Input Mulitplexer

[JCrank]

Hello everyone, First post so take it easy on me

Anyway a little background I'm in the heating, air conditioning, refrigeration, and building control field. I have the need to monitor the temperatures in approximately 35 walk in coolers using are building managment system. I could do this using 35 analog inputs on a few controllers but that get real expensive fast.

So what I need is a device that I can connect multiple sensors to one input on the controller, a multiplexer.

I will use a analog output from my controller controlled through programming to output a voltage from 2 to 10 volts DC. For each different voltage increment I close a relay to connect one of the sensors to the input on the controller.

So basically what I need is a circuit will close a relay with a certain voltage.

For example:
2.5 volts output closes relay 1
3 volts output closes relay 2
3.5 volts output closes relay 3
etc ...
9.5 volts output closes relay 15
10 volts output closes relay 16

Well hopefully this is about as clear as mud

I don't know a whole lot about electronics I do know the basics, but I want to learn.

Thanks in advance,

 

[3v0]

The **broken link removed** and about 8 other 18F micro controllers from microchip have 16 analog inputs. These chips are under $8 each. You do not need a lot other then the processor and the connector on each controller card so the should not be expensive.

Three of these would yeild 48 analog inputs.

There may be a better way.
Look at Dallas 1 wire devices. They use a 1 wire bus. A signal wire and a gnd. I do not recall how many devices you can drop on a single bus but I believe it is greater the 38.


If you want to do a good job run 2 or more 1 wire busses through each cooler. That way if a sensor or a net goes down you can still monitor all coolers.

For temp sensors look at the DS18S20.
They can be had for under $2.50 in quan greater then 50. The DS18S20 is in a T092 package, looks like a transistor. You will need to package it in epoxy. I do this by filling a large soda straw with epoxy and stuffing the sensor and lead into the straw. Dallas also has packaged version know as I-Buttons.

You can buy inexpensive controllers that have 2 or more 1 wire interfaces built in.

 

[Brevor]

I suppose what your discribing could be done with a lot of op amps and logic gates, But I think it would be far easier if the controller could output a binary value (six bits would work) to select the relay. 3V0's plan is far better though if you are able to work with microcontrollers.

 

[FirefighterBlu3]

use the DS18B20 to collect your temperature readings, I agree with the parallel line of sensors, yielding two per cooler. take both of these lines and run them into a PIC. you'll get digital data indicating the temperature values that you can identify per sensor. you can do whatever you want with this information such as sending it up to a PC for logging, turning on a warning alarm, etc.

make sure you read the App. notes for 1-wire networks, pay particular attention to electrically balancing the sensor network since this is likely to be a significant length of wiring involved.

here is where you start: **broken link removed**

the DS18B20 fairly replaces the DS18S20. this should be a good start for you

 

[ljcox]

There are several ways that this can be done.

Use a PIC with an internal analogue to digital converter.

But if you're not comfortable with PICs:-

1. Use Op Amps & logic as someone suggested

2. Use an analogue to digital converter such as the ADC0804 with logic to control the relays.

I don't know what you want the relays to do, but I suspect that you don't need 16 of them.

4 relays can be arranged to provide 16 combinations. So I suggest you give me more info on what the relays are to do so I can advise you further.

 

[3v0]

use the DS18B20 to collect your temperature readings, I agree with the parallel line of sensors, yielding two per cooler. take both of these lines and run them into a PIC.

If you want belt and suspenders use one PIC for each 1 wire net (line). That way if one PIC or the line between it and the world goes down you are still in the clear.

I would guess that the end user wants the system to alert some body via phone if things go wrong with the a cooler.

 

[ljcox]

I've been thinking about your request while doing other jobs.

I think the simplest solution (assuming you don't want to use a PIC) would be to use two LM3914 ICs.

See page 8 of the attached data sheet. This shows how up to 20 LEDs can be lit in the "Dot" mode. The diagram does not show the full details. These depend upon the particular application. In your case, you need to set the levels for 2.5 Volt, 3.0 Volt, etc.

You would use relays rather than LEDs.

HOWEVER, it is essential to connect a diode across each the relay coils to protect the LM3914s from the back EMF when a relay is released.

 

[ericgibbs]

hi,
I would suggest that Len's idea is elegant and simple.
Why install equipment to measure the temperature when a Control Voltage proportional to temp is already available?

Two LM3914's set to detect the change in voltage input levels, driving relays directly or via opto isolators would get the job done.

 

[Super_voip]

I don't think an LM3914 can supply enough current, so an additional driver will be needed.

 

[ericgibbs]

I don't think an LM3914 can supply enough current, so an additional driver will be needed.

hi
IIRC the LM3914 can drive about 10mA.[ checked 7 thru 12mA typical]
It would be fairly easy to drive a transistor or a opto-isolator.

EDITED:
Added LED prog current gif

 

[ljcox]

hi
IIRC the LM3914 can drive about 10mA.[ checked 7 thru 12mA typical]
It would be fairly easy to drive a transistor or a opto-isolator.

Eric,
According to the data sheet I attached, it can drive anything from 2 to 30 mA.

JCrank,
I would use a 15 Volt supply and 12 Volt relays.

I suggest you post a drawing of how the relay contacts are to be connected. You may be able to connect them in a priority chain.

 

[JCrank]

Thanks for the help guys, I really like the looks of the 1 wire networks. I like the looks of the Tri-PLC products. Can these PLCsread the 1 wire network sensors?

 

[FirefighterBlu3]

I'm not familiar with them. The PIC I am designing into my current projects is the 16F877A, it speaks 1-wire.