Guidance on a simple electronics project

I have a tankless water heater, which needs a minimum flow to be activated. If you run the cold water too high or flush a toilet, it is easy drop the hot water pressure enough to get below that minumum flow rate. You won’t find out until the cold water arrives a minute later, and then you have to wait another minute for the hot water to get back through the pipes from the tankless.

The controller for the heater is in a closet, and has an LED that lights up whenever it is active. I would like to stick a photocell over the LED, and set up a device that will turn on an LED in the bathroom, and one in the kitchen whenever the tankless is running, and switch off when it is not. Then I can know instantly when I need to adjust the faucets or shower to keep the hot water coming. A wire from the closet to the kitchen would run about 20ft, and a wire to the bathroom would be around 60ft.

I don’t really know anything about electronics, so I am looking for some advice on what kind of devices and design I would need. I’ve been wanting to learn, and thought this might be a good beginner’s project. I would like to start with a basic design or approach, and then learn what I need to understand and implement it.

You need a light pipe between the LED and detector.

What color is the LED? Reason I’m asking… the detector must be sensitive to the wavelength produced by the LED. A photodiode or phototransistor might work, though (again) you have to make sure it is sensitive to the wavelength produced by the LED. A photocell would probably also work, though I would only use one if you can’t find a photodiode or phototransistor that would work.

Once you detect the light from the LED, the rest of the circuit is pretty simple, and will include a few transistors and resistors.

Might be easier to run a piece of fiber optic toslink cable.

Get the cheapest 20 footer you can find on ebay. Viewing angle and brightness might be a challenge though . . .

This is a pretty simple design.

I would use an OpAmp as a comparator.
This circuit is pretty much what I would use.
You can run additional LEDs off of the output - just use a dropping resistor for each one. remember that you will need to provide power for the OpAmp - batteries will work to start, but they probably won’t last very long. Try an old cell phone charger as a power supply.