Tach LED Bar....little different than most

[aort]

I am looking to make a bar run the length of my windshield with ~100 LEDs going from 50 green, 25 yellow, 25 red. I need to get the signal off of the MSD box and was thinking of turning on 10 leds at a time per 1000 RPM up to 10,000 (we turn 9,600 to 9,800 occasionally). Would this be able to be done with a F-V converter and relays? If so which F-V? I was thinking just put resistors on the input of the relay so as the voltage got higer from the F-V, it would turn the next relay on therefore turning on 10 leds until it gets to the end. Would this work? I would like to do this the most simple way possible without using and micro's or anything if its possible. I have taken some electrical courses in college so I'm not completely lost. Just looking for some advice, Thanks.

 

[picbits]

Microcontroller to capture the time between pulses or to time how many pulses per second (or fraction of a second).

MAX7219 to drive the LEDs.

Not a very hard project (I have a product which does a similar thing) but you need to watch out for dirty voltage supply, spikes on the input etc etc.

 

[picbits]

Or if you want a really quick and dirty method of doing it (and you only want 10 LED resolution) then go for a F to V converter (LM2917) and use comparators to drive the LEDs.

You'd get away with doing the whole lot for <$10

 

[aort]

10 LED resolution wouldnt be a big deal, its just for show. What would you use for the comparators? I have no experience with them, just know they compare two voltages and then switches output to show which is higher....I think?

 

[picbits]

A comparator can be made out of an opamp.

If you want to drive say 10 LEDs then you might have to buffer the output with a mosfet or transistor but just about any opamp would do.

Have a google/wikipedia for comparator and you'll be halfway there.

 

[aort]

I read the wiki article, but I'm having trouble putting it together. I guess I don't see how to stack the opamps together to achieve all sets of 10 leds coming on...and are you saying to use a transistor to "amplify" the signal just to make it more constant?

Its a lot different taking things from the lab and trying to create your own things, so thanks for the help.

 

[picbits]

Nope - an opamp can output maybe 5-10ma. with 10 LEDs you'll be wanting to drive around 200-300ma. The transistor allows you to switch higher currents than the opamp can drive.

In fact, have a look at the LM3914 bar graph driver datasheet - it might give you some more ideas

 

[aort]

I think I have this....The way you are saying it, would there be 10 different opamps (one for each set of 10 LEDs)? Then make the + non inverting pin of the op amp the signal from the F-V converter and the - inverting pin the V right below which the rpm needs to be to turn on that set of 10 LEDs?

 

[picbits]

I think I have this....The way you are saying it, would there be 10 different opamps (one for each set of 10 LEDs)? Then make the + non inverting pin of the op amp the signal from the F-V converter and the - inverting pin the V right below which the rpm needs to be to turn on that set of 10 LEDs?

Yup

I.e. Comparator 1 turns on at 0.5v, Comparator 2 on at 1v, Comparator 3 on at 1.5 volts etc etc.

 

[aort]

So just using resistors to get my +12v supply down thats how I would set the "turn on" voltage of the comparator correct? Also on the op amps, are the +, - and output pins all I would use? Do you recommend any opamps or transistors to use? Thanks again

 

[picbits]

No - you need a regulated supply for the opamp voltage reference (a resistor/zener should suffice)

The opamps I'd get would be rated up to around 40 volts to cope with some of the spikes on the vehicle power rail. Use a resistor divider network to provide the comparator inputs (i.e. +9v -> R -> R -> R -> R ->R -> R -> Ground)

The output transistors are really up to you - I'd personally use something like the ULN2003 array - this will give you up to 500ma per output and 7 outputs per IC.

 

[aort]

Now I'm a little confused. Never used a zener?...Where did the +9 volts -> R -> R...come from? On the transistors, would I be able to run all 10 LED's off of the 7 outputs since they are 500mA per output?

I found a LM118 op amp but they are $5 a piece :/. Also, what amperage do most LED's run on?

 

[aort]

Would it be easier to use the LM3914? Would there just be 10 of those instead? I'm not really sure where to go from here.

 

[picbits]

Ok - you will need a regulated supply for the opamp comparator input. If you look at a typical vehicle supply it can vary between 10v and 15 volt. If you don't regulate it and take it off the "12v" feed then any variances in the running voltage will affect the accuracy/calibration of the tachometer.

I picked +9v as an example - its fairly easy to get a 9 volt regulator for car use (a lot of dashboards have them or an 8v LDO regulator slapped on the back).

You want to google/wikipedia resistor divider network - if you hook up 10 of the same value resistors in series (say 1k each) up to +9v at one and and ground at the other then the voltage at each of the resistor nodes is around 1/10th of the 9v input voltage. You could alternatively use a voltage divider on each opamp input i.e. a variable resistor with one side of the track at +9v and the other side at GND with the wiper going to the comparator -.

A cheap opamp will be fine - if your supply is relatively spike/noise free then a lower voltage opamp will work but be very aware that load dumps on automotive systems can present over 200 volts on the system for up to half a second.

The LED question is really up to what you buy, whether you connect them in series or parallel and how many you use. I have LEDs here that range from 1ma to 2 amps.

 

[picbits]

Would it be easier to use the LM3914? Would there just be 10 of those instead? I'm not really sure where to go from here.

Have you downloaded and read the LM3914 datasheet yet ?

 

[aort]

I guess I should mention, the +12 is just off of a battery that will not be charged from an alternator. It is in an offroad vehicle, therefor it only runs for 2-3 minutes at a time off of just battery power.

 

[picbits]

Still needs a regulator somewhere - the difference between a fully charged 12v battery and a low charge one is a volt or so - this could equate to 1000-2000 rpm on your rev counter.

 

[aort]

Ok, now does the opamp need a + and - voltage going into it as well as the trigger voltage on the - pin and the signal voltage on the + pin?

 

[picbits]

Have you read the LM3914 datasheet ? It would really save you a lot of trouble as you could use one instead of a bunch of opamps .....

But yes. The opamp does need a positive power feed and a ground.

 

[aort]

I am going to dl it now, I was in class earlier and couldn't get anything on those computers.