The photogate circuit shown below is especially effective for fast-moving objects. It will trigger on the passage of a pellet traveling at nearly the speed of sound. The photogate consists of an emitter and detector of light aimed at each other. The interruption of the beam of light from emitter to detector serves as the triggering event. The emitter may be a light-emitting diode (LED), a laser, or a beam of white light from, say, a penlight. The detector is a phototransistor, serving as a variable resistor whose resistance depends on the intensity of detected light.
The circuit shown uses an infrared emitter and detector. A 2N2222 transistor acts as a switch in the gate-cathode path of an SCR. A sensitive-gate SCR is used. The SCR is connected either to the terminals of a flash unit or to a delay unit (to be described later). As long as the phototransistor is illuminated by the LED, the collector-emitter path of the 2N2222 is open. When the light is blocked, the voltage across the phototransistor rises and the 2N2222 conducts, thereby gating the SCR.
The sensitivity of the photogate is controlled with the 100-kΩ variable resistor. This is an important feature for detecting small, fast objects and allowing large separations between the detector and emitter. For maximum sensitivity, the resistance is adjusted low enough to raise the voltage across the phototransistor to the threshold of triggering. Whenever the detector-emitter separation is increased, the resistance must be increased, since the phototransistor's own resistance rises as the illumination on it decreases.
An interesting photograph to try with the photogate is the burst of a popcorn kernel. Aim the photogate across the top of a hot plate and just above the popcorn kernel. When the kernel pops, it will break the beam and set off a flash unit. Be sure to keep your equipment and eyes protected from oil spray.
The photogate above may not work well for slow-moving objects such as liquid drops. The one shown below uses a 555 timer as a Schmitt trigger. When the light from the emitter (LED) to the phototransistor (PT) is blocked by a moving object, the voltage across the phototransistor increases. When it reaches a certain level, a square, 9-V output pulse is produced at pin 3, thus gating an SCR. The sensitivity is adjusted with the 10-kΩ potentiometer.
A component called an interrupter (shown to the right) can be used to replace both the phototransistor and LED. The interrupter has both emitter and detector molded into a plastic housing with a small gap between them. This can be convenient for triggering on the passage of small or narrow objects such as strings and drops.