There are three doors in my house that open outwards, and I have personal experience how easy it is to walk into the edge of one of them in the dark. After doing this a few times, I decided to build a device for providing a flashing LED embedded in the edge of the door to warn you that it is open.
This is really simple to make, and saves getting blood on the bedroom carpet
Step 1: Step 1. Make the circuit
The circuit came from my stash of ancient electronics relics, I have no idea where it originated. It will run off a single AA penlight dry battery, although I used a bigger C size battery. The original unit has been running in my house 24/7 for around 5 years so far with no sign of giving up yet.
I etched the circuit board myself and soldered the parts to it. I used BC107 transistors for TR1 and TR2, and a 2N3906 pnp transistor for TR3, but pretty much any npn transistors will do for the first pair, and any pnp for the third one. The LEDs I used are 5 volt types, but I’m not sure that they need to be, and I suspect any standard LED (and any colour you like) would work just fine.