[SOLVED] Help need for LM2917 Tachometer Circuit Diagram

[vijay s]

Hai
Anybody Got Experienced with LM2917 as Tachometer.. DataSheet Seems to be very Poor... and i want to use 14 pin LM2197 (pulsed DC input)... if anybody has experience in this, please share circuit diagram for 14 pin LM2917....

 

[keith1200rs]

I used the 8 pin one in 1996! It worked fine. All I can see that is different with the 14 pin is 4 NC pins and opamp +input not internally connected to the charge pump and the comparator -input not connected to ground. So with a couple of external links the 14 pin looks like the 8 pin.

Apologies for the low-tech image - the circuit is on DOS Orcad and I couldn't find out how to get a gif out of it. Anyway, that circuit worked!



Keith

 

[vijay s]

@keith

is there any limit on Voltage level of Frequency Input... and do above circuit works with AC input or Pulsed DC...

 

[keith1200rs]

Hmmm. Good question! One difference I have just noticed between the 8 pin & 14 pin is that the input on the 8 pin can be +/-28V whereas the 14 pin can only be 0V to 28V. The sensor I used produced positive & negative pulses (an inductive sensor with a toothed wheel). So, for the 14 pin I think you need to arrange for the two inputs to always be biased between 0V and Vcc. With the 8 pin version I used it would tolerate the negative pulses.

The input filtering I used was to filter out noise. It was a motor running around 3000rpm with 4 pulses per rev, so 200Hz. That filter is around 700Hz.

Keith.

 

[caslor]

here is one page that has what you need i think...

http://www.piclist.com/images/boards/revbar/index.htm

i want to build the same project but as i can find only 8 pin lm2917 i will go with this.


does anyone know what should i change in the schematic to place a 8pin and not 14pin lm2917

 

[keith1200rs]

For the most part, just substitute the pin numbers:

14pin 8pin
1 1
2 2
3/4 3
5 4
8 5
9 6
10 7
12 8

The problem comes from pin 11. This is biased positive in your diagram because the 14 pin chip cannot have a negative input. The 8 pin version has that internal connection grounded and has to have an signal that goes negative. So, your sensor needs to produce a signal that simply goes around 0V. The circuit I used (see yucky screenshot earlier) was for a magnetic sensor that produced both positive & negative pulses.

If you have something that only produces pulses positive (or negative) with respect to ground you will need to capacitively couple it to pin 1 and put a resistor to ground from pin 1. A capacitor on pin 1 would probably be useful to keep the noise out as well.

Keith.

 

[caslor]

I will take the signal from the coil of the car.. i dont remember now what pulse makes (there we connect all the brand tachometers )


Thanks for the answer my friend

 

[keith1200rs]

In which case, try the input circuit on my screenshot.

Keith

 

[luisjuarez]

Hi, this is also my first time using this chip. But i need to know if this chip can convert the frequency in a negative voltage?? i need this to indicate if a dc motor is going clockwise or not, and measure its speed. Thanks for your help

 

[keith1200rs]

Do you mean negative going pulses? The chip cannot detect direction.

Keith.