I have an electrical puzzle

[Perry M]

I need help or advice from you electrical smart people please. I have a trunk mounted battery using a Ford style solenoid system and all works as it should. I want to add a red LED to the system when the battery switch is off and a green LED when the switch is turned on. The green LED is a no brainer but I can't figure out how to turn the red LED off when I switch to the on position. I am sure a relay is required somehow. Anybody got any ideas?

[WallaceMFG]

Can I ask why the red LED is required? All it is going to do is drain the battery when it is turned off. Seems like it would be simpler to just have the green LED when the battery is on and off when it is off.

[dhutton]

You could try wiring the LED across the switch.

Don

[Fierbird67]

It sounds like a 5 pin relay could do the job for you.

https://www.12voltplanet.co.uk/relay-guide.html - Referencing a Changeover Relay

run pin 30 to power, 87 to the green LED, 87a to the red LED. That way, the circuit for the red light is always closed. You would just need to run a line from the switch itself to activate the coil pins of 85 and 86.

[scott_fx]

Can I ask why the red LED is required? All it is going to do is drain the battery when it is turned off. Seems like it would be simpler to just have the green LED when the battery is on and off when it is off.

that led is going to draw like 20ma. It’d take 100 hrs to drain a AA battery. It would take over 3.5 years to drain a 600 amp car battery

[Perry M]

that led is going to draw like 20ma. It’d take 100 hrs to drain a AA battery. It would take over 3.5 years to drain a 600 amp car battery

That's exactly what I was thinking. I wasn't concerned about draining the battery with a single LED. I am still confused on how to wire it however. Wiring the LED across the switch wouldn't work as both sides of the switch are energized when it is turned on. I am going to experiment with the relay as fierbird67 suggested. I will let you know the results.

[scott_fx]

That's exactly what I was thinking. I wasn't concerned about draining the battery with a single LED. I am still confused on how to wire it however. Wiring the LED across the switch wouldn't work as both sides of the switch are energized when it is turned on. I am going to experiment with the relay as fierbird67 suggested. I will let you know the results.
Firebird has a good solution. Just make sure you have the polarity correct on the led. Also, you can get an led that is bi-color (or rgb with some current limiting resistors) so you would only need one led. Just be aware if it’s common cathode (negative/ground) or common anode (positive). If you want to make it very simple you can get 12v led indicator lamps. They have a wide dispersion pattern and aren’t overly bright. Oznium used to have them (they may still have them)

[scott_fx]

PLUG-N-PLAY Instrument Cluster LED Indicator Light Dash Bulbs. Aluminum Pilot Lights. Color Acrylic Lens. Flush Panel Mount 5/16" 8mm 12V for Speedometer Odometer Tachometer (Black Bezel, Green LED) https://www.amazon.com/dp/B01AF5THFS/ref=cm_sw_r_cp_api_i_.tAoEbXB3G5JP

These are the ones in use for my turn signals and high beam indicator

[dhutton]

No relay needed. Wire it across the switch..... There is 12 volts across the switch when it if open.....

Don

[scott_fx]

No relay needed. Wire it across the switch..... There is 12 volts across the switch when it if open.....

Don
he wants an led for both switch states.

[dhutton]

he wants an led for both switch states.

I thought it said he wants two LEDs and already knew how to wire the green one. I’m telling him to wire the red one across the switch. There is 12 volts across the switch when it is open and 0 volts across it when it is closed.....

No need for a relay....

Don

[scott_fx]

I thought it said he wants two LEDs and already knew how to wire the green one. I’m telling him to wire the red one across the switch. There is 12 volts across the switch when it is open and 0 volts across it when it is closed.....

No need for a relay....

Don

I may be wrong but it seems to me that he wants a Green LED lit when the switch is on and a Red LED lit when the switch is off

[dhutton]

I may be wrong but it seems to me that he wants a Green LED lit when the switch is on and a Red LED lit when the switch is off
Which is why I am telling him to wire the red one across the switch. There is 12 volts ACROSS the switch when it is off....

Not from one side of the switch to ground, ACROSS the two terminals of the switch.....

This is not rocket science, it is pretty basic stuff....

Don

[scott_fx]

Which is why I am telling him to wire the red one across the switch. There is 12 volts ACROSS the switch when it is off....

Not from one side of the switch to ground, ACROSS the two terminals of the switch.....

This is not rocket science, it is pretty basic stuff....

Don


maybe I’m not understanding your term “across”. Just so we’re on the same page. Are you talking about a spst switch? I’m at a loss as to how you’re planning on having the red led turn off when the switch is on.

Actually... spdt will get him what he wants. Put 12v on the center leg and the two leds on each of the outer legs

Perry: this will do what you want
Baomain Car Toggle Switch SPDT ON-ON 3 Pin 2 Position 12V 25A with Waterproof Cover for Auto Car https://www.amazon.com/dp/B01J31PRBS/ref=cm_sw_r_cp_api_i_PYDoEbHQ4PPPN

[dhutton]

maybe I’m not understanding your term “across”. Just so we’re on the same page. Are you talking about a spst switch? I’m at a loss as to how you’re planning on having the red led turn off when the switch is on.

Actually... spdt will get him what he wants. Put 12v on the center leg and the two leds on each of the outer legs

Perry: this will do what you want
Baomain Car Toggle Switch SPDT ON-ON 3 Pin 2 Position 12V 25A with Waterproof Cover for Auto Car https://www.amazon.com/dp/B01J31PRBS/ref=cm_sw_r_cp_api_i_PYDoEbHQ4PPPN
Connect it to the two spst switch terminals. This is very commonly done with power switches. Back in the good old days it was a neon bulb.....

12 volt side of the LED to the 12 volt battery side of the switch. Ground side of the LED to the other switch terminal.

Edit, here you go, literal back of an envelope schematic...

Don

[blitzer454]

A LED across the switch is the correct answer but you need to add a transistor as you don't know the equivalent resistance of the load. Something more like this would work. R2 represents the load, in which I used 40k but it could be much less or more depending on your car.
172573

[dhutton]

A LED across the switch is the correct answer but you need to add a transistor as you don't know the equivalent resistance of the load. Something more like this would work. R2 represents the load, in which I used 40k but it could be much less or more depending on your car.
172573

Way to bring the tech but a SPDT switch as proposed above would be easier. I am guessing it would work with my suggestion and some diddling of the resistor value if needed. Worst case add a tiny load resistor. It just comes down to LED intensity...

Anyway, I am guessing we have scared the OP off by now... ��

Don

[Perry M]

Sorry to have made this so difficult. The switch I have is a two post heavy duty battery cut off switch so I don't believe you can wire it across the switch. It is like a light switch in your house, it's either on or off. I used a relay as Fierbird67 suggested and it works perfectly. Thank you. Another question if you don't mind. Can I run a small gauge wire from the battery through a 3 amp or less fuse to keep the computer alive? The fuse would blow if anything was activated which would also serve as a theft deterrent.

[blitzer454]

The problem with just trying to diddle with the resistor value is that this may cause a significant voltage and 20mA across the load even when the switch is off. But I'm with the guy that suggested eliminating the red LED altogether as it's redundant.

[blitzer454]

Sorry to have made this so difficult. The switch I have is a two post heavy duty battery cut off switch so I don't believe you can wire it across the switch. It is like a light switch in your house, it's either on or off. I used a relay as Fierbird67 suggested and it works perfectly. Thank you. Another question if you don't mind. Can I run a small gauge wire from the battery through a 3 amp or less fuse to keep the computer alive? The fuse would blow if anything was activated which would also serve as a theft deterrent.

Yes, but I would use a circuit breaker instead of a fuse so you can easily reset it if you trip it by accident.

[Perry M]

Yes, but I would use a circuit breaker instead of a fuse so you can easily reset it if you trip it by accident.

good suggestion. thankyou

[blitzer454]

On the other hand why do you have a cut-off switch in the first place? If it's because of some racing rule requirement then having the 3 amp circuit probably would not be allowed as that circuit would still have the potential to create a spark after the main cut-off switch is opened.

[scott_fx]

Wouldn’t the absolute most simple solution be just a switch...
172574

The center leg of that switch being 12v

[blitzer454]

Yes, but the OP is using a SPST switch, what you drew is a SPDT switch. I'm assuming that this cut-off switch is pretty large since it would need to be a high current switch so replacing it with a SPDT would be way more expensive than a LED transistor or relay circuit.

[scott_fx]

Yup you could wire it similarly with a light duty spst switch and a relay... which was the first suggestion in this thread. Lol

[dhutton]

So how many are engineers and wannabe engineers does it take to light an LED? ;>)

Don

[scott_fx]

Lol. I’ll let you know after we figure it out