Simple TTL to RS232 adapter

During my spare time, I made a quick TTL design for an RS232 adapter that works with my AVR development board. This adapter helps to connect TTL (Transistor-Transistor Logic) level signals to the RS232 interface. TTL side is a 9-pin female connector, and RS232 sire is a male connector.

The unit is powered from the target board using power pins.

The RS232 TTL adapter circuit:

Put all the in a plastic case and see what I have got:

The inside of the box:

RS232 adapter in action:

Let’s do some tests with this RS232 adapter. First of all, write some example routine for AVR microcontroller:

Compile this program with WinAVR ad burn to chip.

Then download Bray’s terminal or other terminal software, open it and select the required baud rate and different settings like parity bits and so on. Start terminal and reset the AVR microcontroller:

Get the results. Everything is working fine.

Download UART AVR project files here. Don’t forget that in this source AVRLIB library is used. So you need to download it to be able to build.


  1. Adding 2 little LED diodes to TX/RX lines helps much. You can use such converter as RS232 line tester – LED flash for transmission.

  2. Its good idea. I didn’t think about this. It won’t be hard to add these LEDs to existing board. Thanks.

  3. We used to make these ourselves, but you can buy these ready made for $40, and the commercial units are self-powered (you don’t need Vcc). Google for 232LPTTL.


  4. wow, a copy of what can be found in every max232 datasheet, great work

  5. Word press is S*T consider a laxative.

    Why buy a 5 – 11 dollar part you’re not using all the features of when you can use a couple transistors and some discrete parts?

    If you need RS232 levels for some reason you can find a blizzard of cheaper alternatives. I’ve used the ADM202, 75c1406 which are under a buck at some suppliers depending on qty.

    Another way of getting those levels is a 99 cent wall wart. I’ve done that to and it works for extended runs.

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