Hello all! I am designing a circuit to bypass a dash switch. When the button on the dash is pushed, the sensing circuit in the ECU goes to 0v which is recognized by the ECU and it disables the function, then illuminates an LED in the switch. I can't use a short pin across the switch because the ECU will recognize no change in state on the sensing circuit across multiple key cycles, set a fault code, and disable the button.
So, I built the circuit in the replacement module below. The 4.7K resistor is to provide current flow similar to the LED (so the ECU still sees current flow on that circuit, and does not set a fault code). The PNP transistor serves to effectively hold the button closed, allowing current flow, until the the LED signal goes high(which indicates the function has been disabled), after which it stops current flow, allowing the sensing circuit to go high again. Pin 9 is for the LED illumination and goes from 6-11 volts depending on whether the dash lights are in day or night mode. Pin 17 is for the sensing circuit and Pin 22 is a common ground.
My big question is how do I choose the best transistor part number to use for my application? I want something that will be comfortably within its operating specifications, with low cost and long term reliability. I am currently using a S9015, which works, but I do not know how to calculate the best (or correct?) part number to use for my application. Looking around there is so many options I am having a hard time sorting them all out to find the ideal transistor to use. Anyone with more experience that has advice would be immensely appreciated!
So, I built the circuit in the replacement module below. The 4.7K resistor is to provide current flow similar to the LED (so the ECU still sees current flow on that circuit, and does not set a fault code). The PNP transistor serves to effectively hold the button closed, allowing current flow, until the the LED signal goes high(which indicates the function has been disabled), after which it stops current flow, allowing the sensing circuit to go high again. Pin 9 is for the LED illumination and goes from 6-11 volts depending on whether the dash lights are in day or night mode. Pin 17 is for the sensing circuit and Pin 22 is a common ground.
My big question is how do I choose the best transistor part number to use for my application? I want something that will be comfortably within its operating specifications, with low cost and long term reliability. I am currently using a S9015, which works, but I do not know how to calculate the best (or correct?) part number to use for my application. Looking around there is so many options I am having a hard time sorting them all out to find the ideal transistor to use. Anyone with more experience that has advice would be immensely appreciated!