10 LED Shift Lights, using LM3914, NEED ADVICE

[5am]

hi,

i was looking into making a shift light for a friends car (and mine while im at it probably ), and then i came across a thread on here (was only one post with no replies, and i can't seem to find it again now) where someone was using a LM3914, which lights 10 LEDS in a bar or dot graph depending on an input voltage and reference voltage. this was very exciting, so i thought i would look into making a similar style circuit.

the problem i have is, the lm3914 uses a voltage, though i want to have the LEDs light up in response to the tacho feed frequency increasing. i found that my car produces 250 beats per minute for every 100rpm. knowing this, could i use a capacitor to smooth the wave to a reasonably constant voltage, so that the voltage across the capacitor would increase as the frequency increases? the problem would then arise that the tacho may not function properly, as the input it recieves (normally a frequency) would also be affected by the capacitor, so it would now be recieving a voltage, which probably wouldn't have the same effect. to solve this i thought i could use a transistor to amplify the signal, so it should leave the original signal intact, and then smooth out the amplified signal...?

the other option, though less preffered due to cost and complexity, would be to use a LM2907 or LM2917 wich produces a varying voltage relative to a frequency input, which i could then feed to the LM3914.

any advice will be greatly appreciated, and please point out any faults in this idea

 

[akg]

a freq ot voltage converter is what u need at the simplest.
u feed the converter thru a buffer amp, may be an emitter follower, to avoid loading of the tacho line.a smoothing (high value)capacitor with a parallel bleeder resistor would be an approximate F-V converter.

 

[5am]

thanks for the reply

the buffer amp sounds like what i need, though im not familiar with them. r there any transistors/ic's i should be looking at, or a basic circuit design i can start from?

so... the plan is looking like a buffer amp to leave the tacho feed relatively unaffected, then capacitor with bleed resistor to act as the frequency to voltage converter, then the 10 LED bar graph IC...

for capacitor and resistor, i dont really know what im looking for, i will be dealing with about 10KHz to 18KHz frequency, so (guessing here...) 10uF cap with 1Kohm resistor... sound reasonable?

 

[Roff]

thanks for the reply

the buffer amp sounds like what i need, though im not familiar with them. r there any transistors/ic's i should be looking at, or a basic circuit design i can start from?

so... the plan is looking like a buffer amp to leave the tacho feed relatively unaffected, then capacitor with bleed resistor to act as the frequency to voltage converter, then the 10 LED bar graph IC...

for capacitor and resistor, i dont really know what im looking for, i will be dealing with about 10KHz to 18KHz frequency, so (guessing here...) 10uF cap with 1Kohm resistor... sound reasonable?

10kHz? 250 beats/minute is about 4 Hz. If you want to shift at 5000 rpm (as an example), that's about 200Hz.
The other problem with the filter is, it only works if the pulse width is constant. If the pulse width decreases as the RPMs go up (maintaining constant duty cycle), the filter does nothing. You can fool around with F-V conversion, but there is a good reason that the 2907 is being made.

 

[akg]

thanks for the reply

"Ron" +
may be one more method is to trigger a monostable , with the incoming pulses , so that u get a PWM signal wrt i/c freq:, an RC would convert it to Voltage,

 

[Roff]

thanks for the reply

"Ron" +
may be one more method is to trigger a monostable , with the incoming pulses , so that u get a PWM signal wrt i/c freq:, an RC would convert it to Voltage,

You would probably need a Schmitt trigger in addition to the monostable. Unless you just can't get a 2907, you will use more parts and have less accuracy this way. As I said, there is a good reason that the LM2907/2917 is a viable product.

 

[evandude]

In all honesty, I think you ought to experiment with it. One of the harder things with interfacing with the tachometer wire and all that, is finding the wire, and seeing what the signal is like.

You might be able to get away with a low-pass filter, and probably the best way to find out is to hook one up and try it, measuring the DC output voltage with a multimeter. Whatever you do, it's likely that you are going to need some protection circuitry on the tach wire, since it is connected to the ignition coil in most cases, it's likely to have some nasty voltage spikes on it.

I installed a remote starter on someone's car, and it instructed me to verify the tach wire by measuring it with a DMM set to AC volts. The AC voltage it measured was proportional to RPM's. that didn't tell me a lot about measuring the signal to get a DC output, because my DMM is not true RMS so the displayed value doesn't have a lot of direct meaning, but at least it was some inspiration that it should be doable with some simple method.

 

[5am]

thanks guys for all ur input

i think i might just use a 2907/2917, i think it will probably save a bit of time and trouble in the long run.

i've read the manual and found what colour my tacho feed wire is, so i'll try and find it soon and pull out the multimeter on it, see what i get.

thanks again, ill let you know how it goes

 

[hyedenny]

Hi,
I think the datasheets for the 2907 and 3914 have the circuits youre looking for.
**broken link removed**
**broken link removed**

 

[allanseth23]

this might help in your project.

**broken link removed**

hth!

 

[gramo]

if you know someone with pic micro knowledge, its a rather simple project

 

[steev]

this one got good circuit for tach. http://www.fda-technology.de/
i want to built 20LED version with LM2917(14pin) but it uses LM2907(8pin).can someone modify it....(**broken link removed**)