Question for Electronics Wizards.
 

Can someone tell me how to "pull down " a 6 volt signal to 3 volts?
Resistor? Where? How?
P.S. Please don't tell me to cut the wire in half, I know that much. :D

The quick easy answer is Radio Shack.
Go there and buy an assortment of resistors, pretty much any of the standard resistor assortment packages they have are fine since you are talking low amps, low voltage. Grab your DMM (digital multimeter) and 4 d cell batteries. Electrical tape the 4 batteries together flashlight style and tape a red wire to the positive end and a black to the negitive end of your battery stack. You now have 6 volts. Try different resistors attached to the red wire and the resistor to the DMM red lead and the black wire to the DMM black lead. One or two of the resistors will get you very close to 3 volts. that is the elementary method for you.
The expert way is to read the color bands on the resistors and find the one that completes the formula to reduce 6 to 3 volts.
The way I would do it is turn my bench power supply to 6 volts and get my Fluke DMM and try a few of the assorted resistors I have until one gets me from 6 to 3 volts. More than one resistor can be used in line.
I would try a resistor in the range between 100 ohm and 800 ohm is I remember correctly. The DMM will tell you right way if it is even close.
Good luck

@ crhenkel

Thank you, I will do it the way you described. ;)

I don't really see how this question can be best answered with the limited info given. There is the obvious question of how much current will you draw at 3V? If you have more details there may be a better answer. However, you seem happy with the answer given.

That would not work because you have no current. The voltage drop on the resistor depends on the load. What are you trying to do?
Mike

It will work if the load is very low. But without the details it is impossible to make a good recommendation.

I mean the measurement method crhenkel suggested would not work. Assuming meter has infinite resistance there would be no current in the circuit and the meter would still show full power supply voltage.

What sort of signal are you modifying, on/off? If so you can use an LM317T voltage regulator and a few external parts. Cost about $3 at Radio Shack.

Thanks for all the input.
I currently have a signal ( generated by an alternator ), that is 6 volts, going to a tachometer.
I'm trying to " chop " it down to 3 volts. ;)

You might have better luck modifiying the alternator. The signal won't be plain old 6V. It's got to be a wave form, so a simple resistor or voltage regulator isn't going to work.

Yes, you are right. I thought he was describing a divider, but actually it is hard for me to understand what was being described.

Usually "signal" implies some information is on it, such as a waveform, as was just mentioned. But since you say it comes from the alternator maybe it is just the power supply? It makes a big difference! Do you have a tach that needs a 3V power supply? That seems kind of odd. Is it usually battery powered?

Try a pair of back-to-back zener diodes, fed by a resistor.

This is as bad as an oil thread...

Assuming that you don't need much current for a signal, put the 6 volts through a 10K resistor, and then connect a 2.8-3.0 volt Zener diode from that to ground. A 1N4148 diode in series with the 10K resistor would not be a bad idea in case of a failure (don't want anything feeding back to the 6 volts).

It might not be a good idea to use a 317 regulator for this because its response time might not be that good and it's "instantaneous" regulation isn't very good without capacitors-which will smear and distort the signal that you want.

This assumes that the needed signal is "digital" - that is, a number of pulses per second, and not analog. My understanding of an electronic tach is that it is effectively digital.