Input or output selector circuit. - Page 1
Forum section: electronics / projects

author: slaterweezer
author_rank: Newbie
timestamp: October 13, 2013, 07:50:14 pm
content: Hi Everyone,I'm new here. I've a bit of experience in electronics, though more repair of existing analog audio circuits more than coming up with my own designs and very basic understanding of digital electronics!I happened upon Youtube video #262 which brilliant highlighted a single momentary switch switching power on/off in a very efficient manner which brought me here.I'm looking for some help or even just a bit of direction with something I'd like to build which somewhat follows the on/off circuit function but takes it a step further. I hope this is the right place to ask!I work in recording studios and one of the things that I like having the ability to do is listen on a variety of speakers. Most recording consoles offer the ability to select up to 3 different audio outputs, which in turn feed a set of speakers via 3 separate amplifiers.Recently a few London based tech's have been making amplifier output relay boxes that switch the high current speaker outputs of a single amplifier between different outputs feeding different speakers, meaning one amp can be used for multiple sets of passive speakers. It's a very effective solution though one aspect of these units I feel could be improved upon.All of the units I've seen offer a cumbersome rotary switch, providing a simple a OR b OR c solution without any active circuitry. The bonus is that with no active circuitry, they can guarantee that only one pair of speakers can be selected at a time and whilst this does the job, this solution is cumbersome to use and doesn't look very good when sat at a mixing desk.In contrast, most mixing desks use a bank of 3 momentary push buttons which when pressed, select the new option, whilst de-selecting the previous... A far nicer looking design.I would like to come up (with any help you can give me!) with a circuit that performs a similar function to the mixing desk solutions... or comparatively a hifi amplifier select circuit. ie... momentary switches which when pressed, reset the previous selection before selecting the new function.I'm sure there must be a name for this type of circuit, but I looked up "interlocking" and other relevant names and nothing has come up. Can anyone point me in the direction of such an existing circuit, or give me some direction as to the best method that I should pursue.I'd like the circuit to operate at either 5 or 12volts, and output to relays with LED indicators on each output so I can see which selection is selected as-well as the relay carrying the audio.Can anyone point me in the right direction of some reading, or circuits to look at.many thanks in advanceRichard

author: Bertho
author_rank: Regular Contributor
timestamp: October 13, 2013, 08:35:26 pm
content: You can use 3 relays with 4 contacts where 2 are normally closed (NC) and 2 are normally open (NO).The NO contacts switch the high-current speakers. Two NC contacts are pairwise put in series to the activation coil of the third relay.This provides an active fail-safe circuit so that only one relay can be activated at any time.The control circuit can be momentary switches to activate and it takes care of the release of the relay when switching over. Alternatively, an all-off can be implemented with either a fourth button or a "smart" control circuit that disables the active relay when the same button is pressed twice..

author: Bertho
author_rank: Regular Contributor
timestamp: October 13, 2013, 09:14:57 pm
content: To expand on the relays, see image.The control is probably easiest to do with a small micro-controller, which reduces the number of components required considerably, including debouncing (costs under $1). It also makes it possible to decide later how to react on different button press-sequences and add an "all-off" button.

author: slaterweezer
author_rank: Newbie
timestamp: October 13, 2013, 09:43:32 pm
content: Bertho! Thank you so much for the reply.This looks like a great solution.Looking at the circuit, I assume with only 4 poles per relay, three is the maximum number of inputs (or outputs) this circuit could allow for without upping the number of poles on the relays and then if you could get greater numbers of poles (can you get relays with more than 4!?) the NC network around the circuit would get quite complex. Its a pretty cool way to use the NC sides of the relays to protect double switching thoughI'm looking at a schematic from a focusrite mixing desk (which I use) and a system on this for switching between inputs uses quite a complex arrangement. Initially a (large quantity of) momentary NO pushbuttons are wired into a binary matrix creating a 5 bit address output (made up of 29 different selections).The 5 bit address then feeds a latch chip (a 40373???) and then a decoder (which unfortunately is a coded prom (a 2732 but useless to know this without it being coded right? which is labelled Prom Decoding Bin to Select???)The outputs of this then go thru 4051's (are these decoders?) which creates 29 latched outputs to feed relays.Firstly, I realise this is a hugely over spec'd design, but I like the idea of having maybe 8 inputs and I'm sure that doing that with relays alone, would be prohibitive given the number of poles?Is there an in between? or a simplified version of this design concept that I've got here on this schematic? BTW I can email the schematic, but I know that focusrite object hugely to their schematics being posted online.If you want to see what I am looking at let me know your email.many thanks for your help and anyone else reading.R.

author: slaterweezer
author_rank: Newbie
timestamp: October 13, 2013, 09:48:33 pm
content: Bertho,can you elaborate on the micro controller concept and give an example of one which I can look up a data sheet for?I really know so little on the digital front, but this sounds along the lines of what I am imagining I need to know about.cheersR.

author: Bertho
author_rank: Regular Contributor
timestamp: October 13, 2013, 10:45:00 pm
content: Quote from: slaterweezer on October 13, 2013, 09:43:32 pmThe number of contacts you need is 2*NO and (m-1)*NC, where 'm' is the number of outputs/inputs. However, this is only required for hard-wired fail-safe (which is probably a good idea with speakers).You can get more than 4 poles, but they become fast prohibitively expensive for higher currents. You can use two relays, one for the lockout and one for actual switching, but it would compromise the fail-safe.QuoteThis looks like a discrete binary coded matrix for activation.The 4051 is a 8-channel analog (de-)multiplexer (http://www.nxp.com/documents/data_sheet/HEF4051B.pdf) to switch analog signals. Probably the low-voltage audio is directed this way.The relay-solution is only helpful if you are switching high-voltage and/or high-current signals (like directly to the speakers). If you can switch the low-voltage line-signals from the audio source/destination, then the 4051 (and 4052/4053/4066/4067 parts) are much easier.QuoteYes, it is completely over-spec'ed for simple audio-signals and probably not a good idea, as mentioned above. However, I was under the impression that you wanted to switch at the speaker level, which is high-voltage/high-current.QuoteI'll send a PM.

author: Bertho
author_rank: Regular Contributor
timestamp: October 13, 2013, 10:54:53 pm
content: Quote from: slaterweezer on October 13, 2013, 09:48:33 pmIt depends heavily on what you want to achieve. From my original thought, you have three speaker-sets and one amplifier. You use three buttons to select one of the speaker-sets to connect to the amplifier.Buttons need to be debounced and the relay-control is not momentary, but continuous. You also need dead-time between de-activation of one relay to activation of the next. So, having a micro to input three buttons and output the three control-signals would be an easy thing to do. A PIC16F505 (about the smallest and cheapest 14 pin micro) is more than enough to perform this simple function.

author: slaterweezer
author_rank: Newbie
timestamp: October 13, 2013, 11:12:59 pm
content: Hi BerthoThanks again for being so helpful.Yes you're right... I think primarily I would like to switch speaker level, however, the idea that the logic - control element could be implemented for line level as well as speaker output interests me.Originally I planned to make a switcher that simultaneously switched speaker level with line level, allowing me to seemlessly switch between self amplified speakers and passive speakers using one central amplifier. This would require a permanent input to the amplifier alongside switching the line level signals on other outputs and started to feel clumsy.I am currently looking to build myself a new studio, and as a result, I am now planning things which will become part of a permanent setup (rather than historically where I would want a piece of equipment to travel with me and be flexible in different circumstances). I can therefore plan a setup where my current mixing desk line level speaker output A) feeds a single amp with speaker outputs switched and output B) feeds a line level switcher between different "active" speakers. (desk speaker outputs are already interlocking)However, I like the idea of not being limited, Ie I'd rather too many selections and to not use 3 than always wishing I had added 3 more!The design you have already drawn works perfectly, though I think 3 outputs might limit me and looking at the design I (possibly in error!) thought that I'd be looking at more poles to add more outputs.The desk I have uses relays for EVERYTHING... I think there are 120 per channel!!! the result is very clean audio as it's often getting switched on relays rather than going through op amps and other devices that other audio designers use and can arguably colour the sound.So the Relay idea is great, though I know so little about the control side of design that I guess this is what I was looking for help on. The idea of button press = selection + latching, then new button pressing = current Selection zero'd, new selection implemented and latched etc. is what I am looking for. Your design works well from what I can follow, but my question relates to whether I could make it a 1 in 8 hout selection back easily using this design principal.Also you say "the relay-control is not momentary, but continuous" do you mean that in this circuit you would need to hold the button down? or am I misunderstanding that?!So yes, the bit I am most interested in learning about is logic side, ie a circuit that has a number of outputs which interlock and in theory can be extended to 8.many thanks again...schematics being emailed now.cheersR.

author: Bertho
author_rank: Regular Contributor
timestamp: October 14, 2013, 12:05:47 am
content: Quote from: slaterweezer on October 13, 2013, 11:12:59 pmYou can get 5A rated relays with 6PDT which are affordable. More can be done, but they are both hard to get and three times more expensive.With 6PDT you can have 5 channels switched with fail-safe enabled. You could then switch ~100W of power (assuming cos(phi)=0.5), which is probably enough for a studio and kill your hearing at the same time ;-)QuoteAgreed, using small-signal relays is preferable to electronic switching as long as the lines are low-impedance. Less harmonic distortion with relays.QuoteFor the low-level signal side, it is quite easy to make a button matrix and scan it with a micro-controller. The controller then sends the correct activation to a demultiplexer to assert the correct relay. That is actually how the current system works, only it is made with discrete logic.QuoteThe diagram I attached above must have the SELx input active for the relay to be active. Therefore "continuous" signal is required for a selection to be active. The micro-controller would convert momentary presses of the button to the proper activation level.QuoteThe interlocking mechanism is superfluous for low-level signals. They can withstand a beating and can easily be protected by a small series resistor without compromising the quality of the audio.High power amplifiers are often much more sensitive (especially to too low speaker impedance) and there you want to ensure that only one is activated at the same time (you still need an amplifier that can withstand open-circuit situations).However, if you have an amplifier that can tolerate both open-circuit and short-circuit (or too-low impedance), then you do not need the fail-safe. You can just use as many relays as you need with only two NO switches per relay.You should decide what you want to switch and which parts are dependent/independent first. From there a setup can be devised quite easily.