A liquid flow control system for use with infusion sets of the type comprising a liquid supply, a drip chamber 8 downstream of the liquid supply for forming liquid drops and a flexible tube 7 connecting the drip chamber with an injection needle, the system comprising a drop sensor 4 adapted for being arranged adjacent the drip chamber for sensing the passage of drops through the drip chamber, a valve 19 adapted for controlling the flow of the liquid through the tube, valve activating means for activating the valve, electrical circuits for registering the rate of drop formation and controlling the valve activating means, and a battery for powering the drop sensor, the electrical circuits and the valve activating means, the drop sensor, the valve, the valve activating means, the electrical circuits and the battery being mounted on or in a common housing having housing attachment means for releasably attaching the housing to the flexible tube.