tachometer.

[mechtronics]

cool . for protection i could use a 12 volt zener diode from the signal input to ground maybe??

 

[duke37]

I would go with the circuit linked to your first post, using a zener, three resistors and a capacitor.
That circuit shows a regulator at 8V, this may be a bit low to run the 7.56 internal zener, I would push it up to 10V.
Have you thought about more leds, say greens 3000 to 5000, yellow 5000 to 7000, red 7000 to 9000. Better get this circuit working first though.
The spec says 28V max so you should get away without a stabiliser.

 

[mechtronics]

oh right ok, ya il have to build this one first and get it going!!! i could run the circuit at all 12 volts i spose? no i havent thought about doing that i am trying to build it for a friends car and he only wants it to light at 7000 rpm up wards. i could easily calibrate it to start at 0 volts taking away the 35 k resistor on pin 4 of the lm3914 and connecting it straight to ground i spose then varying the resistance to make the top end voltage to be 5 volts. setting the lm2917 to 0.5v/1000rpm if i want it to go up in increments of 1000 rpm like. by doing what u said with the leds can this be done on one lm3914 or would u have to add in more IC's?

therefore

0.5v=1000rpm
1v=2000rpm
2v=3000rpm
etc.

 

[duke37]

In your last post, with 2000rpm/V then 2V represents 4000rpm.
If you want 10 leds, use one LM3914, if you want 20 leds, use two LM3914 and for 30 leds use three LM3914. Start with one LM3914 and get that working but consider how you might fit more leds.

 

[mechtronics]

i get you about the three lm3914's.

but wat i am saying if i could change the circuit to do from 1000 rpm up to 9000 rpm i could adjust the output voltage from the lm2917 to give me 0.5 volts per 1000rpm could i?? could this be done i wonder?? therefore 0.5 mA through each of the 1k resistors.? or i could still even set it like you said to 2000rpm/volt still getting 1000 rpm display intervals.?

 

[duke37]

Make a decision on sensitivity and offset and set the two resistors to suit.
7000rpm seems to be an excessive speed to start the display. I rarely run my engine above 3000rpm but then, I have to buy my own petrol!

 

[mechtronics]

this is for a rally car. when my friend is driving it on the track he has a programmable ecu you see which can do quite a lot of things. he has it programmed to not drop below 7000rpm when changing gear when he is racing round the track. thats the reason i needed it built that way .

but listen thanks for all the help and i will start building the circuit as soon as i get the components for it. will let you know how i get on with it and if it works or not!!!

thanks again.

 

[mechtronics]

im bak again!!! i have this built but when i put my finger on the signal input to pin 1 the led's light up is this supposed to happen??? would a frequency generator be ok to test it with?? i had to use the 14 pin instead of the 8 pin version of the lm2917, my friend ordered the 14 pin by mistake there isnt much ofa difference anyway just 1 or 2 changes were needed in the wiring but thats it.

 

[duke37]

It looks as if it is working. When you touch the input with your finger, you are putting in mains hum, 50 or 60 Hz. I hope you have a protection circuit in the input.
A signal generator would be ideal to set up the calibration.
Good luck.

 

[mechtronics]

i had it working perfect but now it isnt i think its something got to do with the capacitor values at pins 1,2, and 3 of the lm2917. when it was working ok the values i used were:

on pin 1 = 0.1uf
on pin 2 = 0.68uf
on pin 3 = 2uf

but i only had one of each!!!!!

what values would you recommend??

the problem is when it is on and you rev up the car all the led's light up but not in sequence and they seem to be on and off very fast from the bottom led upwards i think myself its the different capacitance values affecting it because now when i try to achieve a 1 volt output for every 2000rpm the voltage isnt a steady output from the lm2917 maybe from capacitance values being different i wonder??

 

[duke37]

The capacitor on pin1 smooths the input signal to stop spurious triggering, it is a compromise. The original reference specifies 22nF and you have used 100nF so it should work at low revs but maybe not at high revs.

The capacitor on pin 2 affects the output volyage, they recommend 47nF and you have used 680nF, that seems very high.

The capacitor on pin 3 smooths the output, they recommend 2.2uF and you have used 2uF. Where did you find a 2uF cap?

If the voltage is not steady, I would suspect poor grounding or a poor input circuit.
What do you mean you have you only have one cap of each, you only need one.

If it worked, why did you change it?

 

[mechtronics]

ok i have it going on my car for testing its working grand, there is just one thing that bugs me about it is the fact that when i have just the ignition on without the engine running and i press the accellerator to the floor very fast and back again the led's light up in a row and go back off again fast this happens if i keep pressing it on and off fast is there any way of getting rid of this i tried low pass filtering but didnt seem to work.??

 

[(*steve*)]

i press the accellerator to the floor very fast

Can I suggest that you stop pumping the accellerator?

Does the unit operate correctly when the engine is running?

 

[mechtronics]

i meant to post this up a while ago have the tachometer working perfectly you can see it on this link on youtube let us know what you all think of it cheers.