# Test equipment for measuring flashlights



## thburns (Aug 13, 2015)

Hi everyone! So lately I've become more and more interested in the physics and electrical engineering aspects of my lights now to the point that I'd like to get something a little better than my cheap equipment that I started out with, partly so that I don't need to do runtime measurements of my lights by locking myself in a closet for an hour and spying on the light meter every few minutes while simultaneously pointing my cheap infrared thermometer at the head of the light 


Thanks to reviews like selfbuilt's and others' here, I've also decided that I'd like to get an oscilloscope and a better DMM to measure the current draw of my lights, check for PWM, measure parasitic drain on standby and other things like the frequency for the strobe/beacon modes, and learn some electrical engineering in the process.


So far I've gotten my list of equipment down to these choices, and I would love to get your feedback!


*Light meters*


Either the Extech SDL400 or the General Tools DLM112SD. Both of these light meters have a thermometer, and one of my questions here is if the built-in thermometer that comes with these is as good (or as good for my needs) as one that I'd get on a dedicated data logging thermometer. I think both of these would be very good to perform runtime tests of relative output from 100% and temperature over time, to analyze things like thermal and timed stepdown. Right now I'm leaning towards the General Tools instrument, because I can get it with a NIST calibration for pretty much the same amount as the Extech meter without a NIST certificate. If I ever decide to try to build an IS, either of these seems like a good option too but I don't have enough first hand experience with light meters to know which one of these might be better, as they both seem to have pretty much the same specs but the Extech is about $100 USD more expensive.


*Oscilloscope*


This is an area where all I know is that I want one of these, but I have no idea what I need because I am an absolute beginner when it comes to electrical engineering. Flashlights have become a segway into a new interest of mine in EE in general, and it seems like I'll be able to teach myself a lot of things once I have one of these. As I said above, I want to measure things like the strobe frequency of my lights, determine whether they're current controlled or use PWM, and learn more about the electrical side of things in general. The only thing I do know right now is that I'm leaning towards a PC oscilloscope for the convenience that would provide (I want to be able to easily create GIFs/screenshots of the waveforms for things like flashlight review posts). I read a very long review of the Pico 2205A last night, and while I didn't understand 90% of what it was talking about, my takeaway was that it seems like a very good oscilloscope for the price and my hobbyist use cases. I've also been told that the Rigol DS1054Z is a good one to get, but $400 is a bit above what's in my budget (< $300).


*DMM*


Right now I just have a cheap Etekcity DMM, and I'd like to make the jump to something better, but not over $250 if possible. I always here Fluke, Fluke, Fluke when it comes to these, and I'm sure one of their more inexpensive models like the 115 or 117 would be good for my needs, but I don't know which one or necessarily what questions to ask myself to make that determination. I will also be using my DMM pretty much exclusively at my workbench, so if anyone has a suggestion for a benchtop DMM that might work well for me in my price range, I'd love to hear what it is and the reason why. I've also seen now that I might be able to get a used Fluke 87V for under $200. One thing I realized recently is that I actually need a DMM that can measure _u_A, for the purpose of measuring parasitic battery drain from standby modes on my lights.


Thank you ahead of time for your feedback, I'm really looking forward to hearing it!

Edit: Is this post more appropriate for General Flashlight Discussion? If so, could a mod please move me over there?


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## CoveAxe (Aug 13, 2015)

Electrical Engineer here.

The first thing is that I would forget about the light meter. You don't need it. If you have an oscilloscope/DMM, you just need a lux sensor that hooks up to it. Then you can datalog everything much more easily. Same for the thermometer as well: there is no shortage of thermal sensors that can connect to an oscilloscope/DMM and almost all DMMs can measure temperature. Having separate equipment just makes it more difficult to manage. This also let's you spend as much or as little on a sensor as you want. Just a quick google search finds a lux meter for $4 and a temperature sensor for $1.50. Those look like they would be more than enough for your needs. I wouldn't spend more than $20 on each sensor for this kind of application.

You also don't need NIST-certified anything unless you are calibrating products on a production line or doing some physics research project and other stuff like that. It's a complete waste of money for reviewing flashlights.

My second recommendation would be to instead look around on ebay or some test equipment resale sites for old scopes and benchtop multimeters if you really feel that you need them. You will find lab quality stuff that works just fine, only it will be a little old, and it will be at a bargain. They will be designed to be much more robust and it's also much easier to resell if you decide that you don't need it anymore. It may even be cost effective to get a scope with 4 channels instead of 2 and you can measure voltage/current/whatever on the scope and not even need the DMM at all.

Third, if you have to buy new, there are combined DMM/Oscilloscope scopemeters in a nice portable package with a screen, making things a lot easier. Hantek makes one that looks decent (I've never heard or used this brand before, so you may want to do some research about it). A pico or some other USB scope/DMM would be fine as well. I don't know the details of how you intend to measure things so I can't offer better help there. If you're doing everything simultaneously, then you would need a 4-channel, otherwise a 2-channel would be fine. Buying the DMM/scope separately is fine as well if the specs/cost doesn't work out with them together.

Personally, I think fluke products aren't bad, but they are extremely overpriced. They have name recognition and they know it so they charge a premium for it. I don't think you will have anything to gain jumping to a fluke from your cheap meter. I've been using my old $20 radio shack DMM for 15 years now and I haven't found any reason to get a new one yet. If you want a high-end benchtop DMM, we use Keithley meters at my workplace for when we need sensitive measurements and they're pretty good. Certainly overkill for what you're doing though.

Peronsally, I think you're also overthinking everything. These are flashlight reviews, not physics PhD research projects. You don't need high-end hardware for this. I work in a world-class research lab. You know what we use most of the time for measurements that get published in research journals? Some simple USB DMM modules that we bought for about $100 a piece.



> One thing I realized recently is that I actually need a DMM that can measure _u_A



I doubt it. The self-discharge of the battery is going to be more than that. Even 10uA range is probably overkill. You can do uA-range measurements if you want, but you'll be paying more for it and it's not going to tell you anything interesting.


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## SemiMan (Aug 14, 2015)

Before answering, can I ask what you want to do? That will define the equipment. 

I have Fluke's, B&K, and cheap meters. The B&K is a bench unit with USB ... like it, but outside your budget. Have the cheap ones cause you often need a few and I like to keep the good one's clean. The Fluke's ... can always trust them, which is why people buy them.

The Rigol are pretty good for the money. Not likely to last forever like a Tek, but you are not going to use it a lot either. Without knowing what you are doing, can't suggest PICO or RIGOL, but the speed of the Rigol limits usage.

I am with CoveAxe on light and temp measurement. You can get NTC chips for pennies, and are accurate to a few celsius. You can buy a cheap light meter for $20-30 that likely good enough for you.


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## thburns (Aug 14, 2015)

So I'd like to take the same kind of measurements on my lights as you find in reviews like selfbuilt's for starters. Relative output runtime tests (but I'd like to combine that with temperature data to see things like thermal regulation kick in), strobe frequency, determining PWM with the oscilloscope, determining standby current, etc. The scope I'm looking at now is the 10Mhz PicoScope 2204A, since it seems from replies I've gotten here and elsewhere that I don't even need anything close to something like the Rigol DS1054Z for the things I'm initially looking to do. I'm happy to pay a bit less now to get myself started and then pay more down the road once I've learned more.

As far as the light and temperature measurement goes, the main reason I was looking at a data logging light meter that included a thermometer is so that I can leave the instrument relatively unattended in my bathroom where I'll be performing the runtime tests, and also so that I can take the measurements at a faster interval than I'm able to do accurately by hand with a non-logging meter. So laziness in part there, but I'd also rather have kind of an 'out of the box' setup initially, and not need to fiddle with custom probes. I don't mind spending a bit more to get a separate light meter/thermometer if it'll make it more convenient to do those kind of tests. But if it's simply a matter of plugging in the probes to the scope and configuring the scope in a certain way to get lux readings and temperature, and be able to log that into a spreadsheet easily, then I'm all for that as well!

As far as the DMM goes, I've watchlisted a bunch of Fluke 87V listings on Ebay now, so I'm going to keep an eye out for one of those and just keep using my cheap $15 DMM for now.

Sorry if I sound a bit ignorant about a lot of this stuff.. Just getting started but I'm very excited to get deeper into the subjects here!


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## CoveAxe (Aug 14, 2015)

> But if it's simply a matter of plugging in the probes to the scope and configuring the scope in a certain way to get lux readings and temperature, and be able to log that into a spreadsheet easily, then I'm all for that as well!



It's not quite that easy, but it shouldn't take more than an hour or so of setup to do it this way.

The reason I recommend doing all of that with the scope/DMM instead of having it separate it two-fold:

1. Eliminating the light meter and thermometer means that you can take the money that would have gone to that and instead buy a much better scope/DMM that you shouldn't ever need to replace. $200-300 for a lightmeter and thermometer seems very expensive to me for something like this. Spending $500+ will get you a pretty good hobbyist scope and the separate sensors will get you the exact same data. Or you can get a pico (not bad in its own right) and just pocket the difference for something else later.

2. I've found controlling instruments through the computer to be infinitely more useful than built-in data logging. The problem with built-in data logging I run into is that you will inevitably find out that it doesn't support something you're trying to do, or the memory card formats change and I can't easily get the data anymore, and I end up having to buy a new sensor anyway.

If all you plan on doing is flashlight reviews and never anything else, than your setup will be fine. But I suspect if you get into this, you will find other uses for your equipment and all of a sudden the built-in features are more of a hindrance than a help. Just my experience, at least.


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## PointyOintment (Aug 15, 2015)

thburns said:


> One thing I realized recently is that I actually need a DMM that can measure _u_A, for the purpose of measuring parasitic battery drain from standby modes on my lights.



No you don't. Multimeter μA ranges are usually very inaccurate. What you need (if you actually want to measure such tiny standby currents) is something like a μCurrent (made by Dave Jones of EEVBlog fame), which you can use with any multimeter to get much more accurate readings.


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## thburns (Aug 15, 2015)

CoveAxe said:


> It's not quite that easy, but it shouldn't take more than an hour or so of setup to do it this way.
> 
> The reason I recommend doing all of that with the scope/DMM instead of having it separate it two-fold:
> 
> ...



For your first point, I did end up getting the Pico 2204A to start out with. I think what I'm going to do is use this to start out with and learn, and then get a better scope later on once I understand all of the fundamentals of what I'm trying to learn, which is general electrical engineering, not just flashlights (but with flashlights as a starting point). Also since the Pico is a PC scope anyway, I figure I'll be able to do the setup you described with the sensors. Are there any resources you can point me to for how I might go about setting this up? I'm probably just googling the wrong things, but searching for things like 'oscilloscope lux meter' doesn't seem to be turning up what I'm after.



PointyOintment said:


> No you don't. Multimeter μA ranges are usually very inaccurate. What you need (if you actually want to measure such tiny standby currents) is something like a μCurrent (made by Dave Jones of EEVBlog fame), which you can use with any multimeter to get much more accurate readings.



So I watched a video that explained burden voltage (which I mostly understood by the end), and now I see what you mean, and the need for something like the μCurrent to measure the standby currents. That's definitely something that interests me, so I'll see about getting one of those.

Thank you everyone for your advice and help! I really appreciate it, and hopefully I'll have a good setup in no time that I can show off the results from.


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## CoveAxe (Aug 15, 2015)

It's perhaps a little low on the bandwidth aspect, but other than that I don't really see a need to get anything better unless you are doing something much more complicated (which unless you're doing this for a living, is probably a "no"). I wouldn't worry about it unless it becomes clear that the scope is limiting you in some way.



> Are there any resources you can point me to for how I might go about setting this up? I'm probably just googling the wrong things, but searching for things like 'oscilloscope lux meter' doesn't seem to be turning up what I'm after.



It's just simple arithmetic: The datasheet for the sensor says that it puts out X amount of current for Y amount of lux. The current goes through a 68k resistor so now if you know the output voltage, you know the lux. You measure the voltage on the DMM/oscilloscope. If you have a table of measured voltages, you can easily write an equation to map it to measured lux.

There are hundreds of different kinds of sensors that all work differently, so it's going to depend on what kind of sensor you get.



> So I watched a video that explained burden voltage (which I mostly understood by the end), and now I see what you mean, and the need for something like the μCurrent to measure the standby currents.



Just an FYI, a 100uA standby current would take over 2 years to completely drain a standard alkaline battery. I'm admittedly not that familiar with flashlights, so I don't know what's considered "acceptable" in this field, but that seems quite long to me. I only bring this up so you keep that in mind when buying high-accuracy equipment. If your benchtop DMM can measure accurately in the hundreds of uA range, measuring high-resolution extremely low currents may not be a useful metric.


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