A modular monitor and control system which monitors the fluid jet and droplet formation region in a stream-in-air cell or particle sorter and controls sheath fluid and sample flows via the first and second valves. The system includes a processor which receives digital video image signals of the fluid jet and droplet formation region in the sorter; continuously analyzes the digital image signals to determine a significant change in the digital image signal; and triggers at least a relay switch activation signal in response to a determined significant change. At least one relay switch which is separate from the cell sorter and in communication with the processor is connected with the first and second valves and configured to close the first and second valves upon activation of the relay switch via the relay switch activation signal.