I connected 8 leds in series and pulsed it via a microcontroller pin while sinking the current using 74hc154 decoder.The led is not bright enough to be seen. Can someone help me with solution. Please
What voltage are you starting from? You might have to reconfigure into a four in series and two strings in parallel arrangement. Dont forget you need a current-limiting resistor in each string.
Are you trying to light all eight LEDs with a single pin on the 'HC154? When operated off 5V, the 'HC154 is only rated to sink ~5mA. The only way you could use it to light 8 LEDs is by being very tricky, and by using another power supply.
You'll need at least 1.8V*8 = 14.4V to drive eight red/orange LEDs in series, in reality more because there needs to be enough headroom for a current limiting resistor.
If they're blue/violet/white you'll need 3.5V*8 = 28V.