Electronic – register a small change of voltage of a photo sensing diode at bullet’s speed

operational-amplifierphotodiodephotosensor

What I intend to do

Estimate Accuracy of a Bullet Hit from the Bull's Eye:

Use a row of light sensing diodes like BPW34 (or you suggest) spanning a feet vertically and horizontally or so with laser light incident on them from above and side respectively. When a bullet is fired between the laser and the diodes it will interrupt the incident light on any one of the horizontal and vertical diodes at speed of approx 2700 km/hour. This will cause a change of voltage/current values of the diodes that shall be registered. 7.2 μs response time depending upon surface area of BPW34 diode.

Problem Areas:

  1. Exact light sensing diode whose response time of light interrupt can produce a noticeable change in voltage/current at above speed.
  2. I intend using it in daylight so bullet interrupt will cause very low change in voltage/current value. Op-amp or something similar which can amplify the CHANGE ONLY.
  3. Passive circuit elements I would need to build.
  4. Connecting Raspberry Pi or Arduino with the circuit that can electronically register the change for further processing.
  5. Last but not the least: I am a starter at electrical circuits.

Additional Question

BPW34

Does this picture mean that the BPW34 diode's rise and fall time in visible spectrum (400 – 750 nm) is below 5 μs?

Best Answer

It seems you are looking to build a "bullet/ballistic chronograph". There's a lot of design discussion on the web. Here are just a few you might get some ideas from:

Uses a laser pointer and mirrors.

Uses a BPW34. See the project's photos for the simple detector circuit.

From Nuts & Volts magazine so it's a project aimed at hobbyists.

  • Search aliexpress.com for "bullet velocity chronograph". There are several relatively inexpensive offerings. You can get design ideas just from the pictures.

I intend using it in daylight so bullet interrupt will cause very low change in voltage/current value.

All of the designs I've seen provide their own illumination source. That way you're guaranteed to have a certain amount of light on the sensor and the bullet will block at least a known amount of light. Shielding ambient light is another way of controlling the level of light seen by the sensor.

Op-amp or something similar which can amplify the CHANGE ONLY.

Audio amplifiers do this, so it could be worthwhile to learn how they work. On the other hand, the first two designs mentioned above have very simple signal amplification schemes.

Finally, you might consider getting an oscilloscope. It has a built-in variable amplifier so it allows you to see very low level signals and it'll make it easier to determine how much amplification you need. You can even read off the time-of-flight directly from the oscilloscope traces.