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As EM-3 coilgun is muzzle-loaded (i.e. the projectile is manually loaded to barrel from the front end), the following hazardous situation may occur: the capacitor is already charged, the shooter puts the arrow into barrel and accidently fires the coilgun while holding his hand in front of it.. Aftereffects may be sorrowful…
I decided to avoid such a threat and protect the shooter from possible wounds. To do this I could for example incorporate to the construction some trigger to be toggled to “off” position after each shot. But I suggested this decision to be inelegant. It would have been much nicer solution to construct a simple detector to block the shot when the hand is near the muzzle cut. So I decided to use the reflected IR-radiation sensor (i.e. IR-radar) by Sharp GP2Y0A21YK0F (see fig. 1). It is primary intended for electronic vision systems (for example in moving robots).
Fig. 1. GP2Y0A21YK0F sensor.
As datasheet says, this module has 5V supply and wide output signal range, so to fit it to EM-3 I made a simple scheme with integrated voltage stabilizer and comparator (fig. 2). Its low output level blocks the shot of the coilgun.
The comparator’s threshold is varied by R2 potentiometer. Taking into account the noise immunity requirements and drift of the constant component of the sensor’s output voltage depending on environmental illumination, the threshold was set to 0.5 V, which allows to locate an adult’s hand at a distance about half of meter. R1C1 chain assures some delay before the unlock of the sensor to additionally secure the scheme. To indicate the sensor’s state LED is added.
The sensor is mounted on the front end of EM-3 in a small plastic bow painted black, its functionality is shown on the video below.
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