.3VDC input to drive LED (automotive)

[Stratocaster]

The ABS indicator circuit sends .3vdc to light the ABD light on the dash. Well, now that the dash is in a box in storage, I need to keep that light. So I have .3vdc when on, and 0vdc (not ground) when off. So what do I need to drive bright LED?

From what I have been digging up on the net, looks like I need an DC to DC converter. Well my high school electronics class was almost 30 years ago. So if someone can draw the schematic, I can build it. But I know I am clearly in way over my head on this.

The race car needs an ABS light

 

[DirtyLude]

It would be far simpler to use the .3volt signal to trigger a switch like a transistor to feed the LED a useful voltage. You have no idea how much current you can draw from that 0.3volts either so a DC converter may not work at all. Most DC up converters I've seen wouldn't even start up at 0.3V.

Are you certain about the 0.3volts? I've never seen that. It seems odd that they would cut the voltage down so much for no reason.

 

[MikeMl]

Yeah, that is not a normal logic signal in any automotive system I have ever seen. Something fishy.

 

[audioguru]

Nobody makes a 0.3V light bulb or LED.

Maybe the 0.3V is actually a 5V or 12V PWM dimmed voltage that a slow meter sees as only 0.3V average.

 

[Wade Hassler]

It could be that the lamp-control line is pulled-down (by a transistor, maybe) for illumination
Like this:https://www.corral.net/images/tech/abs/abs_wiring.jpg

.. if you check the voltage at the lamp socket with no lamp installed, you would read either a low-DC voltage (pulled down) or zero (floating)

 

[Stratocaster]

It would be far simpler to use the .3volt signal to trigger a switch like a transistor to feed the LED a useful voltage. You have no idea how much current you can draw from that 0.3volts either so a DC converter may not work at all. Most DC up converters I've seen wouldn't even start up at 0.3V.

Are you certain about the 0.3volts? I've never seen that. It seems odd that they would cut the voltage down so much for no reason.

I agree it sounds odd. I have meter it later today. I like the idea of using a transistor to feed the LED useful voltage. And No I am not sure, I personally did not meter it. I have a friend in CA that is working the same installation in the same car and we are sharing notes. So I am basing my information on his tests. If I am lucky there is 3v not .3v and I can directly drive the LED.

It could be that the lamp-control line is pulled-down (by a transistor, maybe) for illumination
Like this:https://www.corral.net/images/tech/abs/abs_wiring.jpg

.. if you check the voltage at the lamp socket with no lamp installed, you would read either a low-DC voltage (pulled down) or zero (floating)

I can't check the voltage to the bulb, when the gauge cluster is in a box, next to the car. I am doing a full dash replacement, so I have to use the control circuit. The LED will be a standalone circuit mounted to the dash.



Here is a diagram of from the manufactures ETM.

**broken link removed**

Note: The Gauge Assembly shown in the above circuit is NO Longer installed in the car. So the voltage I have to work with to trigger the LED is the ABS Indicator Control from the ASB computer.

 

[MikeMl]

The pin you are metering is likely an "open-collector output", which relies on an external lamp to pull the pin high (~12v) when the lamp is off, and the internal transistor pulls the pin low (0.3V) when the lamp is on. Since the connection to the lamp is missing, you are seeing only the 0.3V or nothing.

Temporarily connect a 1K resistor from the output pin to +12V. Then measure the voltages at the output pin as it switches. I'm guessing you will see ~0V, and 12V, respectively. If that works, write back and I'll suggest how to connect a lamp or a LED.

I see the diagram now. Which box are are retaining? what pin (wire color) is the output that should drive the remote LED?

Is it the Blu/Red wire?

 

[audioguru]

You need to duplicate the "ABS Indicator Circuit".
Good luck finding out whatever it is.

 

[Stratocaster]

The pin you are metering is likely an "open-collector output", which relies on an external lamp to pull the pin high (~12v) when the lamp is off, and the internal transistor pulls the pin low (0.3V) when the lamp is on. Since the connection to the lamp is missing, you are seeing only the 0.3V or nothing.

Temporarily connect a 1K resistor from the output pin to +12V. Then measure the voltages at the output pin as it switches. I'm guessing you will see ~0V, and 12V, respectively. If that works, write back and I'll suggest how to connect a lamp or a LED.

I see the diagram now. Which box are are retaining? what pin (wire color) is the output that should drive the remote LED?

Is it the Blu/Red wire?

I have the car pulled apart and I am currently running the requested test.

Yes, it is the Blu/Red wire. I will have to pick up a 1K resistor to run the test you requested. Off to Radio Shack

I pulled the voltage with the gauge cluster in place. 0.20v with the light is on, .03 (Not 0V) when off. There is .04 amps.

 

[Stratocaster]

Temporarily connect a 1K resistor from the output pin to +12V. Then measure the voltages at the output pin as it switches. I'm guessing you will see ~0V, and 12V, respectively. If that works, write back and I'll suggest how to connect a lamp or a LED.

OK this test resulted in 11.5v when on, 1.14v off.

So it sounds like I can connect a 12v LED to a 12+ source and connect the ground to the ABS Indicator Circuit.

 

[Stratocaster]

OK this test resulted in 11.5v when on, 1.14v off.

So it sounds like I can connect a 12v LED to a 12+ source and connect the ground to the ABS Indicator Circuit.

Ok this works, however the LED is ON when it should be OFF. I.E. When the gauge light is ON, the LED is OFF. When the gauge light it ON, the LED is OFF. So I need to flip it. Any ideas besides using a relay?

 

[Stratocaster]

So this looks like it will do the trick.

Connect A9 (12+) to +Vs
Connect B20 to Input Signal Transistor collector
The transistor emitter to Ground.
LED to Output Signal and Ground
**broken link removed**

Thoughts?

 

[MikeMl]

Reduce Rc to ~680 and put the LED in series with RC. Hook a 1K resistor from the left end of Rb to +12V (same place as Vs).

 

[Stratocaster]

On further review, I found the circuit in full that drives the Gauge light. I would think that I should recreate the same circuit or is it more complex then I need? The Gauge Assembly box is what will be missing from the car. So drop the Gauge Assembly box and that is what I have to work with.

**broken link removed**

 

[Stratocaster]

Reduce Rc to ~680 and put the LED in series with RC. Hook a 1K resistor from the left end of Rb to +12V (same place as Vs).

Mike, is this the circuit you are talking about?

**broken link removed**

 

[MikeMl]

Except that the LED should be pointing the other way.

 

[Stratocaster]

Except that the LED should be pointing the other way.

I had a 50/50 chance of getting that right.

So if the LED package has a built in resistor, all should be fine?

Thanks for the help Mike (and all others) I will post back when I have implemented this.

 

[bychon]

So if the LED has NO resistor built in, it will work.

 

[MikeMl]

If the LED indicator assembly has a built-in resistor, it will have a voltage rating. Is the one you are planning on using rated at 12V? The 680 Ohm resistor presumes that the LED is just a generic 20mA LED with no built-in resistor.

 

[Stratocaster]

Thanks, that is what I assumed. But you know how that can end up.

thanks for the help and confirmation.