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Articles I read about this tell me to hook up a 1KOhm resistor between the Arduino's TX and the iPhone's RX. I understand this is because Arduino's signal is 5V and I want 3.3V going to the iPhone.

My question is, where did they arrive at this value of 1KOhm? Is it documented somewhere that the serial signals are 1.7mA?

Nick T
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Steven Lu
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  • I do not have any source except knowing people whom have made choices like this before so I am only writing a comment. Often you take a 1k and say 1.8mA cannot hurt my input so lets just use this 1k, it will protect me and I can measure. I am not sure it is always the best choice, but it works 99/100 times. – Kortuk Mar 01 '11 at 07:15
  • See also http://electronics.stackexchange.com/questions/9153 – markrages Mar 02 '11 at 01:41

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someone probably mucked with resistors until the voltage came right, or maybe there's a pullup/down resistor sitting there on the input.

Or it's a high impedance input and 1.7mA is the most they feel comfortable pushing through the input's protection diodes

It depends a lot on what the input circuit and impedance is - you might be better off (and safer) building a voltage divider (2 resistors of your own) you really don't want to be pulling inputs above their Vdd - you end up dumping that current into the protection diodes and making stuff hot (or mildy warm) - better off to start with a simple voltage divider and then looking at the signal levels on a 'scope - tweak to get the result near to 3.3v but lower

Taniwha
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Don't risk blowing up your iPhone by messing about with series resistors. Use a proper 5v-3v converter.

A 74LVX244 buffer chip will accept 5v input and output a proper, safe, 3.3v logic level. This requires a source of 3.3v supply. A sufficient number of fixed-drop diodes (say 2 diodes in series with 1v drop each) is a better passive solution.

Christopher Biggs
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  • A resistive divider is fine. Use 1.5kΩ + 3kΩ in series, take the output across the 3k resistor. Ohms's law works! Nothing sketchy about it. – markrages Mar 02 '11 at 01:40
  • Heh, diodes are definitely *not* "fixed-drop" devices when operating at some unknown current (driving into an unknown receiver). A resistor divider is a much better solution unless we're operating at high speeds (not sure even then) or low power. – Nick T Mar 02 '11 at 04:47
  • Yeah I think at this point I won't even be bothered to put together a divider circuit, but I do know from what circuit analysis i know that that will get the job done. If i did have a convenient IC that did 5v->3v I'd use that too i suppose. I think the actual question I was looking for when I asked this question was, how come a 1kOhm resistor in series was sufficient; why didn't I need a divider? The answer is kinda wishy-washy so I'll just accept it as "use the divider but the resistor in series seems to work fine so might as well use that for now" – Steven Lu Mar 07 '11 at 17:48
  • @Steve, my guess (all I can do given a black box) would be that if you put 5 V into the iPhone through a resistor, clamping diodes on the input would limit it to 3.3 V. You could test this by measuring the voltage on the iPhone side of the resistor when you're giving it 5 V on the other end. – Nick T Mar 09 '11 at 02:16
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I've actually brought this into practice and I can tell you 1KOhm works perfect for as long as I use the project. I know that is no guarantee, but it isn't based on hot-air.