Arduino "Standalone" Serial Issues

I built the "Standalone" version at:

I've been unable to get serial comms working at all. I took it to an experienced engineer and he said that the 7404N chip was inappropriate and that something like a MAX232N should work instead.

1) Should I dump the 7404 in favor of the MAX232N? 2) If not, any thoughts on what I might be doing wrong? I've triple checked pinouts, connections, etc. 3) If I should swap, what might the new schematic look like? He said I'd need several capacitors for the 232 but I'm not sure just where they'd go.


That schematic you linked to has a mistake - the inverter on RXD is hooked up backwards.

The 7404 is just an inverter. RS232 uses both inverted levels and different voltages than TTL. The 7404 takes care of the levels, but the MAX232 will take care of the voltages as well, producing the -12 and +12 levels needed by rs232.

There is a version of RS232 that will operate on 0V/+5V. This circuit could work depending on the particular serial port you connect to.

There is also a game you can play with some transistors and "steal" the RS232 levels from the device you're interfacing with (see This is cheap and usually works, but I have had occasional problems with that method.

To have some thing that will work well, get the MAX232. There are other devices that do the same thing, but the MAX232 is the most common. The MAX233 will let you dispense with the capacitors, but it costs a lot more than the MAX232.


I did catch the mistake on the transposed pins, but everything you've said confirms exactly what my helper said as well. I found this schematic:

Does this look about right?


Yep, I think that one will serve you well. I've probably built more of those than any other circuit. :)


Thanks much!