DIY TACH was: A little help?

[AK91]

Hi Guys,

So my tachometer died about a month ago and I'm not willing to pay £150 + for a another or aftermarket one. So I done some research and came across a few guides on creating a tachometer. One of them stood out from the crowd which is - this one.

So mostly new to this and didn't really understand the schematics so did a little more research and now I actually understand what is what on the schematics. YAY!

I do have a few questions though and was hoping I could get some advise.

1. According to the guide, the tachometer signal is a frequency which comes from the -ve terminal on the ignition coil. However my cars tachometer gets it's signal from a PWM signal. Would it work if I use the PWM singal wire instead?

2. The LM3914's each run 9 led's and there are two on the schematics. I would possibly need at 25 led's run on the LM3914's in bar form. How would I do this?

3. Lastly, does anyone know where i can get a trimpot? I can't find the 220k one anywhere!

Thanks!!

 

[canadaelk]

The schematic shown is barely readable. To me it seems like a lot of effort for little return. With a 20 LED readout, max 8k RPM you get a resoltion of only 400 RPM. Not good. Now to:
Q1: It most likely would work but you may need to do some signal conditioning
Q2: Each LM3924 has 10 LEDs, a pin on the IC sets it to dot or bar mode
Q3: Use a 200k pot
On ebay I found many tachs for US $20, some with quite interesting displays. Check it out. E

 

[AK91]

The schematic shown is barely readable.

Sorry, I forgot to link in the full sized version, it's here

With a 20 LED readout, max 8k RPM you get a resoltion of only 400 RPM. Not good.

It's a 6k RPM and I want to do a minimum of 25 led's so it would be a resolution of 240 RPM, i would like to do more led's but don't know the correct way of using more than 2 LM3914s.

Each LM3924 has 10 LEDs, a pin on the IC sets it to dot or bar mode

Sorry I didn't type this question properly, what I wanted to know is how I add additional LM3914s, the schematics have 2 but I would like to add at least one or two more.

On ebay I found many tachs for US $20, some with quite interesting displays. Check it out. E

I know I can find some quite cheap, but they are really not what I'm looking for. Plus, I'm more into doing things myself.

 

[alec_t]

what I wanted to know is how I add additional LM3914s

Check the datasheet. It shows how to cascade the chips.

 

[Dipak]

AK91,
If you are interested, I have made a LED tachometer for a car which uses 35 LED's and has a resolution of 200 rpm. The details can be found at:

https://www.youtube.com/watch?v=tuJVJYu8Glw

 

[Boncuk]

Excellent work!

Boncuk

 

[Wannabe EE]

i would like to do more led's but don't know the correct way of using more than 2 LM3914s.

I was once wondering the same thing. After I figured out the answer, I posted this:

https://www.electro-tech-online.com...r-dot-displays-a-complete-explanation.126056/

 

[MrAl]

Hi,

There are much cheaper tachs than that on the web. Go to an auto parts store online. They range $30 USD and up.


Dipak:
Yes very nice work there. Looks very appealing. I like the rectangular LEDs they fit nice side by side. As long as it does not distract you from your normal driving.

 

[Dipak]

MrAl,
Thanks for your comments. The tacho does not distract me when I'm driving and it's no different to having a car which comes with a built in tacho.