Control for Two Relays

Thread Starter


Joined Jan 26, 2009
I need to control two relays over a single pair of wires with one wire referenced to ground.

Voltage 24VDC available at the Relays location.

One relay will control temp up, the other temp down.

One switch will activate temp up and the other temp down on a thermostat. The two switches are connected to same pair of wires and need to activate its respective relay.

Any ideas would be appreciated.


Joined Apr 5, 2008

Take the two relais.
Each with a diode in series.
One with the cathode to the relais the other with the anode to the relais.
The other sides of the diodes are put together to one wire.
The other wire goes to the other side of the relais.

With + 24 Volts one relais will work.
With - 24 Volts the other relais will work.



Joined Dec 19, 2007
Is there a time when both switches and their relays are open?
Is only one switch and its associated relay activated at a time, never both?
Can you add circuitry components at both ends of the two-wire line?
Are the switches "dry contacts"... i.e. power applied to them at the thermostat?
Is there a possibility of 24vAC available at the relay end?

I think I have an idea. ;)


Thread Starter


Joined Jan 26, 2009
Yes, both switches and relays are normally open.
There is a possibility that both switches may be pressed at the same time.

Components can only be added to the relay end, possible exception would be a resistor value for each switch closure at the switch end.
The switches are “dry contacts' with one wire common with gnd.
The Relay contacts are also "dry contacts"
24VAC is not an option.

I hope you have an idea!
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Joined Nov 28, 2008
Build a voltage divider at the relay side but with the divider ground leg at the switch side via the single wire available.

By switching in different resistors to ground at the switch end, different voltage appears on the voltage divider junction at the relay side.

e.g. 24V with no switch pressed, 16V with one switch pressed, 8V with the other switch pressed and 4V with both switches pressed.

A simple LM393/LM339 comparator can then check these voltages at the 24V relay end and switches on relays accordingly.
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