# Design and Experiment of Discharge Control Methods for Three-Stage Coil Gun Experiments
## 4 Discharge Circuit of a Multi-Stage Coil Gun System  
### Discharge Control Method Using a Photointerrupter  
The first method proposed in this study is the direct detection of a projectile using a circuit with an infrared light-emitting diode (LED) and a phototransistor. As it entails the direct detection of a projectile, it is more apt to use a photointerrupter circuit by configuring the projectile to pass between an infrared LED and a phototransistor than to use a photodetector circuit to detect reflected light. Figure 6 illustrates how to fit a launch tube for projectile detection using phototransistors with infrared LEDs.  
It is recommended that the launch tube be fabricated with an opaque material to prevent the dispersion of light output from infrared LEDs and to reduce the operating error. A photointerrupter circuit should output the SCR operation signal when a projectile is detected--that is, when the light emitted from the infrared LEDs does not pass through the phototransistor. Photointerrupter circuits must operate in a way that goes against the properties of a phototransistor, which converts light into power. Therefore, a circuit to be used as a photointerrupter needs to be used to fit this logic.  
Photointerrupter circuits can be used easily with an open-source controller such as an Arduino. However, when using an Arduino to configure a discharge control circuit, its control signal output may be lower than the SCR's operating current. Therefore, an additional circuit for amplifying the control signal may be required. In this study, a photointerrupter circuit that can independently operate with only an external power supply along with the capability of SCR operation was used. Figure 7 shows the SCR discharge control circuit with a photointerrupter.  
Figure 5: The prototype of the charging circuit for the coil gun capacitor.  
Figure 6: Configuration of a launch tube, infrared LEDs, and a phototransistor.  
The discharge control circuit receives 12 V DC power, operates devices such as LEDs and transistors, and supplies the current to operate the SCR. The circuit requires an NPN transistor, and a 2N2222 transistor was used in this study. The resistance value should be determined according to the LEDs, phototransistor, and DC power supply voltage used in the circuit. SI5312-H was used as the infrared LED used in the experiment, and ST-5811 was used as the phototransistor. Based on the circuit diagram, a prototype of the discharge control circuit was fabricated for use in the experiment. Figure 8 shows the prototype used in this study.  
The infrared LEDs and phototransistor should be attached to the launch tube as shown in Figure 6 and fabricated to allow the adjustment of the position of the sensors. This is because even the optimal value of the trigger position obtained through simulations can be different from that in the experiment. Unlike in a simulation, during the experiment, the projectile is not located in the center of the launch tube, and friction exists between the projectile and the launch tube. Moreover, even when coil winding in the simulation and the experiment is same, the inductance and resistance values are different. Therefore, in the experiment, launch tests should be conducted based on the trigger position calculated in the simulation while adjusting the position of the projectile during operation of the coil gun. Figure 9 shows the discharge control circuit in the experiment with a launch tube installed.  
Figure 8: Prototype of the discharge control circuit.  
Figure 7: Schematic of the photointerrupter discharge control circuit.  
Sensors should be installed to face each other and send out SCR control signals when the projectile moves through and blocks the light. Because the sensors must directly detect the projectile, the launch tube needs to be installed as shown in Figure 9. As the launcher velocity increases, the discharge position of the projectile moves farther away. Therefore, coils should be sufficiently spaced to secure the distance between sensors.