# Building a circuit to send current to a powered on, powered off valve

#### GregMynrd

Dec 18, 2021
2
Hello! I'm a beginner and only have a general knowledge of circuits, and I'm wondering if this project would be doable for me.

My heating quit a few days ago, I bought a Nest thermostat, then found out my heating system is a little wonky, so the nest doesn't work. Our current thermostat connects to a boiler room steam valve for our condo zone. It has 3 wires R, W, and Y. The valve needs power to both open and close, so the t-stat sends current through R - W, or R -Y, depending on if the heater should be on or off.

I would like to get this working with a Google Nest if I can. My understanding is that the Nest sends current through R and W to turn on the heater, and then stops the current when it wants the heater to stop. I am able to power the Nest by connecting it to a 24v transformer.

I am wondering how hard it would be to make a little circuit board that goes in between the Nest and the steam valve, so that when the Nest sends 24v to the circuit board, the board sends current from R to W, and then when the Nest stops sending 24v to the circuit board, the circuit board sends current from R to Y.

Could I use transistors to detect whether the Nest's 24 volts are being sent, then let the current pass through the appropriate wires, or would I be better off programming something like an Arduino and then using power relays to deliver the 24v to the steam valve? Or maybe it's a lost cause and I should put my energy into something else.

I'll try to diagram out what I'm thinking (I have no idea how to correctly do this).

Thanks guys

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,769
Welcome to EP.
Not half as magic as you think . What you need may use is a spdt relay: Single Pole, Double Throw with a coil voltage of 24 V.

This kind of relay has two contacts (switches):
- one normally open. This means open when no power is applied to the coil, closed when power is applied. This would be the contact between W and R.
- one normally closed. This means closed when no power is applied to the coil, opens when power is applied. This would be the contact between Y and R.
R is the common contact.

#### GregMynrd

Dec 18, 2021
2
Thanks for the help! That looks exactly like the kind of magic I need

#### hevans1944

##### Hop - AC8NS
Jun 21, 2012
4,891
Any sufficiently advanced technology is indistinguishable from magic-*. In this case, the magic is in the relay. Choose wisely, Grasshopper! More may be revealed.

* "In 1962, in his book “Profiles of the Future: An Inquiry into the Limits of the Possible”, science fiction writer Arthur C. Clarke formulated his famous Three Laws, of which the third law is the best-known and most widely cited: “Any sufficiently advanced technology is indistinguishable from magic”.Nov 8, 2018" Google search result.

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