We have a rental property with a small pool that has an RGB pool light powered by 12VAC. The light has several colorful programs consisting of different colors, even programs that cycle through several colors. Very ugly. We just wanted a white light. The problem is, you select programs by cycling the on-off switch N times until you get to the program or color you want. This being a rental property, tenants are often fiddling with the light switches to figure out which controls what, and pretty soon the pool lite is cycling through some ugly sequence of green or purple or something.

We also have LED landscape lights that turn on at dusk, off at dawn. There is enough hysteresis in the photocell circuit that the lights cleanly turn on once at dusk, then turn off at dawn. Perfect for turning the pool light on and off without inadvertently changing its program. The only problem is, the landscape lights are 12VDC, but the pool light is 12VAC. So we couldn't just wire the pool light to the landscape lights.

This relay was the perfect solution. I connected the "input coil" to the 12VDC output of the landscape lights, and used the output relay to control the 120VAC side of the step-down transformer on the pool light. Voila! Each night the pool light comes on with the landscape lights, and goes off the next morning when they do. Since there's no flickering to fool the pool light into a different program, it reliably stays on the last program I set which was the "white" program.

The relay even has the correct threads for a standard 1/2 inch electrical knockout. I just went to to the local "depot for homes" store and buy a cheap fitting to get a rubber washer and thrust nut to let me install the relay on the side of the transformer box for the pool light.

I had situation where I wanted to add LED strip lighting in a pantry closet but the only power was a three way switch. However, power was at the other three way switch and it was nearly impossible to run a wire to the nearest power outlet.
I did some wiring diagrams and I was pleasantly surprise to find this already made relay all encased for easy and safe wiring. I ordered two of them but after a little head scratching only one was needed using the NC and NO contacts
What it solves is when the 3 way switch is operated from the other distant switch, the power changes to the other side of the switch I'm using for power. The relay, when it operates, brings voltage to the load on the NO contacts, and when the power is switched over to the other side of the 3 way switch, the relay has no power, therefore the power connects through to the NC contacts.
The only little issue is when the power is momentarily dropped as the relay operates, the motion sensor turns on as a default when power is first applied. It only happens when the distant 3 way switch is used and after 30 seconds, the LED light turn off. Very acceptable.

I installed this, so I can use my nest thermostat to control my whole house steam humidifier. Works perfectly!

I use them quite a bit in my job as a controls technician to separate our low voltage equipment from 120 volts. I used the one I purchased here when installing a new aquastat on my fathers boiler. The new aquastat that he had purchased was 24 vac and the old one was 120vac. So I used the 24vac for the coil and ran 120 through the contacts to control his circulating pump. Was way easier than sending the aquastat back and now he has a status light to know when it should be on. They seem to work very well.

I used this relay in the wiring of a Panasonic FV-11-15VKL1 ceiling fan to avoid running a separate Romex cable for the fan control wires, which would have required tearing up a lot of sheet rock. This way, the fan runs whenever the light is turned on. NOTE: This is useful only because the fan has a control module installed; if there are no control modules, then just run switched power to the fan instead of using a relay.

The fan's junction box has an extra knock-out that fits the relay's 1/2" NPT nipple and the included nut perfectly. The attached photo shows how to make the connections. It is just barely possible to fit all of the wires and wire nuts inside the junction box, but you can always shorten the relay wires by cutting, re-stripping, and re-tinning them if necessary. Note that there is a delay (20 minutes, unless you install the delay module and set it to some other value) after you turn off the light before the fan turns off. Also, it is normal for the fan to take about 40 seconds to reach operating speed after turning on the light.

Since it's UL listed and fire rated, and I had it installed by a licensed electrician, it should be as safe as any other piece of electrical equipment. The only clear downside to this is reliability and longevity. It's rated for 10 million cycles, which is over 250 years at 100 uses per day, but I suspect that the shelf life is less than 100 years, possibly much less. I'll post an update if it wears out while I still own the house.

I measured the power consumption of the coil as 300mW at 120VAC. The coil current is rated as 28mA, but it is mostly reactive, resulting in a measurement of 2.5VAR at 120VAC.