# Low current 0v to control LEDS

Hi

I have a problem that i have a ecu that puts ov onto a coil to switch it on. I want to put a led in the circuit so i can see when the coil is turned on.

At the minute i attach the cathode to the switched 0v and the anode to 5v and the led won’t light. If i put a a 0v direct from the power supply to the led then the led will light.

Thank you

ullisees: I have a problem that i have a ecu that puts ov onto a coil to switch it on.

I think you meant "ecu applies 0V (GND) to a coil," but it's neither here nor there.

ullisees: I want to put a led in the circuit so i can see when the coil is turned on.

At the minute i attach the cathode to the switched 0v and the anode to 5v and the led won't light. If i put a a 0v direct from the power supply to the led then the led will light.

Are the grounds connected between the ECU, Arduino, and coil? (or does the ECU include the Arduino?)

Maybe post a picture of your setup/schematic. Also, don't forget current limiting resistors.

Where the 5V of the led coming from? It's sharing the same ground with ECU?

Ciao, Ale.

Hi, Have you got DMM, what does the switched point that is pulled to 0V go down to when you have the LED in circuit? You do have a current limit resistor in series with the LED?

Tom... :)

If I understand this right, he is trying to put a relay parallel to a relay coil. Perhaps he can shed more light on it

EDIT: OOPS, that's LED in parallel

If you connected a bare LED to 5V without a current limiting resistor you will have fried it immediately

The coil is a solenoid that has a 12v supply and is switched on by an ecu (not arduino) putting 0v onto the other side of the coil.

I have put the led cathode to the switched 0v and the anode through a 470 ohm to the 12v

Led does not light but the solenoid works.

If i put the supply 0v direct to the led then the led lights.

Inside the ecu the 0v is being switched on by a Darlington then though a 1n5401 diode

Just to get this straight...

You're not moving the anode connection. When you hard-wire the cathode to ground it works. When you wire the cathode to the switched-ground it doesn't work.

Since we know the LED & resistor are OK, we know there is no voltage (or very little voltage) across those components when the LED is off.

Here are the possibilities - 1. The relay coil has a different separate-isolated ground. (most likely) 2. The relay coil is not really switching to ground. 3. The 12V is "going away" when the relay switches-on. (unlikely)

Hi, Please read post #3, measure the switched point, with and without the LED.

Thanks.. Tom.... :)

TomGeorge:
Hi,
Please read post #3, measure the switched point, with and without the LED.

Thanks… Tom…

I have a meter. Do you want me to test the current or voltage

Hi, Voltage at the point which ECU switches to 0V, with respect to gnd/negative.

Tom... :)

Measuring between the supply 0v and the switched 0v the meter is reading 8.5v

ullisees:
Measuring between the supply 0v and the switched 0v the meter is reading 8.5v

Draw a circuit diagram of the system.

ullisees: Measuring between the supply 0v and the switched 0v the meter is reading 8.5v

Is that when you try and activate the "coil", you say it should switch to 0V?

What is the "coil" ignition coil, relay coil, solenoid coil, what does it do?

Tom...

The coil is a solenoid that opens or restricts oil flow.

When the coil is energised the voltage at the switched 0v to the supply 0v is 8.5v

The permanent supply to the coil is 12v

So you are unable to draw a diagram of your wiring despite being asked several times. I don't know what sort of help you want in that case. You have clearly screwed up wiring things as what you describe will work if you implement what you claim to have implemented.

We are asking for a diagram so we can see what you have missed out. OK so your skills with a pencil and paper are lacking. So what about a photograph that clearly shows your wiring. Unless we know what you have actually done how the hell are we supposed to be able to guess how you screwed it up.

When you say the voltage is 8.5V, in relation to what? Where do you have your OTHER meter lead?

Here is a diagram of the circuit.

Inside the controller i cannot change.

Thanks for the diagram. There is no reason why that will not work. Are those two ground symbols actually connected together in real life? If so then what you have is not wired like your diagram.

All the grounds are linked together.