R and C1 is 0.094 uF and C2 is 2.
u
i made the calculation of this formula.
Vout=Vin *C*R*Fi
and i want it to work from zero to 166 H
s
Vout=8*0.00000009*33000*166= 3.
then i have an LM3914 circuit with low reference is zero volts an
high reference is 3.9

will that work ? i mean will the LEDs of the LM3914 run from zero t
3.9v or the LM2907 output rate will increase to over 3.9 whil
increasing the RPM

Thankx :

8. ### John PopelishGuest

That is the way I read the formula, though where you got a 0.094 uF
capacitor I can't guess.

Yes, the LM3914 should indicate linearly from zero to whatever voltage
you connect to the high reference pin. But I think if your cap really
is 0,094 uF, you should calculate with that number. Or make the upper
reference a bit adjustable so you can trim the full scale by experiment.

9. ### ahonda55Guest

the cap. is .074 + .047 uF connected in parallel
i made it today and the result was very good
next step is to connect the comparator-transistor-relay circuit t
make the shift light :

honestly, i made all that and learned many things in electronic
because of the good guys in this forum especcialy ED(ehsjr) Jonatha
Kirwan , Jon Poplish and more :

Thanks you very much

10. ### EddieGuest

I checked the resistor that i have in series with the load and it is
510K. The color band is as follows. Green, Brown, Yellow and then gold
is the last color. What rating do you recommend for that resistor.
Thanks Ed

11. ### ehsjrGuest

Maybe we're lost in terminology. The load is the light
that you want to turn on and off with the LM2907.
If it is in series with 510K, and the supply is at 12 volts,
current will be limited to no more than ~ .0000235 amps.
23.5 microamps through a lamp at 12 volts is not going to
make it glow. So something is wrong with the description.

Ed

Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Continue to site