Clip above shows Attiny85 chip controlling two 10mm LEDs with the single pin. This post discusses schematic and programming of such a solution.
There are two ways to control LEDs with microcontrollers (like Attiny85).
One way would be to connect LED cathode to the chip pin and LED anode through the current limiting resistor to the power supply '+'. To light up the LED you just need to program pin low. To turn it off you need to program it high.
Another way would be to connect LED cathode to the ground and LED anode through the current limiting resistor to the chip pin. To light up the LED you need to program pin high and low otherwise.
Which way to prefer? I can't tell. I myself often choose it based on how well it corresponds to controls of other devices in particular project.
But there are not that many pins on the chip like Attiny85. What if number of pin available is less than number of LEDs needed to be controlled.
On the circuit diagram above you see the method of controlling two LEDs with single pin. It based on three important facts:
- Any LED has some forward voltage. For Green/White 10mm used here it exceeds 3 V.
- Microcontrollers pins may be programmed not only to provide output signal but to collect input as well, If pin is programmed for input it presents itself for external components as a circuit with very high resistance.
- Human eye can't see light blinking if its frequency exceeds 50 Hz. If circuit switches LED off/on fast enough (more than 50 times per seconds) you will see it as always lighted.
Rules to controller LEDs here are next:
- If pin set high green LED is dark and white LED is lighted.
- If pin set low green LED is lighted and white LED is dark.
- If pin set into input mode both LEDs supposed to be dark, because sum of forward voltages on both LEDs exceeds power supply voltage,
I assembled the circuit and programmed it , but result was disappointing. Setting pin into input mode did not turn both LEDs off, They were lighted up. The light was less bright than normally but still visible. Would resistors increase help? Alas not that much. LEDs dimmed a little but not fully. Obviously I need either decrease power voltage or look for LEDs with bigger forward voltage,
Then idea came to mine mind: what if I add to the circuit extra devices with some forward voltage.
Here you see each LED sequentially connected to the silicon diode . Forward voltage of diode reaches 1 V, which makes whole forward voltage of both (LED and silicone diode) ~ 4 V. Obviously it is better to decrease value of resistor to keep current the same (~20 mA). And this circuit solved the problem. I believe it is not bad solution taking in to consideration that silicon diode cost pennies, Below you can see C program to run the test.
#include <avr/io.h>
#define F_CPU 1000000UL
#include <util/delay.h>
typedef enum {OUT_LOW, OUT_HIGH} OutputLevel;
#define pinOut(bit, outLevel) \
(DDRB |= (1 << ( DDB##bit)));\
switch(outLevel) \
{\
case OUT_LOW: (PORTB &= ~(1 << (DDB##bit))); break;\
case OUT_HIGH: (PORTB |= (1 << (DDB##bit))); break;\
}
#define pinIn(bit) \
((DDRB &= ~(1 << (DDB##bit))),\
(PORTB &= ~(1 << (PORTB##bit))),\
(PINB & (1 << (PORTB##bit))))
int main(void)
{
while (1)
{
for (int i=0; i < 100; i++) { // White & Green
pinOut(2,OUT_HIGH);
_delay_ms(5.0);
pinOut(2,OUT_LOW);
_delay_ms(5.0);
}
for (int i=0; i < 100; i++) { // White
pinOut(2,OUT_HIGH);
_delay_ms(5.0);
pinIn(2);
_delay_ms(5.0);
}
for (int i=0; i < 100; i++) { // Green
pinOut(2,OUT_LOW);
_delay_ms(5.0);
pinIn(2);
_delay_ms(5.0);
}
for (int i=0; i < 100; i++) { // No light
pinIn(2);
_delay_ms(10.0);
}
}
}
But one may ask why this? Why not charlieplexing? Indeed charlieplexing is well known technique which utilizes facts of of pin high resistance while in input mode, and non-ability of human eye to see high frequency light oscillation. It can provide LEDs/pin ratio not only 2 but much bigger. To compare let us look at charlieplexing example with the same LED/pin ratio: 2. There is need to have three pins to control six LEDs. But how to control just four LEDs, or two? Not possible. And in addition you need to connect/solder five components to the single dot which for DYI project presents some challenge.
So in a conclusion: even if charlieplexing is great there could be cases when method described in this post is a valuable solution.
Update 07/05/2021. I put link to this post on the
AVR subreddit . And I got some comments that there actually different schematic exists which allows to control two LEDs with the single pin without need of silicon diodes.
Indeed such circuit works perfectly. But again nothing is free. It consumes ~ 20 mA even if both LEDs are off. Meanwhile for circuit with silicon diodes consumption during dark time is much smaller. So which circuit to choose is really question of use case and preferences.
