# Switch circuit

#### Kenny

Joined Oct 11, 2004
55
I need something "like a multiswitch" controlled by an electrical signal.
I'll try to explain:
I need to connect a 2 terminal device to another 2 terminal device (but the last one must be selected from n (aprox. 30) 2 terminal devices. Only one of the n devices must be connected to the main 2 terminal device in a moment.

I know that can be made using transistors as switches, but I believe that there is a better solution.

What is the cheap and less trouble approach?
I attached a simplistic schematic in order to illustrate my problem.

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#### KMoffett

Joined Dec 19, 2007
2,783
Do you intend to select each "device" in sequence (i.e. 1..2..3......28..29..30..1..2), or in some non-sequential order (i.e. 24..2..17..6..24..30..1.....)?

Ken

#### b.shahvir

Joined Jan 6, 2009
457
I think Solid State Relays (SSR) might come of some help.

#### Kenny

Joined Oct 11, 2004
55
The selection of the device is in a random manner, because it depends of a selected entry on another stage of the circuit.

#### KMoffett

Joined Dec 19, 2007
2,783
So you have other circuits that output what? (voltage? current? polarity? contact-closure?)
What's the nature of the MAINDEV and each of the other DEV*'s (voltage? resistance? current?)

It's kind of important when selecting a switching device to know exactly what is being switched into what.

Ken

#### SgtWookie

Joined Jul 17, 2007
22,210
What is the level of the signal(s) that you are switching?
For example, is it a line-level audio signal, or is it medium to high power?
What is the expected voltage level, AC or DC, and current or wattage?

#### Kenny

Joined Oct 11, 2004
55
OK. Many thanks!
Here is the complete story.
The circuit that I'm planning to do is an entry door intercommunication system.
Then, the maindev and the devs are phones connected through a 12V DC power supply, and the selection of the device is made by a microcontroller depending of a code entered with a keypad.

#### KMoffett

Joined Dec 19, 2007
2,783
Better! Do the 12v lines just proved power to enable the remote phones, or does the line carry both power and audio? Would you happen to have a schematic of the system?

ken

#### Kenny

Joined Oct 11, 2004
55
Thanks!

I want to establish the simplest way of communication between the main phone and one of the other phones in a moment.

The communication circuit, in series configuration is: the main phone, the selected remote phone, the 12V DC power supply and a resistor, then, I believe that the line carries power and audio.

I have not yet a schematic, because I'm on a block planning stage where the main thing that worries me is the simplest, cheapest, and less trouble method for select/switch the desired phone connection.

#### thatoneguy

Joined Feb 19, 2009
6,359
Can the end devices originate a call, or only receive?

How much circuitry is allowed or desired at each endpoint?

Simplest and cheapest would be a single pole dual throw switch at each endpoint, like a 3 way light switch. That isn't elegant though.

#### Kenny

Joined Oct 11, 2004
55
Yes, the end phones must be capable of originate a call but only to the main phone.

The circuitry at the endpoint must be minimal, it means, the phone (speaker and microphone) and a ring speaker activated by a subcircuit that rings when the communication is established but the auricular is hanged (but this ringer is not of my interest at this moment).

The SPDT is not a solution, I set at the beginning of my post that the switching must be controlled by a signal sent by another circuit.

My doubt is what is the best multiswitch approach? Transistors, TRIACs, SSRs, Optocouplers, another unknown to me device?

#### thatoneguy

Joined Feb 19, 2009
6,359
The Single Pole Dual Throw could be a solid state relay.

The issue is only having two wires to address 30 units. Digital addressing takes away the minimal at the receiver end.

#### Kenny

Joined Oct 11, 2004
55
Oh no, to the main phone are connected the all 30 lines, not only 2. It must switch between them.

#### thatoneguy

Joined Feb 19, 2009
6,359
Have you looked at PBX designs or intercom designs at the patent shop for ideas?

Modulating voice over power isn't a big leap, it's the addressing of them via a microcontroller.

Do you have any specs for the main unit and each end unit? Or are all of those up for design as well? If so, what gauge wire/current capacity will be used? What is the anticipated longest run? Is the wire shielded? Is changing wiring to Cat 3 twisted pair (4 wire) or Cat 5 TP (8 wire) too late? If wiring already exists, what was present prior to this project?

I'll try to help the best I can, but am a little short on information.

#### thatoneguy

Joined Feb 19, 2009
6,359
If amplification is allowed at the receivers, you could go the old school 4066 or 4016 route with the low level audio, switched to the headset to destination by uC choosing one pin on one of 8 chips (uC and analog switch ICs would be in main unit, with some other stuff). One line would be power/signal, other would be common ground. Shielding suggested.

To ring the base, it could be something as simple as the far end pulls the line to ground, which would light up an LED, or displayed on LCD, as the uC would be scanning a parallel to serial shift register whenever it is idle.

This is working on 5V Logic levels/line levels, with amplifiers at each end station.

The master could ring an extension by doubling the supply to that extension, which would then trip a ringer, once acknowledged, it would revert to 5V.

That's the least complex I can come up with for "Simple", "Low Cost", and "2 wire power + audio".

#### Kenny

Joined Oct 11, 2004
55
Many thanks for your help, the entire info is too interesting.

I'll search for the specifications of the end unit, but it is a premade door entry telephone, and the master unit headset must be something compatible (maybe not to designed for).

The wiring is already made with n * twisted pair, having one common wire and one wire for each end unit. The aproximated longest run is 500 meters, and the shortest is 100 meters.

I don't know if the end unit have amplification, but I suppose that do.

At first I'm focused on the switching stage, I saw the 4066 datasheet (very good) and also I found the DG485. Is there another?

>If wiring already exists, what was present prior to this project?
Prior to my project, there is a complete entry door system for 20 offices.

Is a bit difficult to me explain you what is the system (known as citofonia in spanish), but I found this http://www.aaaremotes.com/lireteensymo.html. And I attached the schematic of the current system installed, taken from the website of the manufacturer. From the schematic, what I want to design is the item labeled as "Tablero de conserje".

Excuse me for give the info of my project in bit pieces, but I thought that would be more easy to understand in this way, and when I was sketching the project in my head I stopped at the switching stage.

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#### thatoneguy

Joined Feb 19, 2009
6,359
If I am interpreting the diagram correctly, each phone is designed for at least 6 wire input, wild guesses here:
A (Tx?)
RT (Ring?)
B (Rx?)
P (Power?)
Z / I - (Extension ID / Line #?) Also, station #3 Is odd and uses I instead.

If the dashed lines are signals modulated on the solid lines, that would reduce wire count to four. There is another clue in the diagram: the colors in the diagram match the colors on 4 pair Cat5 cable - White/Orange, Orange, White/Green, Green, White/Blue, Blue, White/Brown, Brown.