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High power multiplexing help

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#1 mcinnes01


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Posted 26 August 2013 - 03:59 PM



I am just in the process of rewiring my house using a radial system or in fact 2 seperate radial systems (one for the basement/ground floor and one for the first floor/loft). By radial system I mean each socket and light goes back to the distribution board.


The idea is that I can control each socket or light separately and so each of these will terminate on its own solid state 25 amp relay that will be din rail mounted in the distribution board. Each light switch will run on a low power system using cat5 cabling and so all the switching will be done using solid state relays. This not only allows me to manually control the lights using manual switches, a 24v din mounted power supply and CAT5 cable to the relays. But it also allows me to centralised control that I can interface with via the netduino. The reason for this is that you must ensure that there is a switchable light per room and it acts as a backup in case I have a software or electronics issues, most importantly this also avoids the Mrs getting annoyed if she can't switch the light on etc. ;)


What I am trying to do is create some din rail mounted modules that will provide the switching control for the relays. Each module will use CAT5 connections to connect it back to a central module that will house the netduino and will split the SPI which will be used to multiplex the control logic and distribute the switching voltage from the 24v 3.2A 85W din mounted power supply.


Each module will switch 8x 25amp relays using 24v, will have 8 LEDs to indicate what is on, will contain a 74HC595 shift register, a 74HC245N CMOS and a ULN2803 transitor array, plus some resistors or resitor array.


The relays have approximately a 2.5v 30ma voltage and current drop respectively.


The 4 twisted pairs in the CAT5 will carry the 3 connections for the SPI, 1 connection to return QH from the 595s so they can be daisy chained, common Gnd, 3.3v and 24v.


What I need a little help with is as follows:

  • Am I missing any connections on the 74HC245Ns, there is a direction pin (pin 1) and a QE pin (pin 19). I am not sure what to do with these?
  • Can you see any errors in the circuit or see any problems that it might face, and can you offer any solutions if so?
  • As I will have a fair few of these boards and each will have 8 small 3mm leds 30ma 2.5v, would I be better moving the leds to the 24v side of the circuit and if so what will I have to change? Would I need any extra resistors, diodes, caps etc?
  • The resistors I haven't even calculated and this is something I am terrible at, if I leave the LEDs where they are now what resistors would I need? I was thinking of using a resisor array from my inspiration for the circuit (see below). However linked to the previous question what if I moved the LEDs up to the 24v circuit so they aren't running off the netduinos power? Posted Image
  • Would the circuit benefit from any caps to level things out and if so what size and where?
  • Final question do I need any additional protection diodes for the netduino or chips?

Here is a fritzing (see Din Mount Relay_bb.jpg) of what I was thinking for the Din Modules and there is also the netduino shield which distributes the 24v and SPI (see Din Shield_bb), any help moving the LEDs to the 24v side would be much appreciated!


As a key:

Red = 24v

Black = 3.3v (from netduino)

Green = Common Gnd

Orange = Returned QH

White = MOSI

Purple = SCLK

Yellow = SS



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