Electronic – arduino – 3 Volt Active Low Relay

Note: The question was changed out from under this answer from wanting to drive a 6 V relay with a 12 V power supply, to discovering it was a 12 V relay all along. I'll leave this here since it's a good answer to the original question, which might be useful to others.

The simplest way requires no additional parts than what you would have to use anyway, just arranged differently:

This actually controlls the current thru the relay instead of the voltage across it. However, the DC resistance of relays is well controlled, so they can just as well be driven with a specific current. The relay current will be close to 4.3V / R1, which is 43 mA in this example. Look at the relay spec, find the full on current, or get it by dividing the normal coil voltage by its DC resistance. Take 4.3 V and divide it by that current to get the R1 resistance.

Note that while this method is simple, it's not very efficient. Twice the relay power will be used.

A even electrically simpler but more complicated firmware method is:

This has the advantage of being quite efficient. It will draw half the current from the 12 V supply when the relay is on as the previous circuit. The PWM should be fast enough so that the current thru the relay changes little during a half-pulse time. I'd try to run the PWM at a bit above the audio range, like 25 kHz. You don't need much PWM resolution since you want a fixed 50% duty cycle.

Of course another option is to get a 12 V relay. That is one of the common relay coil voltages. Whatever relay you have probably comes in a 12 V coil variant.

From a quick look at the relay board I note the following:

These components provide for a buffered drive circuit of the relay coil. The below schematic is the likely configuration of these components. (LEDs not included).

You should be able to connect your 5V supply between the VCC and GND inputs. Then connect the GPIO signal to the IN1 terminal and also the MCU board ground to the GND pin.

This driver circuit should work with either a 0->3.3V swing or 0->5V swing GPIO. Not knowing just how those two LEDs are connected up it may be that one or the other LED may not work with a 0->3.3V swing of the LEDs are connected into the base circuit of the transistor. On the other hand if the two LEDs are connected up into the collector side of the transistor similar to as follows the the LEDs should work as well. A simple check of the circuit with an ohm meter should be able to confirm how the LEDs are connected.