I am attempting to design a sensor that can non-invasively detect if there is a bullet in the chamber of a gun. To help explain my problem I have put together some models and images for you. First off this is the bullet chamber.
And here are the dimensions(mm) of the barrel which is made of a 4140 steel
This is the what it looks like when the bullet is in the chamber.
The typical bullet weights 4 grams and is made of Pb encased in a thin copper jacket.
The gunpowder is made up of these components.
So my question is if I turn the barrel into and inductor could I detect a change when a bullet is in the chamber?
I have also found some inductor to digital converters that may make this easier to implement. My main concern is that the inductance change from the bullet may be to small to detect. Also as Scott Seidman mentions in the comments the barrel can get quite hot(greater then 800F) but I would like to know if the bullet can be detected at all before I worry about the problem of temperature and whether or not it can be calibrate out.
What do you guys think? Should I build a prototype and try it out?
Best Answer
I believe that at least two methods would be reasonably viable - ultrasonic transducers, or inductive sensing. One which may also work is by capacitive coupling.
You have already thought about inductive sensing, where you could emit a field using the loop and a small constant current source, and the receiver will get a different (higher?) signal strength when a bullet is present in the chamber. Would need calibration for an empty barrel condition, for whatever interfaces with the result of the sensor.
Ultrasound - with a transmitter and receiver pair at opposite sides of the barrel will pick up a stronger signal strength when a bullet is there (less air, more rigid material to transmit through). You can also do it with the sensor only on one side, by getting the reflected sound with a phase shift (or a time difference) due to the distance it has traveled. It will be shorter distance if there is a bullet there. For an interesting read on one version of this, read this page.
Lastly, something which might work is capacitive sensing (uses capacitive coupling). A capacitive sensor approach would transmit a pulse through the material and if the pick-up point is the opposite side of the gun barrel, then if the bullet was there, the capacitance would be very low compared to when there is no bullet. The bullet would act as a conductor of the signal.
I suggest you make a simple test for each of these three methods and see what sort of signal levels with a bullet, and without a bullet, you can observe. If one of the methods works, and is easily above the inherent noise level, then you can deal with the temperature and operating conditions of being inside/on a firearm.