How to ground an input using a high signal

Other Hardware General Electronics
1

I have an existing working system that consists of an Arduino Mega 2560 Output Pin driving a transistor ( 2N3904 ) connected to a relay.

The existing relay is DPDT and Side 1, uses :
Com. 12V+
N.O. signal line to an input pin on a seperate device ( Device 1 ) that requires a 12V+ trigger on the input pin.

side 2 of the relay has :
Com. Gnd
N.O. signal line to an input pin on another seperate device ( Device 2 ) that requires the Ground signal to be removed to activate the device.

I am replacing all the individual relays that I have with a compact 8 relay board, so the DPDT is to be replace with an SPDT.

So my question is, without using up an additional relay, what is the best way to use the 12V+ from the N.O. pin going to Device 1, to break / interrupt a Ground line between Ground and Device 2 input pin ?

2

Draw a schematic, I find the description too confusing about what you have and what you want to do.

3

Sorry. Here's a diagram of what I currently have.

I need to replace the DPDT with SPDT and have the 12V+ ( from N/O when relay energised ) interrupt the ground line to the second device.

reverse_1.jpg

4

I need to replace the DPDT with SPDT

So as I understand it you need one device or the other to be on depending on the relay state.
Then why not connect the common to the ground and the ground of one device to NO and the ground of the other with NC with the +ve of both devices going to 12V.
Or am I not understanding something here.

5

Both Device 1 and Device 2 are powered permanently.

The DPDT relay controls the input lines on the 2 devices.

For Device 1, it send 12V+ to the input pin.

For Device 2, it breaks a Ground line to the input pin.

I need to be able to do both these actions with a SPDT relay.

6

How about a PNP transistor sending 12V into the device when the base is ground (through a resistor).
So that is common -> ground
NC -> transistor base
NO -> other device that needs grounding.

7

Hi Mike

Sincere Thanks for the replies and guidance.

Can you please confirm if the attached drawing is what you are suggesting ?

Regards

reverse_2.jpg

8

Yes but you need resistors on that transistor. One in line from the base to the relay contact and another from the relay contact to +12V

9

Why use a relay at all?