Cheap DMX decoders with cyclic flicker

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vector
Posts: 22
Joined: Fri Sep 23, 2016 9:25 pm
Real Name: Mark Sy

Cheap DMX decoders with cyclic flicker

Post by vector » Mon Apr 24, 2017 12:15 pm

Hi All
Anyone played with these pcbs (attached)
On a known working DMX chain with other "real" light fixtures everything is fine except these boards (driving a single 5mm LED as a test) seem to have a cyclic flicker. Maybe every 1-4 seconds. Other than that they work and dim the LEDS as expected.
I was after a cheap way to control some robotic devices.
The flicker is there even when its the only device in the chain.
I have tried multiple controller types DJ console to QLC+ and Enttec dongle, different cables, just about ruled everything out except the cheap device.
Attachments
MiniDMX3ch_addr.jpg

mlohrey
Posts: 120
Joined: Mon Apr 20, 2015 5:07 am
Real Name: Mark Lohrey

Re: Cheap DMX decoders with cyclic flicker

Post by mlohrey » Mon Apr 24, 2017 2:01 pm

Hi vector,

I have experimented with a very similar board in a jukebox prop I made. I had three of them and they all did the same thing, even when connected to a LSC lighting desk.
I managed to buy some others before the show (inkbird brand) that worked well but a little more expensive. I had to use one and i wired a large capacitor across the terminals that provided enough energy to diminish the effects of the blips.

I was able to negotiate half price for the dodgy ones with the eBay seller. Lol maybe I will find a use for them someday.
Last edited by mlohrey on Wed Apr 26, 2017 4:56 am, edited 1 time in total.

vector
Posts: 22
Joined: Fri Sep 23, 2016 9:25 pm
Real Name: Mark Sy

Re: Cheap DMX decoders with cyclic flicker

Post by vector » Mon Apr 24, 2017 10:11 pm

Thanks for that, confirmation at least proves its not me.
When I have time I will look into the electronics of them but most probably its bad code.
thanks for the inkbird info Ill chase them up

mlohrey
Posts: 120
Joined: Mon Apr 20, 2015 5:07 am
Real Name: Mark Lohrey

Re: Cheap DMX decoders with cyclic flicker

Post by mlohrey » Tue Apr 25, 2017 1:52 am

No worries.

I would be interested if you can find a solution.

Here is an example of the ones I have found to be OK.

http://www.ebay.com.au/itm/New-12-24V-9 ... SwQTVV-i~U

This looks to be a similar product and is a bit cheaper.
https://www.aliexpress.com/item/PX24506 ... .13.E6n41s

Cheers

Mark

vector
Posts: 22
Joined: Fri Sep 23, 2016 9:25 pm
Real Name: Mark Sy

Re: Cheap DMX decoders with cyclic flicker

Post by vector » Wed Apr 26, 2017 3:28 am

Ok here we go,
Summary..these units are no good.
Below are the notes I took whilst digging around.

Problem: Cyclic flicker.
Subject under test: Mini-DMX-3ch-v11 (on back of pcb lettering "designed by mcu_hui"(no net reference found). Using 20pin STC11F04E mcu.

confirmed LED PWM signal loss from output of mcu. Green is DMX into MCU p2. Yellow is PWM output to LED , note gap, long low.
dmxflikr.png
Long low, missing PWM signal to LED
DMX/PWR LED driven by mcu pin 1, sw10 on(Fun Mode). Glitch’s seen @4.2seconds. Green is PWM signal to LED, yellow is glitch every 4.2seconds. Note glitch in sync with PWM gap
dmxflikr4.png
Glitch found on pin every 4.1sec
mcu pinout:
DMX Input on pin 2, PIN3 Nc, pwm out on 6,7,8. PWM pin feeds direct to FET causing distortion.(not the best design)
Sw10 pulls low Pin9 for FUN mode. P11-19 DIPSW1-9 Addr.
DMX looks clean.
24mhz xtal (system clock) max 25mhz. Note: mcu datasheet indicates 4.1s watchdog@24mhz

Flicker pulse occurrence varies. (compared to DMX stream)
Pulse width varies (10-26ms)
Flicker is in sync with DMX/pwr LED. Occurs on the 4th sec on the High to low transition. (same as glitch see above).
Lifted pin1 from circuit, LED signal present on pin,(Sw10off ie DMX mode) no glitch seen on skt1. Flicker still present. Sw10 On(FUN mode) Glitch on Pin 1, skt high.
green PWM (note small black stripe, this is the flicker/gap) in sync with High to low transition of yellow(mcu p1(dmx/power led)
dmxflikr5.png
DMX/pwr LED seems to be "interrupted on its second pulse"
Note the DMX/pwr LED seems to be "interrupted on its second pulse" ie its shorter, there is no reason why you would program it this way.

NOTE: Even the Fun Mode are in cycles of 4 seconds. Changing the sw 1-7 to get more colours fails because after 4 seconds it resets.
ie Nothing works on this unit for more than 4 seconds.

Highly suspect mcu Watchdog is resetting mcu. Quite possibly in mass production(cloning) this has been missed ie left on. In good operation, mcu code that uses watchdog continually clears the watchdog timer so it never resets, If the code breaks or encounters a problem the timer is not cleared and thus creates an internal reset. Code that does not use the watchdog function does not clear the timer. So if the code has no timer clear and the batch chip programmer did not disable the watchdog function we would get exactly what we are seeing here.

If we knew the source of these chips/pcbs it would be a very very simple fix..1 bit out of place in fact, very frustrating.
the Program memory may be unreadable(ie locked) either way I dont have an STC programmer to dig any further.


My blog on the problem

LEDmaster
Posts: 2
Joined: Sat Sep 23, 2017 2:58 am
Real Name: Jan Wullschleger

Re: Cheap DMX decoders with cyclic flicker

Post by LEDmaster » Sat Sep 23, 2017 3:23 am

Hi all.

This is Jan at Custom Effects LED Solutions. We resell LED products and came across the flicker issue in this product as well.

It does indeed seem like a watchdog timer issue.

Anyways, we've re-written the control software and re-programmed the mcu. You can purchase the re-programmed decoders from us at http://www.custom-effects.com/product.p ... id=846#846 or message me for the new software and programming instructions.

Thanks,

Jan

LEDmaster
Posts: 2
Joined: Sat Sep 23, 2017 2:58 am
Real Name: Jan Wullschleger

Re: Cheap DMX decoders with cyclic flicker

Post by LEDmaster » Tue Sep 26, 2017 5:02 am

Well, I'll just public post the code and instructions :)

The compiled hex file is here: www.custom-effects.com/my_files/mini_dmx_3ch.hex (this is only my v1.0 and does NOT have all the fun mode functions programmed yet)
If you want to play around with the source code and make changes you can find it here: www.custom-effects.com/my_files/mini_dmx_3ch.c

To load the hex file onto the mcu you'll need the STC ISP programming software from STC: http://www.stcmicro.com/rjxz.html

Programming is done using serial communication through the RX/TX lines of a comm port.
I programmed the STC11L04E mcu while it was plugged into the MINI-DMX-3CH board. The Rx pin on the mcu that is used for programming is also used to receive DMX512 so this pin should be isolated from the DMX512 circuitry while programming. I just used another DIP20 socket between the mcu and the board and bent the RX and TX pins out. You could also program the mcu by plugging it into a breadboard and providing power to it (I don't think an external clock is needed for programming).
I used a cheap USB-UART converter using the CP2102 IC. Connect RX/TX lines and ground between the mcu and the UART converter. Do not connect directly to an RS232 serial port, the voltages will probably kill the mcu. The RX pin on the mcu is Pin2 and the TX pin on the mcu is Pin3. Remember that TX from the mcu connects to RX on the PC and RX connects to TX ;)

In the STC-ISP software click "Open Code File" and load the HEX file. Then make sure the proper settings are ticked off in the H/W Option tab. You can see the existing options on the IC before programming by clicking "Check MCU" in the lower left on the screen. You'll need to cycle power to the mcu after pressing the button. The new HEX file DOES feed the watchdog so no need to disable it :D :D :D Next click on "Download/Program" to load the HEX file onto the mcu! I noticed that I sometimes needed to cycle power a few times to the mcu before the program would recognize it.

I hope this helps some of you!

Jan

doughadfield
Posts: 29
Joined: Mon Nov 06, 2017 4:40 pm
Location: Herefordshire, UK
Real Name: Doug Hadfield

Re: Cheap DMX decoders with cyclic flicker

Post by doughadfield » Tue Dec 05, 2017 3:53 am

I've just come across this post and yes, this is really helpful! Thank you, Jan, for publishing all this information. Now I can modify the behaviour of the boards I have, to do things like on/off switching of channels, for controlling smoke machines etc. as well as PWM dimming.

thanks again
Doug

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