DIY LED Tacho

**Mattzxr750** · 3rd August 2008, 13:47

Okay i have some questions for all the electrical gooroo's out there. i found this curcuit on the web. And was think about building it.

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It looks simple to build i was just wanting to know.
1. Will it work? does the curcuit make sence?
2. can the voltage regulater LM317T (1-27v output) which is there to limit the voltage to 8 volts, can it be replaced with one that is is set to 8v, removing the 50k pot?
3. and in doing that will it effect the curcuit in any other way?

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I have started to build it, and it doesnt want to work. It may have been some thing I have done. I just want to rule that out

Any help would be great

**dhunt** · 3rd August 2008, 14:03

Replacing the LM317,VR2 & the resistor labelled 510 with a LM7808 or similar 8V voltage regulator would not change how the rest of circuit behaves.

I'm not familiar with the two chips but nothing looks particularly wrong at a glance. What exactly does "it doesn't want to work" mean?

Do you have access to any test gear? Multimeter etc? The first step would be to check that you have 8v on all the approriate pins etc.

Give me a few more details and I might be able to give you some better advice.

**Mattzxr750** · 3rd August 2008, 14:12

Okay, after building the curcuit and connecting it up, i notist that when the revs on the bike change the output from the LM317 changes.

When I connected the input wire upto the tacho on the bike no thing happened, no leds came on, there was a small amount of power at the leds.

As for test equipment i have a muiltimeter but thats about it

**dhunt** · 3rd August 2008, 14:27

OK here's some questions/things to check?

1) What does the output voltage of the regulator fluctuate between?

2) I would suggest putting capacitors on the input/output of the regulator to help improve the voltage regulation. Say a 10uF electrolytic (This can only go one way round) on the output and a 100n ceramic on the input for a start.

3) With the bike off. You should be able to adjust VR2 to get a nice 8V output.

4) What are you using as the input to this circuit?

5) The first chip is a frequency to voltage converter. On pin 5 of the 2nd chip (output of the first chip) you should see the voltage fluctuate with rpm. If doesn't happen the problem is probably with the 1st chip.

This should hopefully give you a bit to get started on. Let me know how you get on.

**Mattzxr750** · 3rd January 2009, 12:32

After having more time to find better better curcuits i came across this one. Im hoping that it will be better due to the fact that it has more info:

http://www.t-r-j.com/Auto/Tach/tachometer.htm

Going to start working on it this week. any one got any idea if it will work or not?

**xwhatsit** · 3rd January 2009, 13:26

Originally Posted by Mattzxr750

After having more time to find better better curcuits

I don't think there was anything wrong with the original circuit -- just guessing, but sounds like you cocked up the voltage regulator somehow (you said output voltage was rising and falling with RPM?) and fried the logic ICs. CMOS is pretty delicate, I always seem to cook them just by touching the pins accidentally

Your new circuit is even more complicated and has even more fragile ICs that need soldering (may I suggest using IC sockets?). I know diddly-squat about whether it will work or not but people generally don't upload non-working schematics with instructions onto websites to trick people.

**Mattzxr750** · 3rd January 2009, 14:34

Originally Posted by xwhatsit

I don't think there was anything wrong with the original circuit -- just guessing, but sounds like you cocked up the voltage regulator somehow (you said output voltage was rising and falling with RPM?)

I think you are right about that.

Originally Posted by xwhatsit

Your new circuit is even more complicated and has even more fragile ICs that need soldering (may I suggest using IC sockets?).

and this time i will use IC socket so i can reuse the ICs over and over, every time i stuff it up

**davereid** · 3rd January 2009, 15:41

First circuit and second circuit are pretty similar, the second one has better protection for the frequecy/voltage converter, and adds extra LEDS with a second cascaded bar graph driver.

I quickly looked up the chip doing the freq to voltage conversion - I could only find an 8 pin device with that part number, not the 14 pin device in the sketch, so I guess they must be using a different variant.

Stick with the first one.

**Mattzxr750** · 3rd January 2009, 16:32

Originally Posted by davereid

First circuit and second circuit are pretty similar, the second one has better protection for the frequecy/voltage converter, and adds extra LEDS with a second cascaded bar graph driver.

I quickly looked up the chip doing the freq to voltage conversion - I could only find an 8 pin device with that part number, not the 14 pin device in the sketch, so I guess they must be using a different variant.

Stick with the first one.

Looking around i found a way to swap the LM2907-8N(8 pin) for the LM2917(14 pin)
http://www.edaboard.com/ftopic186142.html

Ever where i look i find a different version all basicly the same, just with very small differences. I think i will stick with the 2nd one because its closer to what i want. so tomorrow I go and get the parts, and build.

But today i plan and plan and ......
plan

Thanks for the input so far, anymore would be a great help.