The show God of Carnage required that a character projectile vomit all over another character and a coffee table. To accomplish this task on stage I opted to create a device that was triggered by the actor. Having the actor control the effect allowed a more natural look and reduced the chance of a misfire. When the book switch is kicked the Arduino actuates the solenoid and air enters the tank. As air enters the tank the vomit is displaced by air and forced out the hose. The Arduino acts as a timer letting only a certain amount of air enter the tank. Once the book switch is kicked the solenoid will not turn back on until the Arduino is reset. This is to prevent vomit from spewing when it is not desired.
To get the effect looking the best required a lot of experiments and bench tests. A mixture of oatmeal starch without the oats, cream of wheat, and water was settled on for the vomit substitute. One problem was that as the fluid runs out there was a loud sputtering noise as the air started to escape from the tube. To prevent this issue an Arduino was used to switch on a solenoid valve for a set amount of time. By switching on the air for a fraction of a second and then off allowed the pressure to equalize and the fluid stop flowing before it was emptied completely from the system. The final design of the system used compressed air to propel the fluid through a tube that was sewn into the actors sleeve and out through a nozzle at her wrist.
The air tank, fluid tank and electronics were concealed in a table located behind a couch. The tube passed through the couch and terminated in a garden hose quick connect valve. The actors costume had a quick connect valve that was connected when she sat down on the couch. The connecting of the tube was masked using the action of her looking through her purse. To activate the machine the actor kicked a switch disguised as a book with her heel as she stood up. There was a short delay from the time the switch was kicked until the fluid started spewing. The delay was not something that could be eliminated so we had some practice sessions with the actor so that she could work out the timing.
The Arduino was chosen instead of another system such as a PLC for the following reasons: ease of programing, cost, and availability. For small jobs such as this an Arduino is a perfect device; it costs about $20 and is simple to use.