Wednesday, 4 April 2012

Light Canvas - LEDs 6: Dim power

The previous post demo-ed all the LEDs roughly in place in the frame, running on the dimmer circuit - but not running bright enough. I've figured out (I think) the LEDs aren't getting enough power. Firstly I wanted to establish what is happening in the circuit:

  • From it's specifications, the 2N222A transistor has a beta value of about 100
  • The current applied to the transistor base (555 chip output current) measured at: 1.16mA
  • Taking the above, 100*1.16mA = 0.116A
  • This calculation is confirmed by the measured of current going through LEDs (via the transistor): 0.1A
  • This 0.1A is a quarter of the current needed to fully light the LEDs - measuring the current when driving the LEDs directly from the power supply gave a reading of 0.39A (pretty close to the calculated value of 0.42A)
  • The transistor's peak base current is 200mA, so the max emitter current (base current * beta value) should be 200ma*100 = 20A - considerably more than the 0.4A needed.
  • It should be possible to get the 0.4A - I just need to figure out how to get more current into the base.
Answers on a postcard...

Sunday, 4 March 2012

Light Canvas - LEDs 5: All the LEDs

Here are all the LEDs running - 12 panels, each with 5 LEDs. They're not fixed in place yet, just taped in the approximate location.

It turns out that the 555 dimmer circuit isn't quite right, as it seems that the LEDs aren't getting a large enough current - 0.16A, rather than 0.45A (when powering all the LEDs without the dimmer).

I guess it's something due to the transistor not putting/allowing enough current through...


Monday, 27 February 2012

Light Canvas - LEDs 4: Progress!

Having important things to do can be really motivating - for other projects. I've household repairs to organise, but can't figure out an elegant way for it to be done - so I'm ignoring that for the moment, and have made some progress on the Light Canvas :-)

Subsequent to my last post on this project, I satisfied myself that the circuit was ok, and ordered a bunch of components, and took the opportunity of a OxHack session to  breadboard the control circuit, which I did - except for the minor point that the circuit didn't actually work... Oh well, it was a nice event, and good to meet some more hackers and see what they were up to - I need to get along to more of that sort of thing!

Many months later, I've a working breadboard with the 555 astable circuit producing a pulse width modulation varied from 0-100% depending on a potentiometer. The 555 PWM doesn't have enough power to drive the LEDs directly, so it's used to control a transistor which effectively amplifies the power of the PWM enough for the LEDs. W00t!

This happened by:
  1. Getting the 555 PWM working on a single LED directly,
  2. then on a panel of 5 (on a breadboard) via the transistor, then (the next day),
  3. making up 9 perfboard versions of the LED panel, powered via the LED.

Video evidence:

Next steps;
  1. Convert the 555 circuit from breadboard to perfboard.
  2. Modify the frame to house the circuit, controls, and wiring.
  3. Modify the frame so the back can be attached, and hung on the wall.
  4. Stretch the canvas onto the frame.
  5. Install the electronics in the frame, and put it all together.
  6. Put on wall and bask in its glow :-)