Placement of resistors.

Yea I mean more so the logic i'm getting confused. There aren't any resistors in a CMOS dealio but you get me.

still confused though, haha.

When an input is not connected to anything it is known as “floating”. This is when there is no signal on it, so the input cannot know what logic level it is at.

When you connect the input (via a button, say) to either Vcc or GND, you are placing the input into either the HIGH or LOW logic state.

You want to avoid a floating input at all costs. They can cause strange results in programs. To avoid them you place the input into a “default” state - the opposite of the way you want your button to connect the input to Vcc/GND. For instance, if your button connects the input to Vcc (HIGH), then you need to set a default state of LOW. So you need to connect the input to GND for when the button isn’t pressed.

However, you can’t just connect it straight to ground, because then when you press the button there will be a direct connection between Vcc and GND, and that will basically be a short circuit. The whole system will die, and in extreme cases batteries will blow up and wires / traces will melt.

So, you create the default state through a resistor. The resistor can be quite high as you don’t want much current to flow through it when you press the button, but at the same time it needs to be low enough that not too much voltage is dropped across it. 10K? is a typical value.

Now, when the button isn’t pressed, the pin is connected to GND through the resistor and reads LOW. You press the button, and the resistor is connected between Vcc and GND, and the input is connected to Vcc, so it reads HIGH. No more floating.

Hmmm I think I get it. That would explain why there is a 10k ohm resistor on the IREF pin of a TLC5940, b/c you don't want your 5940 pulling unlimited current based off of the 0 resistence IREF. right?

majenko made my tiny descriptions seem useless.... There is one more thing to add: if the resistor value is too high, the input may be suspectable to noise, and if it is too low, it will draw a lot of current, and waste power.

So how would one figure out what is a good medium? Just use 10K for now?

There are calculations you can do revolving around the Rdson of the mosfets in the chip, but 10K is usually "good enough" to work reliably.


funkyguy4000: I've tried the solder drag method before. I ended up with all the pins connected to eachother. I stopped there. Was I supposed to wick away the excess after that?

It depends a lot on the type of flux. I have just about every type you can get; no-clean liquid, rma223 and others. All will leave bridges except this dark brown gunge I've got that burns really easily. It's ability to completely destroy the surface tension of solder is uncanny and not once has it caused a bridge even on 0.4mm pitch parts.

It will even eliminate other bridges if I paste a bit on the bridge and touch it with the iron. I'd like to know what it is, maybe it's plumbers fluxite or something like that.

Ruby Flux brand has that ability as well, its meant for terminals and turns solder into a super flowing material. Never used it on SMD stuff but I use it all the time to tin wire. just a little bit along the length, and wipe a blob of solder and it perfectly tins ribbon cable into breadboard jumper with no lumps or missed spots.

leaves a nice red mess when I have used it on boards, though its easy to clean.