A speed sensor determines the velocity of an object. A first detector detects light emitted by a first emitter. The first detector includes multiple photodetectors connected in series. The photodetectors produce a current that is proportional to the amount of light incident to the photodetector. The first detector additionally includes a current-to-voltage converter and a threshold detector. The current-to-voltage converter converts the current produced by the photodetectors into a voltage and the threshold detector compares the voltage generated by the current-to-voltage converter with a threshold voltage. When an object passes in between the emitter and any of the photodetectors, the light incident to the photodetector is at least partially blocked, reducing the current produced by the photodetector. Any sufficient reduction in current that reduces the voltage generated by the current-to-voltage converter below the threshold of the threshold detector is quickly registered as an output signal of the threshold detector.
