Hi,
I hope you might be able to suggest a solution to this seemingly simple problem I have. Let me explain the circuit first.....
My car is just old enough to have a pin on the diagnostics socket that you can connect to 0v which puts the ECU in 'diagnostics mode'. In this mode, there is a light on the dashboard which flashes in certain sequences to indicate any problems which have been logged in the ECU.
I wanted to make a latching circuit so that when I push a button, it will ground the pin on the diagnostics socket and keep it grounded until I press the button again. I also wanted an LED to come on to show that it was in diagnostics mode. Simple enough so far...
I then discovered that some models of my car were made with traction control, and the button for it fits in a really handy place and looks more 'factory fitted' so, rather than have to make a panel to fit the pushbutton and LED on, I decided to go to my local dealership and buy this traction control switch along with the mounting panel for it. The button has a light built into it as well which is great cos I can use that instead of a seperate LED.
My problem arises from the light inside the button which is a 12v bulb. I am trying to switch it using 5v CMOS logic. Due to the internal wiring of the button, I have to use a PNP transistor to interface between the 5v CMOS logic and the 12v bulb.
Herein lies what I think is my problem - the voltage at the base of the transistor can never rise high enough to turn the transistor off.
Here is the schematic:
http://img.photobucket.com/albums/v330/agua-moose/Electronics/ECUFlash-RevB.jpg
The current drawn by the bulb at 12v is around 40mA.
In the top corner I have a possible solution, to use an NPN transistor and a low value (around 25ohms) resistor to drop only 1 volt, so that the bulb receives around 11 volts. The problem with that is that when the transistor is on, the current drawn will be around half an amp! which is way too much.
I also thought of using a relay, but I don't really want to unless its absolutely necessary.
What do you think? How can I get around this problem?
Thanks.
I hope you might be able to suggest a solution to this seemingly simple problem I have. Let me explain the circuit first.....
My car is just old enough to have a pin on the diagnostics socket that you can connect to 0v which puts the ECU in 'diagnostics mode'. In this mode, there is a light on the dashboard which flashes in certain sequences to indicate any problems which have been logged in the ECU.
I wanted to make a latching circuit so that when I push a button, it will ground the pin on the diagnostics socket and keep it grounded until I press the button again. I also wanted an LED to come on to show that it was in diagnostics mode. Simple enough so far...
I then discovered that some models of my car were made with traction control, and the button for it fits in a really handy place and looks more 'factory fitted' so, rather than have to make a panel to fit the pushbutton and LED on, I decided to go to my local dealership and buy this traction control switch along with the mounting panel for it. The button has a light built into it as well which is great cos I can use that instead of a seperate LED.
My problem arises from the light inside the button which is a 12v bulb. I am trying to switch it using 5v CMOS logic. Due to the internal wiring of the button, I have to use a PNP transistor to interface between the 5v CMOS logic and the 12v bulb.
Herein lies what I think is my problem - the voltage at the base of the transistor can never rise high enough to turn the transistor off.
Here is the schematic:
http://img.photobucket.com/albums/v330/agua-moose/Electronics/ECUFlash-RevB.jpg
The current drawn by the bulb at 12v is around 40mA.
In the top corner I have a possible solution, to use an NPN transistor and a low value (around 25ohms) resistor to drop only 1 volt, so that the bulb receives around 11 volts. The problem with that is that when the transistor is on, the current drawn will be around half an amp! which is way too much.
I also thought of using a relay, but I don't really want to unless its absolutely necessary.
What do you think? How can I get around this problem?
Thanks.