I have the following problem and wonder if anyone may have some inside into what may be happening.

I have an array of 20 leds that i manage to multiplex using the attiny88 using only 9 pins (4 columns by 5 rows). At any given time only one led is on. The different leds are activated with a pushbutton. Initially, i used five transistors with a 1k resistor to turn on and off the rows of the led array.

Everything worked like a charm even at voltages as low as 2.5 vol. I then decided to remove the transistors to free-up some space in my pcb. Now the circuit works well at voltages above four but at 2.5v it makes leds to turn on randomly, when pressing the pushbutton.

I presume this behavior is because the one led is taking to much current, but i wonder why could be the case if the chip is only driving one led at a time. Also keep in mind that the circuit works very well at 4vol.

Any clue about this and how could i fix it? I like the idea of not using the transistors.

  • 3
    Schematics please Aug 27 '15 at 7:17
  • 2
    Do you have the required current limiting resistors? If yes, where in the circuit are they? What value are they? A schematic is an invaluable resource for diagnosing your problem - without it we cannot have a hope in working out what is happening.
    – Majenko
    Aug 27 '15 at 9:53
  • driving one led at a time. Are you sure you aren't lighting 4 leds at a time (one entire row).
    – Gerben
    Aug 27 '15 at 11:59
  • Iwill have the schematic tomorrow. One thing i read is that some pins that do no have pwm
    – Camilo
    Aug 28 '15 at 7:22
  • I will have the schematics tomorrow. One thing i read is that maybe some pins without pwm may turn high at low voltages. Testing this is complicated for me as it is some massive rewiring. Any thoghts on this?
    – Camilo
    Aug 28 '15 at 7:24

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.