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Old Streetfighter Motorcycle Forum Post 10-06-2011, 11:37 PM   #1
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Default Converting a Needle Tach to LED

I was wondering if anyone has tried or knows how involved a project might get to convert a stock needle display tach to a LED tach. My electrical skills are non-existent, and when i look at diagrams i glaze over. i look at schematics, and its over. scrambled eggs for brains. Add to that, i'm not quite sure how a tach really works. i have an '01 ninja 250 to work with, and the tach is one that reads off the ignition coil. Not even sure how that works. i'm thinking either one of two ways. either the tach translates a number of pulses from the coil, or its based on the signal strength. Not even sure that makes any sense, but i tried. if you made it this far, then you understand how much of a complete electrotard that i am. So what i'd like to do is make a series of LEDs that light up progressively as the rpm's rise, maybe one LED per 1k rpms. just so i have a rough idea where i am at. sure, i could buy a tach, but where's the fun in that? this way, i get to save some money, and learn something new in the process.
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Old Streetfighter Motorcycle Forum Post 10-07-2011, 01:31 AM   #2
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Default Re: Converting a Needle Tach to LED

i think im right when i say that the voltage the wire supplys is variable. the higher the RPM the higher the output.

so what you would do is to get a multi meter measure the voltage at that wire at specific points. The points should be the points you want to have on the gauge. an example of this is idle, 200rpm, 100rpm, 1500rpm, 2000rpm, 2500rpm and so on and so on.

Then get your LEDS.

What you will need to do now is to sit down with a calculator and work out what resister you will need to activate the LED at each point. Then complete the processes for each of the points in teh rev range.

So there for if you can work out the correct resister for the LEDS it is possable.

Sounds like you need to go read up on how LEDS work. Im still only figuring it out my self. but thats tha basics to it. you do also have to think of how to stop them over loading as the voltage goes up yu will end up melting the low RPM LEDS

You may notice a few spelling mistakes in my posts. sorry. been trying for years i just cant figure that spelling stuff out.

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Old Streetfighter Motorcycle Forum Post 10-07-2011, 09:18 PM   #3
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Default Re: Converting a Needle Tach to LED

I wish it were that simple. It's not as easy as some resistors & LEDs. You need some sort of frequency to voltage converter and an LED driver. You are going to need the minimum knowledge of soldering and component identification and then go copy one of the many designs on the internet...

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Old Streetfighter Motorcycle Forum Post 10-07-2011, 09:58 PM   #4
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Default Re: Converting a Needle Tach to LED

Frequency to voltage convertor and bar-graph LED driver are both standard chips that cost a couple bucks each. But seriously, if you can't read a schematic you won't be able to follow a diy article. It's not really a complicated project but you won't find a tutorial with pics showing "solder this to that" when all that info is contained in the schematic.

Go to the nearest used book store and spend a few bucks on an intro to electronic circuits textbook. If you're interested in projects like this, once you master schematics you'll feel like you've gained a super power.

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Old Streetfighter Motorcycle Forum Post 10-07-2011, 10:13 PM   #5
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Default Re: Converting a Needle Tach to LED

Ballz. i was hoping it was that easy. guess i'll have to do some serious self-skooling, cuz i've seen some of those tach projects and the schematics got me thinking about scrambled eggs. waaayyy beyond the scope of my brain right now.

so on a related note, i wound up buying some LEDs to replace the idiot light panel on my cluster. and i don't mean use the LED's to replace the bulbs. i'm looking to replace the whole instrument panel eventually, just doing it one part at a time. the idiot lights seem to be a good project to build my knowledge upon. anyway, the LEDs all have a max voltage of about 2.5 volts. according to the wiring diagrams i've seen of my bike, the bulbs in the cluster are getting 12v. that would mean i'd need to lower the voltage going into the LED to avoid meltdown. do i have that concept right?
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Old Streetfighter Motorcycle Forum Post 10-07-2011, 10:19 PM   #6
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Default Re: Converting a Needle Tach to LED

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If you're interested in projects like this, once you master schematics you'll feel like you've gained a super power.
I want that superpower sooooo bad! That's it, i'm buying a book tomorrow!
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Old Streetfighter Motorcycle Forum Post 10-08-2011, 01:29 AM   #7
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Default Re: Converting a Needle Tach to LED

LED's require a resistor. The size of the resistor depends on the LED and operating voltage.
Post up the LED specs and we'll tell you what size resistor you should use... We need the "forward voltage" and "forward current". Sometimes it says on the packet, or you can ask the supplier...

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Old Streetfighter Motorcycle Forum Post 10-08-2011, 08:33 AM   #8
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Default Re: Converting a Needle Tach to LED

Okay here goes. some have detailed info, some don't:

Yellow 5mm LED
2.1 typical, 3.0 max forward voltage
20mA typical, 30mA max forward current

Red 5mm LED
1.7 typ, 2.4 max forward voltage
20mA forward current (no max indicated)

Blue 5mm LED
5.0 typ, 6.0 max forward voltage
30mA forward current (no max)

Green 5mm LED
2.1 typ, 2.8 max
30mA forward current (no max)
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Old Streetfighter Motorcycle Forum Post 10-08-2011, 09:49 AM   #9
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Default Re: Converting a Needle Tach to LED

Quote:
Originally Posted by JLinde1339 View Post
Okay here goes. some have detailed info, some don't:

Yellow 5mm LED
2.1 typical, 3.0 max forward voltage
20mA typical, 30mA max forward current

Red 5mm LED
1.7 typ, 2.4 max forward voltage
20mA forward current (no max indicated)

Blue 5mm LED
5.0 typ, 6.0 max forward voltage
30mA forward current (no max)

Green 5mm LED
2.1 typ, 2.8 max
30mA forward current (no max)
Ok well here is the formula to work out the resistor needed...

R= (V1 - V2) / I
V1> Supply voltage
V2> Forward voltage
I> Forward current

So for your yellow LED:
12 - 2.1 / .02 = 495.
So yellow needs a 495 ohm resistor. The closest you'll find without hunting around will be a 560 ohm.

Red:
12 - 1.7 / .02 = 515
Again the closest will be 560 ohm.

Now your blue LED specs seem a little out dude. 5Vf seems a bit high mate. I say this because I currently have over 400 blue LEDs here that I had no specs for and when I researched for the specs ALL brands were around the same, being 3.2 to 3.4V... Also the calculator below says so
I would double check with your supplier that those specs are right, then use the formula above...

Green:
12 - 2.1 / .02 = 330
330 ohms is a common value so you should easily find it...

Now of course after all that you could just use an LED resistor calculator instead of doing it the hard way!
http://led.linear1.org/led.wiz

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Old Streetfighter Motorcycle Forum Post 10-08-2011, 01:54 PM   #10
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Default Re: Converting a Needle Tach to LED

Yeah, i thot the blue one was a little odd myself, but figured there must be a reason. i dunno. its a radio shack brand, if that helps you out any. i started on their website then went to the store. let me just say, the store didnt have everything the website had, and the way the LED drawer was, it looked more like a lost and found. anyway, props for the help on this, and pointing me in the right direction. big thx
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