From Saurabh Gupta and Dhananjay V Gadre, Netaji Subhas Institute of Technology, Dwarka, New Delhi, India
This method allows 2×N×(N1) LEDs using only N I/O lines and a few additional discrete components. The technique exploits the fact that each I/O line has three states: one, zero, and high impedance. Thus, with two I/O lines, states 00, 01, 10, and 11 of eight possible states control the LEDs.
Here is an example with two I/O lines controlling four LEDs. P1 and P2 are the inputs. The circuit requires that the LED turn-on voltage should be slightly more than VCC/2. For red LEDs with a turn-on voltage of approximately 1.8V, a suitable supply voltage is 2.4V. For blue or white LEDs, you can use a 5V supply voltage. For N I/O lines, this technique requires N1 transistor pairs.
Here are the states of P1 and P2 as they relate to which LED lights up:
P1 | P2 | TP1 | LED |
0 | 0 | Vcc | D3 |
0 | 1 | Vcc | D2 |
1 | 0 | 0 | D1 |
1 | 1 | 0 | D4 |
Z | Z | Vcc/2 | None |
A second example shows 24 white LEDs with 4 IO pins and 6 transistors controlled by an ATMEL AT Tiny 13 Microcontroller.
And here is the C source code for a bargraph display with the analog input comming into PB2 (pin 7, Ain).
/*Saurabh Gupta and Dhananjay V. Gadre*/ /*Tiny13 Processor*/ /* January 2008*/ #include<avr/io.h> #include<avr/interrupt.h> #include<avr/pgmspace.h> typedef unsigned char u08; void delay(u08 a) { u08 i,j; for(i=0;i<a;i++) { for(j=0;j<250;j++) { asm("nop"); asm("nop"); asm("nop"); asm("nop"); } } } const unsigned char portb_value[] PROGMEM = { 0b00000000 , 0b00011000 , 0b00000000 , 0b00010010 , 0b00000000 , 0b00010001 , 0b00010000 , 0b00001000 , 0b00010000 , 0b00000010 , 0b00010000 , 0b00000001 , 0b00000000 , 0b00001010 , 0b00000000 , 0b00001001 , 0b00001000 , 0b00000010 , 0b00001000 , 0b00000001 , 0b00000010 , 0b00000001 , 0b00000000 , 0b00000011 }; //port_value[0]-->for L0 (1st led) //port_value[23]-->for L23 (24th led) const u08 ddrb_value[] PROGMEM = { 0b00011000 , 0b00011000 , 0b00010010 , 0b00010010 , 0b00010001 , 0b00010001 , 0b00011000 , 0b00011000 , 0b00010010 , 0b00010010 , 0b00010001 , 0b00010001 , 0b00001010 , 0b00001010 , 0b00001001 , 0b00001001 , 0b00001010 , 0b00001010 , 0b00001001 , 0b00001001 , 0b00000011 , 0b00000011 , 0b00000011 , 0b00000011 }; u08 value=0,count=0; ISR(TIM0_OVF_vect) //1200 Hz { //delay(100); TCNT0=-32; ADCSRA|=(1<<ADSC); // if(value!=0) count=(count<value)?(count+1):0; // else // count=0; //count=(count<24)?count:0; DDRB=pgm_read_byte(ddrb_value + count); PORTB=pgm_read_byte(portb_value + count); } ISR(ADC_vect) { int temp; u08 temp2; temp2=ADCH; temp=temp2*15; temp=temp>>7; value=temp; value=(value<24)?value:24; } int main() { DDRB=0x00; PORTB=0x00; TCCR0A=0X00; // TCCR0A - Timer/Counter Control Register A // --------------------------------------------------------- // |COM0A1|COM0A0|COM0B1|COM0B0| | | WGM01| WGM00| // | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | // --------------------------------------------------------- TCCR0B=(1<<CS02); //9.6 MHz divided by 256 // TCCR0B - Timer/Counter Control Register B // ------------------------------------------------- // |FOC0A|FOC0B| | |WGM02| CS02| CS01| CS00| // | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | // ------------------------------------------------- TIMSK0=(1<<TOIE0); // TIMSK - Timer/Counter Interrupt Mask Register // --------------------------------------------------------- // | - | - | - | - |OCIE0A|OCIE0B|TOIE0 | - | // | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | // --------------------------------------------------------- TCNT0=-32; //1200 Hz ADMUX=(1<<REFS0)|(1<<ADLAR)|(1<<MUX0); // ADMUX - ADC Multiplexer Selection Register // ------------------------------------------------- // | - |REFS0|ADLAR| - | - | - | MUX1| MUX0| // | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | // ------------------------------------------------- ADCSRA=(1<<ADEN)|(1<<ADIE)|(1<<ADPS2); // ADCSRA - ADC Control and Status Register A // ------------------------------------------------- // |ADEN |ADSC |ADATE| ADIF| ADIE|ADPS2|ADPS1|ADPS0| // | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | // ------------------------------------------------- ADCSRB=0x00; DIDR0 = (1<<ADC1D); //Disable Digital input buffer on PB5 // DIDR0 - Digital Input Disable Register 0 // ------------------------------------------------- // | | |ADC0D|ADC2D|ADC3D|ADC1D|AIN1D|AIN0D| // | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 | // ------------------------------------------------- sei(); while(1); return 0; }
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