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  • Author Author: djfraz
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Smart Home: Movie Lighting, I Broke it

djfraz

So, just to set the scene. I had dimmed the lights, got a cool one out of the fridge and sat down to watch a film. I said "Ok Google, Turn Movie Mode On", the notification came up on my Living Room Kodi, telling me movie mode was on. But no Lights?

 

My first thought being the power cable had come loose as it was a cheap usb cable with the connector cut off and put into a screw terminal, but it wasn't. I tried the tried and tested method of unplugging and plugging it back in, waited a second and tried turning it on again, and again nothing. At this point I noticed that the on-board LED of the nodeMCU board which I am currently using was glowing very faintly, and i had a horrible thought, that I had bricked the board.

 

If you don't know what I am talking about, I have written up another blog post on this at Smart Home Movie Lighting

 

So I unplugged the LEDs and the power and started doing some investigation.

image

 

I started by flashing the board with the Example Blink sketch, and it worked; this got me even more confused. So i tried uploading the Movie Light arduino sketch, which still has Serial output for debugging, it uploaded fine. I open the Termite (a Serial Monitor) and start sending commands, it returns the correct information back out. So at this point I assume that the LEDs had blown, which would be understandable as they came from a bargain store and where part of a £3 light.

 

After leaving the board for a while to work on my Party Ball (Smart Home: Let the Party Begin) I decided to have another look just incase I had done some irreversible damage. I first started by seeing if the LED has blown, as I though it was strange for them both to blow when the system was turned off. I plugged them into a cheap breadboard power supply (I don't have a benchtop power supply yet), at 3.3V the had a faint glow, adn at 5v they light up.

 

So I now hooked my oscilloscope up to the output pins of my board, and get a noisy signal of a constant voltage at around 4.5V, when the output should be off. I start to worry again that I have blown the digital pin I had connected the output to. So I try taking NodeMCU out of the board and  the probe further back, to the pin on the NodeMCU, and I started sending commands to the device, and there is my PWM output. I think, is there a short circuit in my board, which wouldn't have surprised me. But nothing.

 

Then I have a though, has my transistor blown? I gave a quick test with a breadboard and another transistor just connected to a 5mm led, and it worked. So my transistor output had blown.

 

The transistor I had originally used was a BC547, and checking the data sheet it has a max collector current of 100mA, I had never checked the current in the circuit; but i know I am using a 500mA supply, so most likely I have blown the Transistor.

 

The moral of this story, for me anyway, start thinking; and don't get too excited by it working on the breadboard and put it straight into a perf board and inplace. So now I have to wait until i can order some beefier transistors, or design some current limiting for the transistor output stage. REally i should have seen this coming, as the whole time i have been using the device, I have noticed it flickering when on, even at low levels. I will most likely wait until I order the ESP-01 before remaking this circuit, or just through transistors at it until I remake it. And when i Do, some more care will be taken when designing it.

  • in reply to djfraz

      wrote:

     

    ... I am more glade the transistor did its job and protected the board from the high current draw, as i only have a couple of the nodeMCU boards, and most of them are sat in breadboard with components attached for other projects. 

    That is luck. A typical transistor has undefined behaviour when you let it handle more current than it can handle.

  • in reply to genebren

    I think I mostly just got excited when my first test with just a 5mm LED attached, and when i saw the COB Lights in the bargain store the next day I got even more excited, then having a test board built using the transistor, which i had bought a bulk pack for general purpose, and tested it behind the TV, my dream had come true, and that was me satisfied.

     

    I knew i would need to make a more permanent arrangement eventually, and at least now, i know where that would have failed and can hopefully avoid it. I am more glade the transistor did its job and protected the board from the high current draw, as i only have a couple of the nodeMCU boards, and most of them are sat in breadboard with components attached for other projects.  

  • in reply to mcb1

    Yeah, im going to sit down when designing the next one a find the current draw of the leds at full brightness, as I am not sure and stats about these as they where bought from a cheap light. I can only assume that they are just standard COB lights.

     

    I will probably order  load of higher current transistors as i will need some for the Party Light and Party Laser projects for controlling the motor, probably a TIP120 which is used in the arduino tutorial. which may be over kill for the cheap 5v motors in those devices, but would rather be safe than sorry again. I will probably add some BC337 to my list, i have some BD139 which looking at the datasheet are rated a 500mA.

     

    I may also look into having a constant current driver in the circuit so I could use some of the more high power LED modules if i wanted.

  • This happens more than you would think.  It is sometimes difficult to slow everything down and check everything while we are bringing up a circuit.  The light turns on and we think great, time to move on to the next thing.  When you are working with known parts, with good specifications, you are far less likely to make this kind of mistake.  When using recycled components, we need to do a little more work, like learning it's behavior and in a sense generate our own specifications (i.e. current flow and voltage drop when working with LEDs) so we can be sure that the circuit is stable and all components are operating inside their normal conditions.    is right, this was a learn-able moment, one that you are unlikely to repeat.  We learn from failures as much as from successes, just remember to keep learning.

    Gene

  • And when i Do, some more care will be taken when designing it.

    Cheap lesson to learn (small transistor overloaded), and one I guarantee you'll never make again ...

     

    Try a BC337, it can handle 500mA and are cheap/plentiful

     

    Mark