When building multistage EM-3 coilgun I needed an optical sensor of the projectile. I decided to use an ordinary IR-based sensor. Such sensors are much written about (for instance, here), and I was experienced in working with them. Demands to the sensor were:

1. High noise stability and protection against an accidental triggering
2. Suitability to the construction of EM-3 which suggested sensing of the projectile moving inside IR-transparent plastic tube
3. As EM-3 is build on SCRs, the sensor must generate relatively high current pulses (up to 1 A) to activate them

These requirements defined the sensor to be strobed, i.e. activated by an external signal during the shot time. Among other things this allowed to feed IR-led by large current pulses, increasing the noise immunity, but reducing overall power consumption of the system.  

During production of the sensor I decided to add the indication elements to visualize its state. As a result, the scheme on fig. 1 was elaborated.

Fig. 1. Optical sensor circuit for SCR-controlled coilgun. OUT is connected to a gate of corresponding SCR.

As one can see, two power supplies are provided – for illuminating (IN1) and for detecting (IN2) parts, which allows them to be strobed by different signals. For example, in EM-3 IR-led is fed by 50-ms pulses of 40-mA current. Detector consists of the phototransistor driving the SCR by the power key VT2. Led VD2 indicates an activation of the sensor, and VD3 – high level signal on the output opening the SCR.

R5-C6 chain is build to prevent driving current of the SCR from flowing during all the time when the phototransistor is open. The reason is when the illuminating pulse finishes, the charging converter begins operating immediately, and all SCR must be reliably closed by that time. This is what is realized by R5 resistor limiting the current on a level not enough to open the SCR. But in the first moment after phototransistor and VT2 are open, the driving current is sourced from C6 and limited only by R6. The values chosen for R6, C6 ensure current pulse of 1A amplitude and effective duration of about 500 mcs.

EM-3 includes 5 such sensors used in 2-6 stages. 1^st’s stage sensor has some varied construction (described here). The video below shows the testing of the sensors, and the option of their separated activation through IN2 inputs is demonstrated. To activate all sensors simultaneously the paralleled IN1 inputs are used.