My parts list is here: http://a.co/fltiKey , and the light sensor is one of those Chinese MH-Sensor-Series Flying Fish with a photodiode. The difference between space and '#' at a normal terminal/editor font is around 20 counts (out of 1024 total), which is plenty enough to detect reliably, and the response time is easily <1ms, as cross-checked by the 1000fps camera.