An advantage that a photogate has over a sound trigger is that the former triggers on the precise location of the object that breaks the beam. For example, the shape of a snapped elastic cord can be photographed as it passes through the beam. While a sensitive sound trigger could also capture the shape, the location of the cord when the flash discharged would be much less predictable.
A complication when using a photogate occurs if the event to be photographed occurs after the breaking of the light beam. For example, if a liquid drop is to be photographed as it splashes into a pool of liquid, the photogate must be located above the pool. If the passage of the drop through the photogate were to be the event that triggered the flash unit, there would have to be a delay to allow the drop time to reach the pool. This difficulty can be overcome by using a delay circuit.
The delay circuit shown above uses a 556 timer, an IC consisting of two 555 timers. Grounding the input starts the first timer and produces a square pulse at output 1 (pin 5). The width of the pulse is determined by the choice of capacitance, C, and the setting of the 1-MΩ variable resistor. Output 1 is coupled to the trigger (pin 8) of the second timer through a 0.005-µf capacitor. When output 1 falls to zero, the second timer starts, producing a 10-ms pulse at output 2 (pin 9). This pulse gates an SCR to discharge a flash unit. Output 1 can also be used to gate an SCR in order to provide an immediate flash discharge. The smallest time interval between the immediate and delayed discharges is about 0.2 ms.
When using the photogate and delay unit together, the output of the photogate becomes the input of the delay unit. The value of C chosen for the delay circuit depends on the experiment being conducted. If, for example, a drop of milk passes through the photogate and falls a distance of a half meter into a pool, a delay of about 0.3 s is required to capture the splash. Using a 0.5-µf capacitor for C provides a selection of time intervals up to 0.5 s. (The time delay in seconds is approximately equal to the product of the variable resistance in megohms and C in microfarads.)