Function Generator App for Smart Phone (What jack/cable to use) - Page 1
Forum section: electronics / beginners

author: mhs2xs
author_rank: Contributor
timestamp: February 11, 2011, 03:04:46 pm
content: I've installed FuncGen on my Android based phone (Samsung Galaxy S). What's the best way to get from the 1/8" TRS female (phones jack) to what I'd need (probes and/or clips) for circuit testing? Can I use a TRS male and solder both the Tip and Ring to the positive probe/clip and the sleeve to the ground?Your help is much appreciated.Cheers!Mitchell

author: Unknown
author_rank: Unknown
timestamp: February 11, 2011, 03:16:37 pm
content: I would first check the output with a scope or DMM (on the phone set frequency to 50 or 60 Hz and the meter on ACV) to see which of the channels it uses, I wouldn't just short them. Maybe it can use both for dual channel operation?I would make a headphone plug to female BNC connector (you're unlikely to find them commercially, I think), then you can use standard BNC cables (eg. BNC to grabber hooks). You could also directly attach clips/probes, in that case you would indeed use sleeve for ground and one of the others (see above) for signal.Some cheap toys like the DSO Nano also use headphone jacks I believe, you may be able to use one of their cables.

author: mhs2xs
author_rank: Contributor
timestamp: February 11, 2011, 03:21:17 pm
content: Excellent. That's what I need to know. I tried to search for a "pocket function generator" to see what was out there but none of the things I found indicated, or even pictured, the cable.Edit: Yes, the DSO Nano uses a 3.5mm audio jack for the probes. Good call....Thanks again.

author: the_raptor
author_rank: Regular Contributor
timestamp: February 11, 2011, 05:09:32 pm
content: The DSO nano probes are pretty crap (I own one). You could easily knock up your own of the same quality.They aren't custom with nice strain relief or anything, and will probably kill you if you touch high voltage with them.However some people do sell audio jack to BNC connectors for the DSO Nano market, those are probably worth the money.

author: saturation
author_rank: Super Contributor
timestamp: February 11, 2011, 06:00:30 pm
content: Just some thoughts on this approach.TRS jacks are made for audio applications. One needs the right sized minijack to alligators and you are set.Putting better clips, i.e., BNC, shielded cables etc., will unlikely be meaningful unless you can output MHz from this in a quality worth maintaining its fidelity, which will require access direct to the PCB, in which case interesting problems can occur if it doesn't work out. So, alligators are good enough.The output of amps to TRS jacks are typically 8 ohms, most electronics equipment is at least 50 ohms, so be aware the impedance mismatch. Also the output voltage was made to drive mini-earphones or a preamp input, not that required for projects, so beware overloading the output.Insure the output is AC coupled, so no DC can inadvertently enter via the output jack. Phones are typically low power/low voltage logic, and may not take kindly to 5V.If phone gets damaged by misadventure in electronics, a non-contract Galaxy S replacement is ~$500. If a phone is insured make sure the insurer can't find out its been tampered with. Lastly, consider the quality of the Android app output, is damaging it worth the risk? Or should one get a function generator kit for < $30, $50 assembled [ Elenco] ?Some thought for safer experimenting, enjoy!Quote from: mhs2xs on February 11, 2011, 03:04:46 pm

author: Unknown
author_rank: Unknown
timestamp: February 11, 2011, 07:54:40 pm
content: Quote from: saturation on February 11, 2011, 06:00:30 pmI believe there's a standard in the professional audio world of 120ohm output impedance for headphone outputs, but I wouldn't expect consumer electronics to follow them. I would expect most amps to have some degree of short-circuit protection, since the jack shorts the output when you plug it in. Headphones range from 16ohms or so to 300ohms (although I wouldn't expect the amp in a cellphone to be designed for large headphones), so 50ohm should be ok. Apart from loading, all this transmission line stuff is completely meaningless for these kind of frequencies (probably not far beyond 20kHz), unless you plan on using kilometers of cable. The reason why I'd use BNC is because it's the standard connector for relatively low-frequency signals in the electronics world, not because it will improve performance.Quote from: saturation on February 11, 2011, 06:00:30 pmI'd expect the output to be AC coupled anyway (I know computer soundcards are, never checked cellphones), but the voltage rating for the cap may be quite low. Feeding a signal of any kind in the input is probably not very healthy, AC or DC. It's not a lab-grade instrument.Quote from: saturation on February 11, 2011, 06:00:30 pmSure, cheap FG is probably much more practical, but this looks like a neat toy, too.

author: mhs2xs
author_rank: Contributor
timestamp: February 11, 2011, 08:34:32 pm
content: Thank you Saturation, raptor and alm. Glad you took the time to chime in.I do appreciate the candor and concern. I still have much reading and studying to do before I embark into anything treacherous w/ my phone. I'm playing a show this weekend and, if I get paid enough money, I see a dedicated generator in my near future.The scope is here, I've so much to read and understand before I even begin to truly use it. I was referred these video tutorials and, from what I've seen, should be very helpful. Tektronix also has some pdf's for download as well that I'm looking at.http://www.allaboutcircuits.com/videos/index.htmlI also bookmarked page of someone upgrading the DSO probes as well.http://www.justblair.co.uk/upgrading-the-probes-on-the-dso-nano.htmlI do know I need to be mindful of what I'm doing. I take what I'm doing here seriously and will be most careful as I realize losing a phone could be the least of my worries if I do the wrong thing. I don't plan on doing too much IWO high voltage. At least not in the near future. Electricity is something that I learned early on commands respect and vigilance or the consequences can be mortal for most anything in it's path.I ordered this as well as the probes:http://www.robotshop.com/seeedstudio-trs-to-bnc-probe-adaptor.htmlSaturation:I have read your reply here twice now and understand completely. I've archived the page so I'll have it whenever I need to refer back to it for the impedance mis-matches and voltage/output differences you indicated. Again, thanks so much.If you all think of anything else I need to be aware of, please don't hesitate to tell me. Have a good weekend.Regards,Mitchell

author: saturation
author_rank: Super Contributor
timestamp: February 11, 2011, 08:43:07 pm
content: thanks alm, news to me, in the 120 ohms area it is! I agree with your points too.http://en.wikipedia.org/wiki/Headphone_amplifier#Output_ImpedanceChecking on this found this:Should make connections much easier. From robotshop.com, but others carry them too. Seems like a lot of folks have been into this so some folks have already made premade cables.Quote from: alm on February 11, 2011, 07:54:40 pm

author: mhs2xs
author_rank: Contributor
timestamp: February 12, 2011, 05:45:20 pm
content: Ok. Dang, once again doing something w/ software in the digital domain, just because it can be done, is a LOSER....I'll be grabbing the Instek SFG-1003 at my first opportunity. Thanks a lot for trying it out.The only thing that I bought I'll really not be able to use is the 3.5mm to BNC and the probe deal, but those were cheap, and with good reason apparently. But, the 3.5mm to BNC actually may be useful now that I think about it.BoredAtWork, I suffer from that every now and again, but it's mostly just stress. Wish I could find that elusive Rock Star job and get out of engineering altogether sometimes. But I've had that wish for over 35 years now but refuse to starve for it. Maybe I need to lose some weight...LOL...Cheers!Mitchell