Detecting Bullets with Reflected Light - Page 1
Forum section: electronics / beginners

author: mribble
author_rank: Regular Contributor
timestamp: November 06, 2017, 01:54:20 am
content: I found this white paper about building opamps for photodiodes:http://www.ti.com/lit/ug/tidu535/tidu535.pdfI certainly don't understand everything in the paper, but it is very well written and even without understanding parts I could still adopt the circuits from 5V to 3.3V.I did have several questions about how important the PCB layout is and was wondering if you guys could look at that? I've attached svg files for the schematic/pcb, and the kicad files for those who use kicad. My main questions revolve around R1+R2 and the traces attached to them. In the white paper they paid a lot of attention to those traces to reduce impedance. I removed the ground planes around them, but didn't include the vias like they did. Could people comment on whether that part of the layout of my board looks reasonable? And if you see anything else that could be improved let me know.If something is wrong please be gentle. I'm a software guy that just struggles through the hardware as best I can.Thanks for any help!

author: rstofer
author_rank: Super Contributor
timestamp: November 06, 2017, 03:19:45 am
content: It would be better if you could attach a .pdf. Not everybody uses the same applications.

author: mribble
author_rank: Regular Contributor
timestamp: November 06, 2017, 01:53:49 pm
content: There are svg files in the zip. Any modern browser can open those. Does that work for you?

author: StillTrying
author_rank: Super Contributor
timestamp: November 07, 2017, 04:46:37 pm
content: I thinkDetecting Bullets with Reflected Light using a photodiodeis going to be difficult.I'd guestimate that the output from the opamps will only be around 5 to 10mV when the bullet passes an ideal position above the photodiode and LEDs. Assuming my guestimate is close will that be enough?At these 1MHz speeds you don't have to worry too much about the impedance of the track between the 50R and the board's connector.On the PCB layout the opamps +Ve input and output appear to be shorted together?

author: floobydust
author_rank: Super Contributor
timestamp: November 07, 2017, 08:59:00 pm
content: I can't see how the bullet makes it exactly past a single photo-diode..22 bullet diameter is 5.7mm so a single photo-diode cannot sense much wider an area. There's some optical engineering needed. Ambient light will also get in the way.Using.22 bulletvelocity of 330m/s (1,083 fps) ~sonic, that's about 1mm in 3usec and a 8mm long bullet gives an event say around 25usec max.Maybe look at safety light curtains, they use a photosensor array and lenses to cover a wider area.

author: rstofer
author_rank: Super Contributor
timestamp: November 08, 2017, 04:20:40 pm
content: This kind of thing has been done for decades and usually optical. There was a time when the bullet was shot through metalized tapes but that was a long time ago and done in a laboratory. I have no idea which photodetector is used.Here is one incantation:https://www.midwayusa.com/s?userSearchQuery=ced+chronograph&userItemsPerPage=48I don't imagine I'm going to take mine apart...The sensors aren't worried about background light, they want to see the glint off the passing bullet. That said, there are sun positions that are less than optimal: early morning, late afternoon, winter (in northern hemisphere) and so on. In those situations, there are the infrared skyscreens which place IR LEDs above the sensors.When things get really difficult, people build plywood boxes to exclude all external light and just use the infrared skyscreens. This might be common at pistol matches where ammo is tested for rules compliance. A fold-up box would be ideal.The sensors are 2' apart so for a bullet travelling 3000 fps, the pulses are 666.7 uS apart.For rifle, the MagnetoSpeed is very handy (no messing around down range):http://www.magnetospeed.com/products/chronographs/If you want the right answer, there is Ohler. Their model 35P uses 3 sets of screens. My 1980s incantation uses 2 screens 10' apart. Counters didn't run as fast back then.The Ohler site has a problem so here is a review of the Ohler 35P versus the MagnetoSpeed.https://rifleshooter.com/2015/11/magnetospeed-v3-versus-oehler-35p-chronograph-comparison-and-review/

author: mribble
author_rank: Regular Contributor
timestamp: November 18, 2017, 12:46:05 pm
content: Sorry, I didn't see any replies after a day and didn't think anything was coming. So I went ahead and made the PCB and assembled it. It is kind of working, but I'm having some issues.First let me address the previous comments:Stilltrying - I don't understand what you mean by "On the PCB layout the opamps +Ve input and output appear to be shorted together". I don't see the +Ve input (which is 3.3V) connected to the output. I'd be interested in how you estimated the 5mV change. That said, I do think I could detect that.floobydust, I don't need a wide detection area. The typical 10-20 degree light emission from an LED will give me enough range. I am blocking ambient light so that isn't an issue.rstofer, I have seen those and many more. None of them meet my needs which is why I'm trying to design my own.So now to the issue I'm seeing with my current circuit. I have a 12 bit ADC reading these values (0->4095). I see values around 3950 with no reflected light and that jumps to 4020 when there is some reflected light. I'm trying to figure out why there isn't more of a voltage shift (which basically is why is my base voltage with no reflected light so high). I don't have experience with transimpedance amplifiers (which is the type of circuit I'm using) so if someone with experience there can tell me what is happening that would be great. I am going to go an do some research on my own now, but any help others can provide would be hugely appreciated!I used the info in this whitepaper to design the circuit:http://www.ti.com/lit/ug/tidu535/tidu535.pdf

author: mribble
author_rank: Regular Contributor
timestamp: November 18, 2017, 09:39:35 pm
content: I figured out what was wrong. I needed to switch the resistors in my voltage divider that I'm using as the positive offset voltage to prevent over saturation. Switching R5 and R3 in the schematic I attached addressed the issue. I might need to adjust the gain resistor to make it more sensitive (I'd rather do that than increase my IR light), but overall I'm pretty happy with how it is working now.

author: StillTrying
author_rank: Super Contributor
timestamp: November 19, 2017, 02:55:38 am
content: Quote from: mribble on November 18, 2017, 12:46:05 pmI might have been looking at the wrong package. On the layout, it looks like there's only 3 connections to U1."I'd be interested in how you estimated the 5mV change. That said, I do think I could detect that."I didn't do a lot of calculating! I've been playing with photo diodes lately.https://www.eevblog.com/forum/chat/20w-halogen-bulb-viewed-by-a-photodiode/"So now to the issue I'm seeing with my current circuit. I have a 12 bit ADC reading these values (0->4095). I see values around 3950 with no reflected light and that jumps to 4020 when there is some reflected light. I'm trying to figure out why there isn't more of a voltage shift (which basically is why is my base voltage with no reflected light so high)."Assuming the ADC Vref is 3.3V, you're already getting a 56mV change which is larger than I thought you would.Use an old de-skinned electrolytic cap or similar to represent the bullet."I don't have experience with transimpedance amplifiers (which is the type of circuit I'm using) so if someone with experience there can tell me what is happening that would be great. I am going to go an do some research on my own now, but any help others can provide would be hugely appreciated!"Transimpedance amps are always difficult, although for your use a bit of overshoot won't matter at all as long as it doesn't keep oscillating. You could flash a LED at it from a FG or 555 to test/tune its response.I didn't realize you were usingIRphotodiodes.