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Try to get an LED wash light circuit to be remote controlled via DMX51


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I'm having a problem with this circuit. I am trying to get an LED wash light circuit to be remote controlled via DMX512. The problem is, after I designed the circuit for the RS485 (which will deliver the DMX signal to my LED circuit) my professor told me that there's no time to make it and that I should just buy it. I couldn't find a descent one to buy. I am also not sure if I should connect the LEDs with parallels or using a darlington pair?




I have attached the circuitry for the RS485. Also, the options I have to connect the LEDs, not even sure which microcontroller is the best for them. Any help? It's also 5 - 9 LEDs.

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Your DMX/RS485 circuit looks ok except pin 3 of the connector should go to A and pin 2 to B.


As for the Leds, what type of Led are they? A Darlington driver such as ULN2803 is good for small 5mm leds. Connect the leds to the positive rail, switch the negative side. For higher power LEDs you need a driver circuit for each colour.


If there's no time to build, you can buy all this hardware as an arduino for about £30

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We've been talking about these on the forum recently for LED tape, is this not what the OP wants? If a higher drive output stage is needed then we need a spec for how high.


(Lots of other sellers for similar products available.)



Thats a Constant Current , CC driver, for big 1W + LEDs which sounds more appropriate for an LED wash, can wire the LEDs in series up to limit of supply voltage.


If OP is looking for a Constant Voltage CV driver for 5mm LEDs/5050 / LED tape this would be sort of thing



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Given that this is a design project, couple of critiques:


1) Your DMX receive circuit is not fully compliant with either RS485 or the E1.11 (DMX512) standards.


The Common of the DMX wiring must not be directly connected to ground.

RS485 (EIA 485 really) permits non-isolated transceivers, however there must be a small resistor of at least 0.2 ohms between the RS485 cable common and ground to limit fault current.


However E1.11 says that it should be isolated.

eg using a small DC-DC converter and triple optoisolator or one of the RS485 transceivers that have integral isolation. (I'd suggest the latter because it's easy)


This is to avoid current from flowing through the shield/common of the DMX cable, which causes interference and in some circumstances can be dangerous.


- See section 5 of E1.11 2008 R2013 for details.


2) There's no surge suppression.

It's recommended to fit TVS diodes between Data +, Data - and Common to absorb small surges - eg static shock.

See this TI paper for a summary: http://www.ti.com/lit/an/slla292a/slla292a.pdf


Figure 5 in that PDF shows an EIA485-compliant layout (note that it's not fully compliant with E1.11)


Of course, neither of the above are strictly necessary for your prototype to work.

In a "small" system, it'll be absolutely fine without them.


However, from what you've said above then it seems this is an educational design project.

You will get a lot of extra marks for referencing and following the ANSI standards in your formal design, even if the actual prototype you build doesn't follow it due to lack of time or resources.


For your prototype, I'd suggest Arduino.

It's a really nice and cheap hardware platform to plug stuff into, and the 'prototyping' boards that plug in are really easy to get hold of and solder your 'custom' bits into.


Driving the LEDs:

The classic "Constant Voltage" PWM setup is a MOSFET and current-limiting resistor on the 'low' side of the LED strip.

- Don't use bipolar transistors, they have a really high power dissipation in this scenario and require the microcontroller to source a relatively large current.


Driving a 500mA LED strip, a reasonable MOSFET will dissipate around 0.05W, while a bipolar transistor will dissipate at least 0.35W.

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Good data there Tomo.


However as seems to happen quite often on BR, we might be talking to ourselves. The 1-post OP doesn't seem to be interacting with any of the comments/feedback/suggestions in the thread and their profile info suggests they haven't been back since the first post...


I'm learning stuff though at least http://www.blue-room.org.uk/public/style_emoticons/default/biggrin.gif

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