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HVAC Monitor

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Jeff Hastings

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I am trying to build a monitoring device for my HVAC so I can see which stages are active. There a 5 24vac signals that could be on or off at any time. would like to have 5 LEDs, one on each signal, to monitor this. I would like to use the fewest parts possible, like driving all 5 individual LEDs off one IC??

Any ideas? Diagram would be nice. thanks.
 
Are the signals isolated from ground/neutral?
 
no, I don't believe so. I'm taping into a standard thermostat which has a ground and common lead which are both tied together at the thermostat. then I am trying to build a monitor that will tell me when the thermostat put 24vac on any to the 5 outputs.
 
So do all 5 outputs share that common/ground connection and swing ± relative to ground? That would simplify things.
Can you post a sketch of the output connections?
 
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View attachment 68928
Yes, they all have the same ground and all and currently used to trip relays in the HVAC system. I have no access to the relays and need to tap in at the thermostat terminals.

I tried to scan a sketch but have never used this board before and may not have done it correctly. If not I'll try again.
 
Your (+) terminal is actually (R) and your (-) terminal is actually C. You missed W which is usually heat. Here is typical thermostat wiring: https://wiki.xtronics.com/index.php/Thermostat_signals_and_wiring

So, it looks like two-stage cooling and no heat or is AUX a strip heater?

Thermostats of yesteryear didn't bring the common terminal to the stat. You will need this.

Fortunately, your request is easy:

It just consists of an LED, A resistor and a diode (e.g. 1n4002). LED Vd is usually about 2.1 V, and usually you need abut 10 mA of current. A resistor sized to be about (24-2.1-0.6)/If. If typically is 10e-3 or 20E-3 Amps. 10and 20 mA respectively. The power rating of the resistor should be checked against (I^2)/R or the current in amps squared divided by the resistor. The resistor has to have a rating rater than the calculated power. e.g. 1/2 W resistor.

Knowing the LED specs, If, Vd, and Ipeak we can possible get the brightness higher.

So it's just a LED, a resistor and a diode in series and you pick the order. Only the polarity of the LED and the diode polarity matters. Make the arrow of the symbol face the same way.

One end of the simple circuit connects to (C) and the other to the specific terminals.

Depending on your construction technique, the circuit could be messy or easy.

An easy construction technique, but more expensive, is to use LEDs that are 12 V or 5V and have pigtailed leads on them. If you connectorize the wires, you can attach everything to a perf board with say depluggable terminals for the connection to the system. Modifying the resistor value using these LEDS may not be straightforward.

You can use other mounting clips for the LEDs or get ones that mount integrally on the panel. You can also connectorize this and make the connections to the leads directly.

I recently purchased a couple of IP67 rated LED's. Basically rain proof.

Another construction technique might use a SIP header, so you just lug the SIP header into all of the mounted LED's. That might be the easiest.

I'll reccomend the Metal Oxide type of resistor.
 
If you want greater perceived brightness and less flicker, then rather than use a LED in series with a (silicon) diode you could use two LEDs in reverse-parallel with each other. A 2k2 resistor in series with the 2-LED combo would give an RMS current ~ 10mA for any LEDs with a Vf of 1.8V-3.5V.
 
Here is another idea. Make 5 of these four-component networks and put them in-series with the five wires going to the HVAC system. The relays used inside the HVAC draw ~150mA, so the network reduces the current through the LED to a peak of about 30mA. The network reduces the voltage that gets to the relay a bit (+4V, -0.8V), but that will not effect the operation of the relays. The LED gets a peak current of 30mA on the positive half-cycles, and nothing on the negative half-cycles. R2 shunts the peak relay current around the LED. D2 minimizes the voltage drop during negative-half cycles and protects the LED from reverse voltage.
 
Mike:

Not a good idea because some thermostat's do "power stealing" and need the low resistance of the coil.

That wont be a problem. The thermostat will not care about 15Ω forward, a diode drop in reverse. I have done this before where the LED was the LED in an opto-coupler and it worked like a champ.
 
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It only matters with certain stats. i.e. Ones without a common.

They can "power steal" from say R and W, meaning draw a small amount of current through the relay coil without turning the relay on which charges a supercap. In the above case, the stat isn't charging when it's calling for heat.

The stat may say use an OPTOMOS relay.

The configuration is rare.
 
It only matters with certain stats. i.e. Ones without a common.
...

Mine doesn't have a "common". The stat can steal all the power it needs through the 15Ω resistor (in series with the real external load). The 15Ω/diode shunt around the LED prevents it from glowing dimly because the stolen current level is so low.
 
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