Voltage to frequency using LM2907

[magos]

Hi there I have troubles with LM2907. My problem is that I have to do a voltage to frequency converter which is 466.67Hz = 3.5V where 3.5V represent 14000rpm.
Its for an R6 engine, please please help

I have tried the circuit of the data sheet and does not work. I also tried to insert the correct value of RC which is

V=finxVccxRxC
R=10K
C=0.1uf
RC=1ms

and Vcc=7.5

 

[Klaus]

I'm not sure about the LM 2907 but the tacho I once built used a LM 2917 chip. it worked fine....

Klaus

 

[FRIED]

Are you using the 8pin version or the 14?
edit; Frequency to voltage?

 

[magos]

Version

I am using the 14 pin version

You can see on the attachement

 

[magos]

Re: Version

I am using the 14 pin version

You can see on the attachement

 

[audioguru]

Are you certain that your car has good old breaker points that the circuit is designed for, and not the newfangled spinning magnet thingy? The datasheet shows a different IC for the variable-reluctance type of RPM sensor.

V0 in your circuit should be about 13V, and pin 9 is regulated with a zener diode to about 7.5V. Is pin 9 your 7.5V Vcc?

Did you use a voltmeter to measure the output, not the dead short of a current meter?

The circuit in the datasheet has different capacitor and resistor values. Are these the ones you tried?

 

[magos]

My engine is an R6 engine with the following specifications:

RPM(max)=14000
number of cylinders=4
But I am getting a feedback from the ECU which is 2 pulses per revolution since is a four cylinder. Therefore
14000 at 60seconds
x at 1seconds

x=233Hz
output=233x2=466Hz

A more accurate output is of 466,67Hz

My Vo is 7.5 Volts


I also use the voltometer to measure the output voltage
My circuit is using the capacitor values that I have in the diagram i designed to produce the following characteristics

V0=7.5 V @ 466.67Hz or 14000 ERPM (4 Cylinder Engine)
Vout=3.5V

 

[audioguru]

Hi Magos,
1) In your 1st post you used an LM2907. But your schematic shows an LM2917. What do you have?
2) Most circuits in the datasheet use a 9V-16V car battery and the 470 ohm resistor you are using drops it for the zener diode regulator in the LM2917. You say that your Vo supply is only 7.5V but the IC's current through your 470 ohm resistor drops the 7.5V to about 6V for Vcc (pin 9). This 6V isn't regulated nor decoupled and is probably swinging all over the place. If you really do have only a 7.5V supply voltage (where from?) then your 470 ohm resistor should be replaced with wire only if you are not using an LM2917 with its zener diode.
3) Most circuits in the datasheet use a 20nF cap at pin 2 but yours has 100nF. I think that 100nF can't charge and discharge properly with input frequencies above about 200Hz. Your frequency is much higher, requiring a cap with a lower value.
4) Most circuits in the datasheet use a 100K resistor at pin 3 but yours has 10K. If you use a 100K variable resistor you should get a 3,5V output with it near half, when the cap at pin 2 is 22nF, and the 470 ohm resistor is shorted out.
5) What is the voltage of the circuit's input pulses from your car's ECU?

 

[magos]

Hi audioguru
1million thanks for helping with my problems

1) You are right my post way for the LM2907 but finally I used the LM2917.
2)I am using a 7.5V source for testing which will be changed initially with a regulator which will change 12V to 7.5V. I will use a wire instead for connecting pin 9 your idea seems very reasonable.
3)So you think using a 20nf capacitor and 100K resistor and finally short pin 9

5) What is the voltage of the circuit's input pulses from your car's ECU?
The supply voltage for the ECU is 12Vots and before feed in a relay is used, don’t know if this answers your question

 

[audioguru]

Hi Magos,
By using a 7.5V supply with an LM2917 you risk blowing its zener diode.
Why not use the 470 ohm resistor on its pin 9 and use a 12V supply like it is supposed to be? Then the zener will regulate its Vcc at 7.5V.

It will work fine when built like in the datasheet, but you don't know the voltage level of its input pulses from the ECU.

 

[magos]

ECU tacho output

hi audioguru

The ECU tacho output is

12 volt square wave 50% duty cycle.

 

[audioguru]

Hi Magos,
Your circuit's input from the ECU is perfect. With the couple of changes I recommended, it should work fine.

 

[ido2]

Sorry for bumping such an old thread, I might have started my own, but here's my question:



at no rpm (in input from Fin) should the output voltage be 0 ?

if not, I'd like to post my circuit which is very similar to this one and ask what could be wrong

 

[steev]

i have seen one circuit in which LM2917 is powered directly from 12v.so is it possible to power LM3914 with 12v.actually my bike(dirt) uses 12v supply and i don't want to use the voltage regulators bcz of space problem.
so is it possible??

 

[audioguru]

You can use a 12V supply for an LM3914 if it is in the dot mode. If it is in the bar mode and the LED currents are high to make them bright then the LM3914 will melt when all the LEDs are on.

The datasheet shows how to connect a series resistor to the LEDs to share the heat dissipation and a bypass capacitor.

 

[student1616]

thats cool.
i am planning to built below circuit.can u guys tell me what components shld i change to make it work on 12v(12-15v) supply.
https://members.tripod.com/~foz11/DIY_MOD_s/Digital_Speedo/digital_speedo.html
just name the components i'll modify the diagram and will provide eagle board so that other can make one easily.

 

[audioguru]

The circuit in your your new link will bpow up the 7.5V zener diode voltage regulator at pin 9.

The circuit you had needed a 12V or higher supply. It also needs to have a 10k ohms output pot because the 350 ohm pot is dragging its supply voltage way down.

It is very important to know if you have breaker points or a swinging magnet thing called variable reluctance as an input to determine if you have the correct IC.

 

[student1616]

thanks for the reply guru
my vehicle uses CDI ignition system.so i am sure i'll be getting pure sine wave.
and it will be great if some one could give me the rough diagram of LM2917,LM3914 powered with 12v supply
forgive me if i sound dumb
regards

 

[audioguru]

If your car doesn't have breaker points (nothing to do with CDI ignition which also could have breaker points) then it has a low level AC input and you should use the LM2917-8 which has only 8 pins. The LM2917 with 14 pins has a positive input only for use with breaker points.

Get the tachometer working before adding an LM3914 display.