Hi. Please forgive me. Starting out today!
I am trying to rig up something but I am struggling to source the correct part. I need a stand-alone relay switch which will be normally on but will switch off when it receives 3v. The supply which is to be turned off is only 12v. I have 3 different relay switches but I can’t get any of them to work. Some work but need more than 3v to switch, some switch on instead of off when current is received. I have looked online for hours and then there are so many options. Can anyone help it would be much appreciated. I am a construction worker and the device I’m trying to put together will save lives in the construction industry. If I can get it to work this device would have stopped my friend being blown up by an electric cable last week (he’s burn but will be ok). Thanks
Schematics, datasheet, code ...
The rig needs to be standalone. The relay switch will receive power from an existing source so no need for Arduino power supply. I only need to know where to find the correct type relay switch. The relay switch needs to turn off when it receives 3v, a positive and negative only received to activate switch. Then the switch will intercept a 12v power supply. When the switch receives the 3v it turns the 12v power supply off.
Show us a good schematic of your circuit.
Show us a good image of your wiring.
Give links to components.
Posting images:
https://forum.arduino.cc/index.php?topic=519037.0
Heelis:
The rig needs to be standalone. The relay switch will receive power from an existing source so no need for Arduino power supply. I only need to know where to find the correct type relay switch.
You just need an SPDT relay. There will be a normally closed contact that will open when the coil is energized.
The relay switch needs to turn off when it receives 3v, a positive and negative only received to activate switch.
What does a receiving a "positive and negative" mean?
Then the switch will intercept a 12v power supply. When the switch receives the 3v it turns the 12v power supply off.
You have been asked twice to post a schematic and info about the relays you're trying to use. I'll make that three times.
You can't talk intelligently about circuits without schematic and component information. What is the coil voltage of the relays you're trying to use? Is it 3V? We need part numbers and/or datasheets so we can see what the pick-up voltage is.
Schematics, datasheet, code ...
Hi,
Welcome to the forum.
Please read the first post in any forum entitled how to use this forum.
http://forum.arduino.cc/index.php/topic,148850.0.html .
Can you tell us the part numbers of the 3V coil relays that you have?
Or post pics of them?
Most have a change over output, that is Single Pole Double Throw that will provide the correct switching action you require.
Tom.. 
Google "3V NC relay". NC means normally closed i.e. opens/breaks a circuit when energised. Many relays have both NC and NO (normally open) contacts and you just have to connect to the right one.
There are loads of them but the other parameter you need to specify before you can choose the right one is how much current at 12V you need to break/switch/intercept. There's a world of difference between a relay that can switch 1A and one that can switch 50 or 100A.
Steve