IR dectctor too slow? - Page 1
Forum section: electronics / projects

author: Tajjbe
author_rank: Newbie
timestamp: March 09, 2023, 01:24:13 pm
content: I just started building an ballistic chronograph to test bullet speed of an air rifel... Mostly as a fun project.My idee is to set up two IR emitters at a fixed distance from each other, and on the other side of each emitter, set up a row of detectors.When a bullet comes throught it will breifly break the IR light to one of the detctors and then measure the time it takes to break the other sensor-array.I set up it up for testing and it works as I intended. The IR-detectors produce a voltage of about 300mV and when I drop a bullet between the emitter and sensor it goes down to about 100mV.The problem is now that it does not register a bullet when I shoot it from the rifel. As it is to fast to registered?IR-detector, bought on Digikey: LTR-323DBIR-emitter: VSMB3940X01-GS08The rise and fall-time of the detector is 50nSec.. The the bullet should break the light for about 10uS (200m/s).Can anyone help me what I am doing wrong or point me in a direction to solve my project in another way?Thanks!

author: MarkS
author_rank: Unknown
timestamp: March 09, 2023, 01:34:16 pm
content: Three questions come to mind: 1) Are you sure the bullet is breaking the beam? 2) Are you protecting the beam from ambient light? 3) Are you sure your math is correct in respect to the time the bullet breaks the beam?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: March 09, 2023, 02:31:11 pm
content: For something like this I'd use a phototransistor not a PD. 10K pullup resistor. That should give you a signal that can be used directly as a logic-level inout

author: Marco
author_rank: Super Contributor
timestamp: March 09, 2023, 02:35:36 pm
content: The speed in the datasheet is for use as a photodiode, are you using it in photovoltaic mode by any chance?Also the speed is for 1K bias resistor. Increase the resistor, decrease the speed.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 09, 2023, 04:10:43 pm
content: The speed specs are with a reverse voltage of 10 V and thus photocondictive mode. The photovoltatic mode is slow with photodiodes - more in the 5-50 µs range, corresponding to the minority carrier lifetime.This circuit how to use the photodiode makes quite some difference.

author: Terry Bites
author_rank: Super Contributor
timestamp: March 09, 2023, 04:11:07 pm
content: You want to be in photoconductive mode as marco and kleinstein say. In the data sheet gives you rise times with 10V reverse bias.seehttps://e2e.ti.com/blogs_/archives/b/thesignal/posts/illuminating-photodiodes.Its a good idea to use a photodiode calculator tool. (dont forget to add the cable capacitance to the photodiode)https://tools.analog.com/en/photodiodeOnce you have your amp designed, you can servo out the ambient light.https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/218058/photodiode

author: Tajjbe
author_rank: Newbie
timestamp: March 09, 2023, 06:19:02 pm
content: This field in electronics is a pretty new for me so thank you all for info!Yep, I have just tested it in photovoltatic mode.. This is likely the problem..In any case I will double-check my math also.I'll try photocondictive mode and see if I can detect my bullet!@Terry Bites, good links, will come in handy!