Each of the given circuits contains a symbol which does not exist:
Some of these symbols are immediately recognizable as letters, while others require further decryption. In order, they are
- D
- O (Morse Code)
- O
- R (Flag Semaphore)
- B
- E
- L (Braille)
- L (Pigpen Cipher)
The letters give the answer DOORBELL. This puzzle is designed to be deliberately easy to backsolve from just a few letters, as an offset to the sheer number of electrical circuit symbols that exist.