Temperature gauge for inside house (Kitsap)

I’m on a tight budget at the moment and with winter I’m fine just putting on a bunch of sweatshirts and blankets. When my son is home from his dad‘s, I want to make sure I don’t let the temperature get too cold and I’m trying to keep the heat off as much as possible. If anyone has an indoor temperature gauge so I know when I need to turn the heat on or not I’d appreciate it. Thank you for your time.