168. The Brain said: You can directly hook an LED upto 110VAC, provided you use the proper resistor to limit current. However, as you have identified this would be a little more dangerous then some other solutions. The snag with the resistor method is, you're wasting over 100V at say 15mA average (50% duty), over 1.5W.

\$\begingroup\$ I think what people's answers have missed is that the Zener diodes need a minimum current to work properly (usually quite a lot, in the range of 2-10mA at least) and this DOES dictate the resistor used, other than the power rating of both components, and also makes the zeners less useful as a regulator and more useful as a (normally low precision/accuracy) shunt reference

The NAND gates do an AND operation with the value and that of an NE555 that is used as debounce for a switch. All components are fed from a 5V voltage source. I read about combing different logic families and am a bit confused about when to use resistors and how many: I read connecting TTL to CMOS I need a pull up resistor.

^{Thanks for the replies, Yes the ATX PSU is rated @ 3.3V (3.5V measured with a multimeter) The LEDs are actually slow colour change LEDs (blue, green, Red) when connected to the PSU the voltage fluctuates between 3.5v and 3.35v during the cycle.} There are, in fact, LEDs that have the resistor for a particular operating voltage already built-in, so you don't need an external one. For instance, here's a spec sheet for LEDS that can be directly attached to a 5V pin. I have a few of these directly connected to my Arduino Nano (5V operating voltage) - no problems so far, and they're on yjfFCU.

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do i need a resistor for led