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Can I do this without issues?

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strokedmaro

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The attached schematic shows the current setup on the left and the proposed setup on right. Currently my car needs to see four 25Ω solenoids..each grounded through a switch in the computer. Is it possible to use just one dummy resistor instead of four as Ive drawn? Its also important to still be able to tell which switch in the computer is active so Ive also included a pull up resistor one one switch as a demonstration...the test area is where a PIC will be connected with a resistor. SoOOOoooO, the PIC needs to be able to tell which of the four switches is closed, while the computer just needs to see a 25 ohm "solenoid" to prevent a check engine light from coming on. Is this possible? THANKS!
 

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strokedmaro

New Member
Each switch will see 25/2 ohms, if R=25ohms.

The purpose is to get away from using four 25 watt resistors and use a single one for all four switches. I was thinking a much higher value for each pull-up (R) so the paralleled value would be negligible. Lets say I used 10K, 1/8 watt, pull-up resistors so the computer would see 24.938 (paralleled). Anybody else have input?

EDIT:: I guess the question should be if all four switches are closed would the computer be able to see the approx .5 amp current draw from the single resistor on all four switches?
 
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BrownOut

Banned
If you are truly measuring current, then each switch would need it's own resistor. However, if all switches are connected to the same soliniod, the current would depend on how many switches are active, as well as the connected resistor. One solution would be to use one transistor per switch to isolate the switche's current from the soliniod. Drive the base of the transistors with a 25 ohm resistor.
 
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strokedmaro

New Member
If you are truly measuring current, then each switch would need it's own resistor. However, if all switches are connected to the same soliniod, the current would depend on how many switches are active, as well as the connected resistor. One solution would be to use one transistor per switch to isolate the switche's current from the soliniod. Drive the base of the transistors with a 25 ohm resistor.

Im not sure I understand why thats the case...if one switch is closed (grounded) the the computer knows the solenoid is good (25 ohms) if it see's a current load of .5 amps (this is at 13.5 vdc car power).....why would a second ground from the computer (same potential as the first switch) cause the current to be anything different?

EDIT:: could you draw out the transistor idea? That sounds promising.
 
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BrownOut

Banned
First you said the computer needs to know which switch is closed. Then you asked if the computer can see .5 amps, which I assume means at the switch(s). Sorry, I cannot understand your problem. Maybe someone else can help.
 

strokedmaro

New Member
the computer doesnt need to know which switch is active...its whats controlling the switches. The pic however still needs to tell the difference between which switch is closed which is why I want to add the pull up resistors and diodes. Without the pull up resistors the first switch thats closed will remove the high signal from the other 3 PIC inputs. However, the computer needs to be able to see .5 amp draw when each switch is closed....if the first switch is closed that switch needs to show .5 amps drawn...if 3 of them are closed each one of the 3 needs to show .5 amps.
 
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