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Simplify Voltage I/O - if possible.

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rage

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Hey all!

I'm creating a circuit for my motorcycle that is driven off of the Gear position Indicator, but that's not my problem. My problem is with the power. I have 2 wires that I can use for power, to simplify the project. One wire provides 5 volts when in neutral, but much less when in other gears. The other wire provides 13 volts in every gear, but provides no voltage when in neutral.

I'm running a Pic microcontroller, so I need a consistent 5 volts.

My thought was to run each wire to a LM340T5, then a Diode, then connect them both and them and then run them through another LM340T5 to get a consistent 5 volts. I'm wondering if I'm making this too difficult and if not, can someone suggest the right diode for the job OR if there is a much easier way - please don't hesitate to point it out - clearly.

Thanks,

darrin

 
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kchriste

New Member
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Using the regulators that way isn't going to work.
What you need to find out before proceeding any further is if you can draw the required current from the 2 wires and still get your 5V and 13V. ie: Find out how much current your PIC project will draw and then try drawing that many milliamps from each of those wires while monitoring the voltage at the same time. Another thing: The 5V from the the one wire would have to be very stable and noise free to power the PIC. I highly doubt that it would be so.
It really begs the question: Why can't you just use the power from the battery via the ignition switch?
 
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rage

New Member
It really begs the question: Why can't you just use the power from the battery via the ignition switch?
To be honest, it has to do with ease of installation for the end users. Another company is already using this type of power connection.

Using the regulators that way isn't going to work.
What you need to find out before proceeding any further is if you can draw the required current from the 2 wires and still get your 5V and 13V. ie: Find out how much current your PIC project will draw and then try drawing that many milliamps from each of those wires while monitoring the voltage at the same time. Another thing: The 5V from the the one wire would have to be very stable and noise free to power the PIC. I highly doubt that it would be so.
I only need 5 volts that is powering a 7 segment display, so it's not drawing a lot of current.

My only issue is when the motocycle goes into NEUTRAL. I have 13 volts through all the other gears with the "RED" wire, but when it's in NEUTRAL, I need to take the voltage from the "YELLOW" wire (5+ volts in NEUTRAL). The "Yellow" wire changes voltages to signal to the motorcycle's computer as to what gear it is in. I am simply reading those values, from the "Yellow" wire to display the gear via the Pic's ADC. But, those voltages go down to ~2 volts or slightly less when in certain gears and that's where I have the concern.

Thanks for the help.

darrin
 

kchriste

New Member
Forum Supporter
Another company is already using this type of power connection.
How much current does their circuit draw? If yours draw substantially more, you may have a problem.
The "Yellow" wire changes voltages to signal to the motorcycle's computer as to what gear it is in. I am simply reading those values, from the "Yellow" wire to display the gear via the Pic's ADC.
If you draw too much current from the "Yellow" wire you are going to cause the voltage to drop too low and upset the bikes computer by making it think it is in a "~2 volt gear" when it really is in neutral.
 
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rage

New Member
How much current does their circuit draw?
I'm not certain, but it can't be more than 15mA per LED - 7 LEDs total + whatever the PIC uses.

If you draw too much current from the "Yellow" wire you are going to cause the voltage to drop too low and upset the bikes computer by making it think it is in a "~2 volt gear" when it really is in neutral.
That's why I wonder if I can't create something that will only take the voltage from the "Yellow" wire when the bike is in Neutral as to not upset the readings.

Will a Transistor be able to help in this situation? Use "Yellow" input voltage when bike is in NEUTRAL otherwise use the "Red" wire voltage?

darrin
 

kchriste

New Member
Forum Supporter
The issue with the 5V from old Yeller is that there is no headroom for a voltage regulator or even a Schottky diode there if your PIC is running from 5V. You would need a buck-boost regulator for that which is more complicated than a LM340. If you ran your PIC at 3.3V you could get away with a simple low drop out linear regulator and a couple of diodes.
Bill's idea is good. Keep the current consumption low.
 
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