Project Description

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Project description
Note, the component values were what I had ready at hand, not optimized for anything but expedience! :) Control signal input can be driven by any uC's GPIO, I used an Arduino for all the usual reasons. In the absence of an input, Q2 does not conduct, so R2 pulls Q1's gate high, and because it's P-channel, it doesn't conduct either. In this state, crank voltage (typically 6-7 volts) flows through D2 to the DUT. Drive the control signal high to turn on Q2, which pulls Q1's gate low, and run voltage (typically 13-14 volts) flows through D1. D2 becomes reverse-biased and no longer conducts, so the DUT is powered exclusively by run voltage. Bypass SW1 allows the DUT to be manually forced onto the run supply, averting the next scheduled crank dip while the operator works on the DUT. An LED on Q2's base might be nice to add.
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Design Files

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Component (7)
Qty
Description
SPST Toggle

SW1

1
Generic General Switch
Resistor

R1

1
Generic Resistor (100)
2n3904

Q2

1
Generic BJT (NPN)
1n5822

D2

1
Generic Schottky Diode

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