Is the design correct?? - Page 1
Forum section: electronics / projects

author: subhadeepbiswas!
author_rank: Newbie
timestamp: October 09, 2017, 02:46:25 pm
content: I'm making an amplifier which supports USB and Bluetooth as well. The bluetooth device I've used is an USB device. So I connected both the device in parallel and used a DPDT switch to interrupt the 5V power lines of the USBs so that one remains on and other idle at a time and vice versa but that didn't work the USB controller is not recognizing any of the USB devices. I did some research on this and then connected the DPDT with the D+ and D- pins of the both USBs by doing this the controller is recognizing only one USB (mostly the sd card reader not the bluetooth device) so I'm thinking to turn on/off both D+ and D- pins digitally using transistors I'll use the dpdt switch to apply the voltage to the base of the transistors that would select one of the USBs at a time.Here is the circuit I've designed. Will it work? please tell if the design is correct/wrong or I've missed something? Thank you.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 09, 2017, 02:50:54 pm
content: No, not as shown, and also not with bias resistors as you'd normally use with BJTs as analog switches.You're not completely off in the woods, though. There are USB switch devices, nothing more than high performance analog switches, available. Drop one in and you'll be good to go.Otherwise, consider a simple USB hub IC. These are cheap and plentiful, and allow both devices to connect without a need for switching.Tim

author: BigBoss
author_rank: Regular Contributor
timestamp: October 09, 2017, 03:01:47 pm
content: Look at this..It has also very tiny package.http://www.ti.com/product/TS3A5018?dcmp=dsproject&hqs=pf

author: subhadeepbiswas!
author_rank: Newbie
timestamp: October 09, 2017, 03:04:16 pm
content: Thank you Tim for the quick reply. I'm trying to solve this problem by designing the switching circuit myself. Is there any way I can design the same using transistors/MOSFETs or by using optocouplers? pls suggest.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 09, 2017, 03:10:13 pm
content: You can, but why?Tim

author: subhadeepbiswas!
author_rank: Newbie
timestamp: October 09, 2017, 03:19:00 pm
content: Because I find pleasure in makingSubhadeep

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: October 09, 2017, 05:44:46 pm
content: No.USB data bus is bi-derictional and often tristated. It's not trivial to deduce which side is driving it at each moment.Here is a good introduction into USB basics:http://sdphca.ucsd.edu/lab_equip_manuals/usb_20.pdfIf you want to be adventurous (and weird) you can use SD211 in place of BC547s.Leo

author: subhadeepbiswas!
author_rank: Newbie
timestamp: October 09, 2017, 05:55:58 pm
content: Thanks a lot , I'm sleeping on it.. I'll update my work here as well as post problems that arrived

author: ebclr
author_rank: Super Contributor
timestamp: October 09, 2017, 06:03:12 pm
content: That's is an easy an trouble-free way to dohttp://www.analog.com/en/products/switches-multiplexers/analog-switches-multiplexers/adg774a.html

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: October 09, 2017, 06:15:14 pm
content: Quote from: ebclr on October 09, 2017, 06:03:12 pmAre you sure it's happy at 3.3V signal levels? DS says 0V to 2.5V but I haven't read it further.Leo

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 09, 2017, 07:48:00 pm
content: Headline "Single 3 V/5 V supply operation"? (Haven't read either.)If you're heart set on doing it discrete, then studying how these ICs do it is key.The downside: you can't buy MOSFETs with a free substrate (or "back gate") connection anymore, they're obsolete. This makes it more difficult to realize an analog switch with off-the-shelf parts.The next best method, then, is a bidirectional switch, like this:You need either a floating gate bias supply, or a bias supply about 5V above the highest signal level (i.e., >= 8V for USB). 2N7002 would be sufficient here.This is a single SPST switch, so you need two for each USB device, and logic to switch them alternately (to make a DPDT switch).Tim