Between the laundry in the basement, the kitchen (sink and dishwasher) and a full bath on the first floor, and a full bath on the 2nd floor, sometimes taking a shower can be interrupted with the cold/hot game that gets played when someone elsewhere in the house starts using the water also. Instead of running up and down the stairs to warn others that a shower will commence, we need a signaling device that the water is or soon will be in use. The house is old and big. Upstairs and kitchen can not hear when the water is already being used. A point of use heater is on the list for later. We don't live with our phones attached to us so a group text would be useless. I think this would be cheap to make. This is what I need. It will be wireless The oval on the left is an on off button The oval on the right is a light. I think it can be the size of an outlet cover. Like a wireless doorbell there will be a receiver but instead of ringing it will pass the signal to the 3 other locations. When someone is preparing for a shower or loading the dishwasher, they can see if the hot water is available. If it is, they press the switch and the light lights up on all the panels. If they see one of the lights is already on, they can wait. So what do I need to make this?
I think I would just fix the problem. By this I mean add a point of use water heater usually at the shower. Most of the time with on demand water heaters I have found most customers want more hot water for the exact same reason you have.
Adding a 30 amp 240v on demand water heater brings the tepid water temp to the nice warm water most home owners want, yes even when there are other demands that drop the water temp 15-25 degrees in the shower.
It is all based on flow but adding a small electric point of use electric water heater is a inexpensive way to have almost endless water at a comfortable water temps.
I can personally say this method works having a home with 3 full baths and 3 daughters , and up to 6 granddaughters at the same time, I started with a point of use electric in the master bath, the next year in the upstairs bath and before we bought a different home the 3rd bath. Now even with the entire family home the main water heater a gas on demand warms the water and the point of use electrics bring it up to temp even with all 3 showers being used at the same time.
I loved the gas tankless but it did not have the flow needed for even 2 showers at the same time, after adding the point of use electrics on each of the baths the power bill actually went down and everyone could shower back to back without running out of hot water and the space saved by removing the main 100 gallon water heater provided more room.
Having a point of use solves the same problems with a tanked or tankless hot water supply, I have added similar small heaters for other customers and most have really thought they did the job.
I would consider an electric point of use tankless on the shower. It may be the solution you need.