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# Shift Light - Using LM2907, beginner in need of help.

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#### David Bartlett

##### New Member
Hi all,

You have a great forum going here and I'm hoping you can help me out.

I'm looking to build a simple shift light for my car, so at 5400rpm the light comes on.

I'm hoping to use National Semiconductors LM2907 (more info on the chip here - **broken link removed** )

My car outputs 270hz = 5400rpm, power souce is auto 12v(but not a steady 12v remember)

Here's the plans from National:

As a begginner, I could easily build the board but I can't for the life of me figure out which capacitors/resistors to use. Would it be possible to use a varaiable resister as R1 so I could adjust the shift point? I plan on using a standard 3.5v Blue LED, so I imagine a resistor will be needed to drop voltage. Maybe some of you guys could look over and provide me with a list of parts, suggestions or any direction.

Thanks,

Dave

National Semi got back to me on some of my questions, heres what they said:
The 270 hz has a time periond of 1/270 = 3.7ms. Table 10 shows a range of
capacitors and resistors to give different time periods.

I would recommend starting with a 220K resistor [selected because it is a
standare value] and a 0.01uF capacitor. The resistor value could be
replaced with a 150K fixed resistor in series with a 100K pot.

Don't be afraid to adjust both the resistor size and the capacitor size for
best performance. You almost can't hurt the part. Page 4 of the
application note gives details of how to select R1, C1 and C2.

The 10K values of the two resistors in the voltage divider and the 500K
hysteresis value resistor should be just fine.

Figure 9 does have one error in that it has two different "R1's." The R1
referred to in table 10 and in the equations is the R1 connected to pin 3.
The two 10K resistors should have been named RA and RB to avoid the
confusion.

------------

Now my question is, what kind of cap to use ->voltage wise and polar or non-polar?

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