LED tachometer project

[dmc0162]

I am trying to make an LED tachometer to use in my car.

I was trying to go by this circuit here:

I built it on a breadboard, and for the life of me I couldn't get it to work. No matter what, I I would only get the first LED to light up, and the second LED would be dimly lit, with no change with the input signal.

The tachometer signal on my car is a +5-volt square wave. Has anybody ever done any sort of similar project, and maybe has a different schematic I can build, or any advice on what might have gone wrong in my circuit?

Also, I can't tell if pins 8 and 9 in the bottom LM3914 are supposed to be connected or not, because the lines cross, but there is no dot. It is inconsistent with the rest of the drawing.

 

[duke37]

I think the lines should cross but not connect, you can check this by looking at the data sheet.
You should separate the F to V convertor from the display (use a potentiometer as substitute) to get the display working first.
Is the 2917 producing output pulses, should there be a capacitor across C9 to give a smoothed output?

 

[Rleo6965]

Maybe it’s better if you isolate first if LM2917 have varying dc output w/ corresponding input signal from car signal. This is to simply troubleshooting your circuit.

 

[mechtronics]

Pins 8 and 9 on the lm 3914 should not be connected. pin 8 is only for a reference adjust output voltage. pin 8 can be connected through a resistor to give a different output reference voltage. i have built one of these with one lm 3914 and it works perfectly for me. but i used two variable resistors: one to set when the led's come on for a specific rpm for when the engine is at low revs and two: for when the engine is at hight revs. try using a 10nf capacitor going into pin 1 of the lm2917, a 47nf on pin 2 of the lm2917 and a 2.2uf on pin 3 of the lm 2917 these caps have to be spot on to get proper speed of linearity display to appear. if cap values are too high the display will not work or could be too slow when working. i never used a voltage regulator either becuase the lm3914 can withstand up to 35 volts without damage or false output readings, got from the data sheet. and the lm 2917 can withstand up to 28 volts but using one should not effect it. try using a 100k pot for trimming on the lm2917. i also would suggest that you disconnect the F to V converter from the display and just connect that up to the coil of the car to see if u r getting a change in voltage with respect to frequency when revving the engine. and testing the trimmer pot to get a certain voltage to suit a certain rpm for example 2000rpm/volt.

 

[verisian9]

Sorry if this thread is a little old, I am attempting this circuit as well. Just to clarify, Mechtronics, your suggestions to the circuit are:


20120728_171118 by benji325, on Flickr

Edited: fixed picture url

 

[mechtronics]

my version of it.

the 8 pin version of the chip would be a lot easier, it also minimizes a lot of wires that would be on the circuit.

use a 12 volt zener diode, forgot to put the value in diagram sorry!!

 

[verisian9]

I have drawn up the freq to voltage section using the 8 pin lm2917, i had a look at the datasheet to make sure i had the correct pins


f to v LED Tacho by benji325, on Flickr

How do you attach images without linking to another url?

 

[(*steve*)]

How do you attach images without linking to another url?

See Here.

 

[mechtronics]

i put wrong values in the first time here is the proper circuit with component values sorry!!!

 

[verisian9]

Thanks Steve, I realised once you posted that I should have searched on the forum/read beginner threads.

Mechtronics, I am relatively new to circuit design (at uni we have only done some circuit analysis but we have not had to choose certain values for components yet) and I would just like to check with you about the caps and zener diode ratings:

Is it ok to use:

-0.022uF 50V Ceramic Capacitor
-0.047uF 50V Ceramic Capacitor
-2.2uF 50V Radial Electrolytic Capacitor

and a 1 watt 12V zener diode?

I am not sure about the use of ceramic vs. electrolytic caps, apparently it depends on the frequency and capacity required by the application.

What would be the best resource for learning about this kind of thing?

Thanks very much for your help

 

[mechtronics]

yes those values should be ok to use its the same thing any way just specified in microfarads etc. zener diode should be ok to use too.

i would recommend using google to learn more about capacitors, ceramics and electrolytic. plenty of info on them to read up on.

the ceramic material on the capacitor acts as the dielectric, and as for the electrolytic capacitors they use an electrolyte.

with respect to the values here is a nice conversion chart to look at just found it on google there.

http://www.justradios.com/uFnFpF.html

 

[verisian9]

After waiting a very long time for parts, and getting distracted by work and other things I finished this circuit. I couldnt get the lm2917 to work properly (lm3914s worked well with a pot).

The data sheet says:
"Two options (LM2907-8, LM2917-8) have one input internally grounded so that an input signal must swing above and below ground and exceed the input thresholds to produce an output. This is offered specifically for magnetic variable reluctance pickups which typically provide a single-ended ac output. This single input is also fully protected against voltage swings to ±28V, which are easily attained with these types of pickups."

I am fairly sure that my car has uses a magnetic variable reluctance pickup in the distributor (It is a 1986 carbureted honda civic).

I measured the voltage of the tach signal wire (from negative coil terminal) and it is around 28V AC (which seems a bit high compared to other people's accounts on the internet).

The lm2917 is not showing any variation in output voltage w.r.t a change in frequency, any ideas?

Maybe I inadvertently fried the lm2917 or have incorrectly connected the circuit

Edit: I checked everything and when I apply power all of the LEDs light up, if i remove the signal from the lm2917 to the lm3914 and replace with a potentiometer i can cycle the leds as a bar graph. Would I possibly just need a potentiometer with a larger resistance for the reference voltage on the lm2917?