The singing fish, a popular gag gift, is actually a complex yet easy to understand mix of motors, springs, and circuits.
A singing fish is at its very base a small robot. If you strip away the outer shell that gives it its fish shape and the foam padding that fills out that shape, you'll find two segments that work together to create the singing fish, one for the head and one for the tail. The head mechanism can be further broken down into one machine that swings the head forward and back to make the fish turn in your direction and another that opens and closes the mouth to simulate singing.
All three of the fish's movements are created through the same process. There is a motor at each joint and a spring holding that part in its standard position. The motor starts for a brief moment when it is triggered and it pulls the mouth open, swings the body out, or moves the tail forward. When the motor stops and is no longer providing the force to move the part, the spring snaps it back into place. This process is repeated for the duration of the song.
These motions are all controlled by a circuit board inside the plaque on which the fish is "mounted." The board is then programmed to run the motors at such intervals that the fish's movements are in sync with the rhythm of the music. The programming adds to the illusion that the fish is singing when the mouth opens and closes at the right moments. The circuit board also contains the song clips that the fish will sing along with, stored digitally. It can actually hold a surprisingly large amount of music since it's not recorded at high quality. A speaker in the back amplifies the music while remaining inconspicuous to those walking in front of the fish.
The highlight of the singing fish is perhaps its ability to sense when a person walks in front of it and it's time to give a performance. This is accomplished through a photocell. The photocell registers the amount of ambient light in a room and senses the moment that a person walking in front of it blocks the light. This triggers a circuit that begins the music and the fish's synchronized movements.
In addition, most fish have a button that you can push that will trigger the same circuit. They also usually have a button or switch for those times when you want your singing fish to remain silent.
Most singing fish run on battery power with a battery pack hidden like the speaker on the back of the plaque. Some singing fish have an AC adapter that allows you to save batteries by plugging it into the wall, although that ruins the illusion of it being a real mounted fish.
