# Multiplexing Signals

Discussion in 'The Projects Forum' started by Roiw, Nov 22, 2012.

1. ### Roiw Thread Starter New Member

Nov 22, 2012
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Hello guys,

I'm new to the forum and to eletronics, I've made some n00b stuff at home but now I have a college assigment in eletronics.

And to be honest I'm totally lost. I would love if you guys could point me at the right direction, my professor is allways busy and don't have time to help "poor mortals" like myself.

THE PROJECT

In my project I have 8 (simple strain gauge) sensors connected to a circuit (I will call it circuit A ) and that circuit is connected by a single wire to another circuit ( let's call this one circuit B ) and circuit B is connected to 8 LEDs.

And finally I have to light up the LED corespondent to the right sensor.

The challenge is that A and B, are connected only by a single wire and It's possible to have 2 sensors pressed at the same time, that would imply in 2 light up leds.

*The sensors must be the same, only diferents in value of elements.

I have to design the circuit A and B

My Strategy

I was thinking about, in circuit A, creating a signal for each sensor and summing up all the signals. Passing that signal through the wire, and in circuit B I only have to filter it to light up the right LED.

Why I cant do it

- I don't know how to create a signal. Don't know how to create a circuit that sends a signals that can be filtered later.

Any ideas of how to do it? Or where I can see a example of something similar?

I can only use :

-OpAmps
-ANDs, NOT's, OR's, XOR, etc..
-Transistors
-1.5v Batteries for circuit A. For B I can use a 12v source
-Diodes
-Resistors
-Capacitors
-Inductors

If I'm doing everthing wrong please point me to the right direction please!

2. ### tubeguy Well-Known Member

Nov 3, 2012
1,157
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Sounds like the Instructor is giving you sulfur, diamonds and coal...

3. ### Roiw Thread Starter New Member

Nov 22, 2012
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I don't know what your expression means. Hope is not something bad. D:

4. ### JohnInTX Moderator

Jun 26, 2012
3,335
1,671
Circuit A could consist of 8 sinewave oscillators, each at a different frequency (and none an integer multiple of any other).

Search this forum and the google for transistor oscillators / op amp oscillators and you'll get lots of circuit ideas. You want something approximating a sinewave so the output does not contain lots of other freqs, just the fundamental for easier filtering but experiment and you may be able to get away with a simpler to build non-sinusoidal oscillator.

As you said, sum the 8 outputs and send the composite freq down the wire. At the B end, buffer it and feed it into 8 active bandpass filters which sort the individual frequency components from the composite signal. The filter output is sensed (rectified and integrated with an RC) to drive the LED driver.

Microchip filter lab software is an easy tool to design active filters. Also check out Analog Devices, Linear Technology etc.
Texas Instruments has lots of general info on linear design in the appnotes section (they bought Burr-Brown and National Semiconductor's linear stuff) like this, this and this.

Try searching on Color Organ for other ways of controlling individual lights with different audio frequencies.
DTMF signalling is also a possibility if you can use some more advanced chips.

Going digital is a possibility if you can use counters/multiplexers etc. but not so much if you are limited to simple gates.

Have fun!

Last edited: Nov 22, 2012
Roiw likes this.
5. ### Roiw Thread Starter New Member

Nov 22, 2012
12
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Thanks! I really appreciated that! I will do the research you told me ! I'm glad to be moving to the right direction. Many thanks!

6. ### tubeguy Well-Known Member

Nov 3, 2012
1,157
198
Just some humor for the holidays... Ask a Sci-Fi fan to explain.

7. ### JohnInTX Moderator

Jun 26, 2012
3,335
1,671
I promise to be merciful.. and quick.

Happy Thanksgiving!

8. ### Roiw Thread Starter New Member

Nov 22, 2012
12
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Sulfur, diamonds and coal.. Now I'm supossed to need a rope and a bamboo like plant?

Is that right?

hahahah

9. ### tubeguy Well-Known Member

Nov 3, 2012
1,157
198
Followed by a tuck and roll....

10. ### THE_RB AAC Fanatic!

Feb 11, 2008
5,432
1,312
Nobody saw mythbusters do that one? It did not end well.

Now for the OP; it sounds like your project is begging for 2 microcontrollers, and simple serial digital comms between them.

Otherwise a good solution would be to just buy a slightly larger cable, it's quite easy to get 8 core cable + gnd+shield, it is sold as multicore alarm cable as one common name.

11. ### Roiw Thread Starter New Member

Nov 22, 2012
12
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Thanks for the answer THE_RB ! But thats a college project so I can only use those components listed on the top.

I was doing some research and I think the right thing to do is what JohnInTx suggested.

Thanks anyways ;-)

12. ### tubeguy Well-Known Member

Nov 3, 2012
1,157
198
What strain gauge sensors are you using ?

It is possible these will also require some interface circuitry to provide a usable signal.

13. ### Roiw Thread Starter New Member

Nov 22, 2012
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I don't know whitch strain gauge I'm going to use, didn't thought of that untill know.

I think I'm gonna pretent that I have a usable signal and when I get everthing running well I get back to it. Seams like a good strategy right?

Thanks for reminding me that!

14. ### tubeguy Well-Known Member

Nov 3, 2012
1,157
198
Strain gauges produce very small, variable voltages based on the force applied - on the order of 10mv per volt (per National's site), so yours would produce 15mv or less with a 1.5v battery.

Edit:And the strain gauge is used as pat of a Wheatstone bridge to accomplish this.

You should clarify with your instructor whether the strain gauge is to be used like a switch where the LED just turns on as the force applied reaches a certain amount or the LED needs to get brighter and dimmer based on the applied force.

Very important as far as the circuit design.

Last edited: Nov 23, 2012
15. ### Roiw Thread Starter New Member

Nov 22, 2012
12
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Thank for all the support guys!

I really appreciated it!

Here is how I done the circuit!

It's a video of a simulation that I done on Falstad. ( a simple circuit simulator made in JAVA http://www.falstad.com/circuit/ )

This simulation is made for my professor and since I'm from Brazil it's all being explained in portuguese. ( Sorry for that guys. D: )

But if you guys have some questions or something just let me know!

Thank you again !

And cya!

16. ### tubeguy Well-Known Member

Nov 3, 2012
1,157
198
Very nice! And very kind of you to post the simulation.

I do have a question - Did you figure out the joke ?

17. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,357
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absf likes this.
18. ### Roiw Thread Starter New Member

Nov 22, 2012
12
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Sure! I think is about that bamboo bazooka on star trek ;-)

Though I'm not a big fan of sci-fi hahaha

19. ### tubeguy Well-Known Member

Nov 3, 2012
1,157
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It's a classic !!!