Looking for a light sensor with fast response time - Page 1
Forum section: electronics / beginners

author: is-this-taken
author_rank: Contributor
timestamp: July 11, 2023, 07:49:36 pm
content: Hi!I want to replace a switch with a light sensor with a fast response time to use a switch. As a noob I'm not sure what to look for.Response time should be in the range of microseconds.What am I doing? I want to precisely geotag photos from a drone (DJI), and only have an LED indicator that the photo is being taken. Using a dGPS so I have 1-2cm precision and the drone is moving so timing is important.The GPS unit has connector for a simple switch and I want to use a light sensor to log the time.Thanks!

author: Kim Christensen
author_rank: Super Contributor
timestamp: July 11, 2023, 08:07:30 pm
content: A simple phototransistor may do the job. I'm assuming you can place the phototransistor right against the LED and have it shielded from ambient light?Also depends on how much current it needs to switch. Might take a bit of experimentation to ensure it's reliable.Another option would be to remove the LED and wire an opto-coupler's LED in it's place and use the output of the opto to trigger the GPS. This would be less hokey. Again, we need to know how much current the "switch" needs to pass to trigger the GPS.

author: TimFox
author_rank: Super Contributor
timestamp: July 11, 2023, 08:48:13 pm
content: If you use a phototransistor with a resistive load in its collector, you may get more Miller effect capacitance than you want for fast response, with the inverting voltage gain from B to C.Look at the capacitance specifications for any photodiode or similar that you are considering.For a simple photodiode (including the BE diode of a phototransistor), the simple model is a current generator (proportional to light input) in parallel with a "parasitic" diode, which has some capacitance.If the DC voltage across the diode is zero, that capacitance will be a maximum.If you apply a reverse bias across the diode, that capacitance will decrease dramatically, but you will have some dark current due to the bias voltage.You can use a simple op-amp circuit to amplify the photodiode current (into a virtual-ground "short circuit"), with or without bias voltage and follow it with a comparator to derive your trigger signal.

author: is-this-taken
author_rank: Contributor
timestamp: July 11, 2023, 08:49:27 pm
content: I'll be 3d printing a light proof housing for the sensor and attaching it right up the LED.As this is a DJI consumer drone I do not want to get into disassembly.Can you give me some examples or how to word my query?

author: jc101
author_rank: Frequent Contributor
timestamp: July 11, 2023, 09:03:34 pm
content: Don't forget that whilst you get a very accurate location of the drone in the geotag, it won't be what the camera is actually pointing at unless you take a picture directly below the drone.It gets more complex if you want to geotag what the camera looks at, as angles, height etc., all need to be calculated. Depends on your requirement.Helping with a downlink of the Police heli-telly link many years ago now, I was always impressed when they could point the camera at a house whilst flying around, and the computer would pop up the house's address.

author: David Hess
author_rank: Super Contributor
timestamp: July 11, 2023, 11:14:17 pm
content: A phototransistor or photodiode with appropriate signal conditioning will be more than fast enough.

author: is-this-taken
author_rank: Contributor
timestamp: July 12, 2023, 12:02:56 am
content: Quote from: jc101 on July 11, 2023, 09:03:34 pmI'll be doing photogrammetry, so no issues there, for me it's important to know the position of the camera.Quote from: David Hess on July 11, 2023, 11:14:17 pmCan you please elaborate a bit?

author: ArdWar
author_rank: Frequent Contributor
timestamp: July 12, 2023, 03:17:13 am
content: How fast do you need it to be? Even a badly biased phototransistor will still give you at least in the hundreds of microsecond response time. That'll still give you millimeter-level accuracy. Surely at least not worse than typical DGPS (RTK?) walk.At this point I probably more concerned if the LED is even properly in sync with sensor readout timing, or if the GPS position is even updated that fast.

author: Faranight
author_rank: Unknown
timestamp: July 12, 2023, 02:01:45 pm
content: I'm assuming you don't want to tear apart the drone to get access to that LED and run some wires from it to whatever circuit you're using, yes?And you're planning to use the LED light as a trigger source sort of in an optocoupler configuration?I'm not sure, if this is going to be useful to you or not, but inanother threadI shared a circuit that I use to monitor the light level by using a photodiode and an opamp.You could maybe remove the RC filter and add a Schmidt trigger afterwards to limit the sensitivity to changes of light (also watch out for stray light sources that may accidentally trigger the switch).

author: David Hess
author_rank: Super Contributor
timestamp: July 12, 2023, 05:51:15 pm
content: Quote from: is-this-taken on July 12, 2023, 12:02:56 amPhotodiodes and phototransistors are fast and can easily respond within 100s or even 10s of nanoseconds. Speed then depends on the signal conditioning circuitry, but switching times of hundreds of nanoseconds are not difficult.

author: TimFox
author_rank: Super Contributor
timestamp: July 12, 2023, 09:26:58 pm
content: An example of a packaged "photosensor" (photodiode plus logic-level output circuit) with low microsecond delay:https://www.mouser.com/datasheet/2/308/1/QSE159_D-2320048.pdf

author: liaifat85
author_rank: Regular Contributor
timestamp: July 14, 2023, 11:27:04 am
content: I find LDR satisfactorily fast.

author: is-this-taken
author_rank: Contributor
timestamp: July 14, 2023, 09:40:40 pm
content: Well I found an NPN phototransitor in local shop, SFH 300 3/4, hooked it up and it works, but it's just not sensitive enough for the LED I got on the drone.I'm guessing I need a more sensitive phototransistor or is there another way to work around it?From the specs sheet:PhotocurrentVCE= 5 V; Std. Light A; Ev= 1000 lxIPCEtyp. 4800 µAI have no clue how to interpret that. I looked at their other available phototransistors, but I'm at a loss as which ones are more sensitive.What values should I look for?Here's a list of what they have available:BPV 11BPW 40BPW 77NBBPX 43-5BPY 62-3IRE 5SFH 300-3/4SFH 309PSFH 310FA

author: TimFox
author_rank: Super Contributor
timestamp: July 14, 2023, 10:00:41 pm
content: "1000 lx" is fairly bright: a TV studio lighting is around 1000 lux, office lighting is 200 to 500 lux.At that level of light, the spec says the collector current (with +5 VVce) will be 4.8 mA.That is presumably for some form of white light.What is the spec for your LED? The output will be proportional to the diode current, other things equal.Note that the photocurrent is a function of wavelength, and will be less at red than at blue, other things equal.Back of the envelope calculation:A typical red LED has a total output of 40 mcd (millicandela) at 20 mA diode current.1 cd at 1 m distance is 1 lx.Assuming 1 cm from the LED, 1 cd would give 104lux at the detector (inverse-square)Therefore, 40 mcd would be 400 lx at 1 cm, so your first spec would give 1.9 mA collector current.I have not included any spectral sensitivity here.How much do you need?

author: Terry Bites
author_rank: Super Contributor
timestamp: July 27, 2023, 10:39:19 pm
content: Exactly, chill...