# LM2907 using questions

#### davidbr

Joined Apr 25, 2018
60
Hi all,
I am using a LM2907 chip to read a wheel rpm. Basically I have a metallic wheel and a whole in it, where I put an inductive sensor which is closed or open depends if is on the whole or on the wheel. The circuit is working but still I have some "issues / questions" about it.
The schematic I use is this - lm2907.png
The senor is also operating at 12V. I have the wheel connected to an electrical screw driver and when is not moving I read about 0.3V and when is moving on maximum speed I read about 2.1 V. The readings are done using an ads1115 and an arduino nano.
Now my questions :
1 - When I start moving wheel the voltage is increasing pretty fast till maximum, but when I stop the wheel it needs about 2 or 3 seconds till get back at 0.3V . How can I change the circuit for get a faster response ?

2 - How can I calculate the frequency I read using voltage I read?

Thank you very much

#### ericgibbs

Joined Jan 29, 2010
15,726
hi David,
What is your best guess at the actual frequency of the sensors pulses, also the duration of the pulses.?
You could use the ADC input of a an Ardunio and then calc actual frequency, but you need some ref rotation/speed point.

E

#### jpanhalt

Joined Jan 18, 2008
11,088
From the datasheet:
It appears R1 can be chosen independent of ripple, however response time, or the time it takes VOUT to stabilize at a new voltage increases as the size of C2 increases, so a compromise between ripple, response time, and linearity must be chosen carefully.
Your C5 (datasheet = C2) may be a bit large.

From a broader view, you are changing a frequency (digital) to voltage and voltage back to digital. Why not use the arduino to measure the frequency directly?

Joined Mar 10, 2018
4,057
Note C2 (datasheet C2) being electrolytic typically has very loose tolerance
from one part to another. So consider than when choosing a cap. Maybe a
MLC would be a better choice. Datasheet shows a typical of 1 uF.

Regards, Dana.

#### davidbr

Joined Apr 25, 2018
60
Maybe sounds stupid but what is "MCL" ?
Thanks

#### AlbertHall

Joined Jun 4, 2014
12,072
Maybe sounds stupid but what is "MCL" ?
Thanks
Multi-layer ceramic.

#### davidbr

Joined Apr 25, 2018
60
Thanks a lot
Will try with 1uF for some tests and see if change something

#### eetech00

Joined Jun 8, 2013
3,231
Hi all,
I am using a LM2907 chip to read a wheel rpm. Basically I have a metallic wheel and a whole in it, where I put an inductive sensor which is closed or open depends if is on the whole or on the wheel. The circuit is working but still I have some "issues / questions" about it.
The schematic I use is this - lm2907.png
The senor is also operating at 12V. I have the wheel connected to an electrical screw driver and when is not moving I read about 0.3V and when is moving on maximum speed I read about 2.1 V. The readings are done using an ads1115 and an arduino nano.
Now my questions :
1 - When I start moving wheel the voltage is increasing pretty fast till maximum, but when I stop the wheel it needs about 2 or 3 seconds till get back at 0.3V . How can I change the circuit for get a faster response ?

2 - How can I calculate the frequency I read using voltage I read?

Thank you very much
Hi

1. You can get a faster response if you lower the value of C5 and increase R7 while retaining the same RC constant.

2. Frequency calculation. I believe the equation would be:
Freq=Vout/(Vcc*R7*C5*K)
Where K=I2/I3
And
I2=AvgCurrent Pin2
I3=AvgCurrent Pin3

eT

Last edited:

Joined Mar 10, 2018
4,057
Should have been MLC, my typo.

Regards, Dana.

#### ScottWang

Joined Aug 23, 2012
7,244
Hi

1. You can get a faster response if you lower the value of C5 and increase R7 while retaining the same RC constant.

2. Frequency calculation. I believe the equation would be:
Freq=Vout/(Vcc*R7*C5)

eT
Freq=Vout/(Vcc*R7*C4), C5 is a bypass capacitor, Pin 2, 3 all Charge Pump pins, please check the circuit in page 9 and the equation 2 in page 10 of the datasheet.

LM2907 datasheet.

#### eetech00

Joined Jun 8, 2013
3,231
Freq=Vout/(Vcc*R7*C4), C5 is a bypass capacitor, please check the circuit in page 9 and the equation 2 in page 10 of the datasheet.

LM2907 datasheet.
The equation I've shown refers to reference designators used in the circuit posted by the TS in post #1.

eT

#### ScottWang

Joined Aug 23, 2012
7,244
The equation I've shown refers to reference designators used in the circuit posted by the TS in post #1.

eT
You should check the R1, C1 in the original circuit (datasheet page 9) first and then comparing with the circuit in post #1.

#### eetech00

Joined Jun 8, 2013
3,231
You should check the R1, C1 in the original circuit (datasheet page 9) first and then comparing with the circuit in post #1.
Page 9 is a LM2907/2917 16 pin. That's not the same circuit. Look at page 1 or page 14.

eT

#### ScottWang

Joined Aug 23, 2012
7,244
Page 9 is a LM2907/2917 16 pin. That's not the same circuit. Look at page 1 or page 14.

eT
Yes I saw it, but they still are the same Pin 2, Pin 3 and I what I emphasized were the CP1 (Pin 2, Capacitor), CP2 (Pin 3, Resistor).