An electronic device may have control circuitry and input-output components. The input-output components may include audio components, sensors, and other devices. A proximity sensor may supply the control circuitry with proximity sensor data. The control circuitry may adjust the audio components or take other suitable action in response to proximity sensor readings from the proximity sensor. The proximity sensor may have a light source such as an infrared laser diode and a light detector that measures a reflected portion of infrared light pulses emitted by the infrared laser diode. The control circuitry may include circuitry for safely producing pulses of emitted light with the light source. This circuitry may include a signal generator that produces ramped pulses, a differentiator that differentiates the ramped pulses to produce differentiated pulses, and an output driver that produces current pulses for the light source based on the differentiated pulses.