When building microcontroller projects it’s often desirable to connect them to a computer. Most RS232 computer ports are able to read 0 and 5 volt TTL data levels however there are some computers that require voltage levels that are closer to the true RS232 spec. Most people use a MAX 232 chip when there is a need to follow the spec. correctly, but it’s possible to build your own circuit. uC Hobby has an article that shows and describes a circuit that does just that.
“The basic problem we have to solve is that the uC UART is a logic level device and RS232 is not. To send data over RS232 the voltages need to be about +/-10V while the output at the uC will be 0-5V or 0-3.3V. To see a zero signal the receiving RS232 device needs to see a negative voltage. We also need to invert the phase of our logic signal so that a 1 (5V) is converted to the negative voltage (-10).
Typically the threshold for 1 vs. 0 in the RS232 receiver is slightly above 0V we can get away with using less then +/- 10V. +/-5V should be sufficient for almost any devices. We usually have +5V available in our electronics projects but the negative voltage is a problem.”
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That’s a nice solution for those that don’t have a MAX 232 or similar level-translation chip available, or don’t want to wait to order one.