Screen Flicker problem!

Discussion in 'General Electronics Chat' started by rougie, Apr 25, 2008.

  1. rougie

    Thread Starter Active Member

    Dec 11, 2006

    Yesterday I posted a post called "Noise from 556". I Thought it would be better off if I re-posted a detailed version of my problem... Here it is.

    If you open my attachment, you will see my schematic. Some fellows where concerned that the way I connected my LCD was the cause of the flickering. I must say that the LCD has working perfectly for about 6 months without any flickering problems. It is only when I attach wire A (Please view schametic) that the flickering starts. In reality the 556 timer has nothing to do with screen refreshes or any timming for the LCD. The LCD is completely controlled by the MCU.

    I was wondering if I should use some other transistor instead of the 2N2222. Like I said, I am not that good with semies. I sort of left this stuff along time ago, and would appreciate some ideas on how I could modify the 2N2222 driver circuit. The 2N2222 will be intended to drive an IR LED. But weather I plug the LED or not, I still get flickering on the LCD.

    PS. The timer's specs says that the current version of the 556 can be powered from 2 to 18 VDC!

    All help appreciated.

    With greatest regards
  2. SgtWookie


    Jul 17, 2007
    The problem is that when the 2N2222 transistor conducts, the 43 Ohm resistor is the load; the emitter drops about 0.65v, and the remaining voltage (3.3 - 0.65 = 2.65) is dropped by the 43 Ohm resistor. This is causing in the neighborhood of a 60mA-70mA surge in the current on the power supply at a 38kHz rate.

    I think you connected it that way because you wanted positive logic.

    Instead, remove the 2N2222. Take a 2N2907 transistor, connect it's emitter to the 3.3v supply line, and the base to the 33k resistor, and the collector to your load.

    However, your load is not resistive; it is an IR LED. You normally must use a current limiting resistor to prevent burning it out.

    What is the Vf (forward voltage) of the IR LED at it's rated current?

    There's the possibility of using the gain (hFE) of the 2N2907 to control the current through the IR LED, if you can verify what the gain of your particular 2N2907 happens to be.
  3. mik3

    Senior Member

    Feb 4, 2008
    Can you tell us what voltage the IR led needs to work and what current?

    Do you really need to switch on the led when the 38KHz wave is low and turn it off when it goes high?
  4. rougie

    Thread Starter Active Member

    Dec 11, 2006
    Hello Mik3 and SgtWookie,

    Thanks for your prompt reply. Boy, I need to crack a book on these semies and quick! But there is good news at the end of all this.

    Okay guys, the circuit I showed you was not the entire one. What comes after the output of the 2N2222, is it is connected to the plus side of the led (anode I believe it is.) The cathode of the led is then connected to the collector of another 2N2222 and a 470 ohm resistor to ground. The emiter of this second 2N2222 is grounded also. The Base of the 2N2222 is a Voltage delta signal to attenuate the current through this transistor. If the emitter doesn't have enough EMF, then the led stops conducting through the emmiter. The 470 ohm resistor is just there to keep an offset so the emmiter doesn't need to go all the way down to 0 to turn off the IR LED. In other words what transistor #2 is there to limit the brightness of the IR LED via an analogue voltage from my MUC all while turning it on and off at 38KHZ. (By the way it really isn't exactly 38KZ, it is 38KHZ on for 0.5 ms and off for 5ms, hence the use for the 556 timers!!!)

    Anyway, getting back to what I was saying, by varying the the voltage (Delta V from 0 to 5VDC) at the base of T2 the IR illumination *signal* will vary, and therefore when reading in a signal from my IR reciever, I know at what distance the object will be.

    Now, the delta voltage signal connected to the base of T2 was comming from a variable DC power supply. The circuits were powered by another DC power supply connected to the power rails of the circuits. I Removed the power supply that provided the delta voltage signal and replaced it with a voltage divider made from a simple potentiometer and used this instead. 95% of the flickering went away. I further connected 2- 300 micro farad capacitors and a couple of 0.01 micro farad tantalum capacitors on the power rails and presto, No more noise at last!

    By the way the IR LED is a TSAL6100 from Vishay with a FV of *approximately* 1.5 volts. At full drive it draws appoximately 100 ma.

    Probably you guys know a better way of doing the above circuit (with a few op-amps a guess!), however I tested it all morning and I am impressed on how clear the TFT LCD is and how all signals are well captivated from the IR recievers. Maybe later on, I may test the 2N2907. Yes, yes, the hfe! something to do with the amplifcation of the transistor or is that beta? Gawd! Sorry if I am very rusty on this.

    I am very happy, thankyou for your feedback it has been a pleasure to speak with you both.

    Please feel free to get back with and feedback. I don't come on this chat very often, it is actually the second time in a very long time. I should come here more often, I am sure to learn quite a bit. :).

    With sincere regards to MIK3 and SgtWookie.