# Resistors and VCC requirements on 74xxxx ICs

Discussion in 'The Projects Forum' started by kidpreacher, Aug 8, 2010.

1. ### kidpreacher Thread Starter New Member

Aug 8, 2010
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I am building a 4 player game show circuit with 74LS04, 74LS20, and 74LS74 chips, which will turn on LEDs as well as trigger an optoTriac?.? to provide AC current for lights..

The original schematic had no values listed for the pull down resistors on the switches, so I just assumed a 1k would work based on another project I found in a blog.
I have breadboarded this five times, and even wire wrapped it, but it will not work.
I have downloaded the IC data sheets, but don't really understand what they mean.

I am a pastor, not an electronics guy, but am willing to learn. I understand BASIC concepts, but am still working on the Ohms law and so forth.

My two questions are A) if you have your base input current from your transformer, and know you need a resistor, where on the data sheet do you get the required info to calculate the size of the resistor? V=IR, so voltage is 5 from transformer, R will be the resistor, so I need the # from the data sheet to say what is the "I" value to calculate R... but don't get where it is listed. Does each pin of an input on a chip need a resistor because of different requirements?

Question B) related to A, in that I can not get my circuit to work, and wondered if I just am not giving it enough amps. I am using an old phone charger - 5v 20mOhms. ICs are 5v, but how do I figure how many amps the circuit needs? 5 ICs, plus power for triacs, and LEDs. Is it cumulative (a sum of all required power for each chip, led, and triac combined) or just supply based on chip with highest requirement? Where on the data sheet find this?

Any help here so I can understand this IC stuff would be greatly appreciated.

Please feel free to e-mail me directly - <snip> (Eastern time, USA)

Last edited by a moderator: Aug 8, 2010
2. ### marshallf3 Well-Known Member

Jul 26, 2010
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203
The circuit clearly lists the resistor values as 1K ohm (1,000 ohms) although 10K is a more commonly used value for 74xx series logic ICs.

As a rough idea for how much current the circuit needs (with some to spare) count the number of LEDs, ICs and optocouplers then multiply that sum by 0.03 to get the amperage requirement.

If you want more exact numbers you'll have to look each IC up, each LED etc then you'll have to be able to estimate just how much load each one is taking to operate which can vary wildly.

You are dealing with a circuit meant to control live 120 VAC through the optocouplers, extreme care will be required if you're unfamiliar with electronic construction and safely grounding the case. Then again according to the output rating of those optocouplers they were meant to drive relays, not full load light bulbs.

Last edited: Aug 8, 2010
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