Latching Photogates for the Right Angler, Popsicle Dragster, Ramp Climber and Tube Competitions

The timing of moving objects for these competitions require two photogates that once triggered can no longer start or stop the timing process. The objects often have gaps in their structure that could cause multiple start and stop triggering of the timing. Photogate computer software available from companies such as Pasco or Vernier can be programmed to no longer search for a particular photogate once it has been initially blocked. The problem with using software of this type is that it requires connection to a computer and proprietary interface hardware.

To avoid using cumbersome hardware and connections to a computer for our competitions, we constructed a latching photogate timer with two photogates (start & stop). The photogates make use battery powered red laser diodes and laser detectors that are powered by a 5.0 volt supply. When either photogate is blocked, a pulse is sent to the latching circuit that locks out further pulses from that photogate while simultaneously generating a 30 millisecond pulse that momentarily closes a relay connected across the "start/stop" switch of a digital stop watch (interval timer), thus starting the timing. When the second photogate is blocked, a similar action stops the timing. The time interval can be viewed on the large numerals of the interval timer (hanhart prisma 200). After the time is recorded, the interval timer and the latching circuit can be reset by a push button switch.

College of Saint Benedict
Saint John’s University

Jim Crumley
Chair, Physics Department
SJU PEngl 107
Physics Department