Electronic – Can a M74HC595 shift register directly drive LEDs

currentledshift-register

I'm going to use a shift register to turn off and on some LEDs. However, I just read on the M74HC595 datasheet that it can only output 6mA on each pin. The LEDs I got are rated at 20 mA. Do I need the 20mA to power the LED or is 20mA the limit?

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Leds are rated for a given Current for ""full"" brightness for a specified life time. Most "regular" leds are 20mA typical, for x number of thousand hours. They can be driven at a lower current for extended life, or at a higher current at the cost of life (There are maximums to how much current can be used, as the current causes heat which then kills the led).

You can, and often should drive them with less current, unless you need full brightness. Just use Ohm's law to get the correct resistor for the 6mA current.

(Voltage Source - LED Forward Voltage) / Current (In Amps, so 0.006).

For the most part, leds at full brightness can be obnoxiously bright at straight angles. I've gone in to change the resistor for some gameboys because those things will blind you. 6mA is plainly visible in room lighting.

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Electronic – Shift Register output to Relays

How is the shift-register wired? Do you have a 0.1uF bypass capacitor across the power leads close to the IC package?

It sounds to me like a noise issue, particularly since it's only triggered when you have a load on the relays. The fact that it results in the shift-register register-state getting reset makes me think it's a power issue.

Also, how are you wiring the shift-register.
With a 74HC595, you need to:

Tie the two register clocks together (Pins 11 and 12)
Pull the master-reset pin high (tie pin 10 to VCC)
Pull the output-enable line low (tie pin 13 to ground)

Lastly, you need a 0.1 uF bypass capacitor between pin 16 (Vcc) and pin 8 (Gnd).

Electronic – PWM driven LED array over a shift register

I believe you need to set PWM to off while you are changing content of the 595.

OCR0A = 255; // set PWM to no brightness (off)
_delay_a_little_more;
SPDR = 0b00000001; // select first LED
while(!(SPSR & (1<<SPIF))); // write to shift register
PORTB = 0; // latch outputs
PORTB = 1;
OCR0A = 0; // set PWM to full brightness
_delay_ms(1);

OCR0A = 255; // set PWM to no brightness (off)
_delay_a_little_more;
SPDR = 0b00000010; // select second LED
while(!(SPSR & (1<<SPIF)));
PORTB = 0;
PORTB = 1;
OCR0A = 255; // set PWM to no brightness (off)
_delay_ms(1);

On a side note - there is more efficient way of doing what you want (i.e. controlling brightness of multiple LEDs with 595 and EN pin). It's called Binary Code Modulation. http://www.batsocks.co.uk/readme/art_bcm_3.htm

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Electronic – Shift Register output to Relays

Electronic – PWM driven LED array over a shift register

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