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 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. Pin1 is a PWM signal for the LED illumination (waveforms are below). Also Pin1 is floating in the normal switch circuit, R2 serves to provide a path to ground for the base of the transistor and to drain the capacitor after the key goes off.
I am looking for help with component selection and design improvement for my switch bypass circuit. The circuit below works, but I still have some voltage ripple through my transistor. I am not sure if this is going to negatively effect component life, or if it's fine for some ripple to pass through.
On the schematic below, the dash switch module would be removed, and replaced with the replacement module.
The PWM signal looks like this, depending on whether the headlamps are on or off:
Headlamps OFF:
Headlamps ON
My biggest questions are:
Is there an obviously better way to do what I am trying to accomplish that is simpler?
Should I have any concerns about component life given the PWM from pin 1?
How do I choose a transistor that is BEST for this application? Or are the conditions broad enough that basically any old PNP will do?
Any feedback/questions/thoughts/etc. from people that have more experience or knowledge in these areas would be immensely welcomed!
So, I built the circuit in the replacement module below. 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. Pin1 is a PWM signal for the LED illumination (waveforms are below). Also Pin1 is floating in the normal switch circuit, R2 serves to provide a path to ground for the base of the transistor and to drain the capacitor after the key goes off.
I am looking for help with component selection and design improvement for my switch bypass circuit. The circuit below works, but I still have some voltage ripple through my transistor. I am not sure if this is going to negatively effect component life, or if it's fine for some ripple to pass through.
On the schematic below, the dash switch module would be removed, and replaced with the replacement module.
The PWM signal looks like this, depending on whether the headlamps are on or off:
Headlamps OFF:
Headlamps ON
My biggest questions are:
Is there an obviously better way to do what I am trying to accomplish that is simpler?
Should I have any concerns about component life given the PWM from pin 1?
How do I choose a transistor that is BEST for this application? Or are the conditions broad enough that basically any old PNP will do?
Any feedback/questions/thoughts/etc. from people that have more experience or knowledge in these areas would be immensely welcomed!
