rs232 converter

Other Hardware General Electronics
1

Currently I`m using max232 to convert true rs232 signal to ttl. Voltage level from rs232 is +-8v and other machines +-12v.

On ttl side is nodemcu(esp8266). Was thinking of modifying circuit to make it cheaper. I need only TX line of RS232 i.e. I`m reading only data and not sending anything back. So max232 chip plus extra capacitors for charge pumps is overkill if simple circuit can replace it.

I look at max233 with integrated capacitors but its very pricey and I will still need voltage divider on esp side to reduce 5v to 3.3v.

RS232 speed is fixed at 9600bps.

Found sample circuit of rs232 converter and as per ltspice it must work without problems. It is based on bc847 bjt and diode to protect bjt from negative voltage. What I want to ask if this circuit has some flaws that I cant see ?

Thanks

rs232 converter.jpg
rs232 converter.jpg751×520 53.5 KB

2

I've used a simple circuit like below in the past and had no issues.

ezservo.jpg

ezservo.jpg

3

@zoomkat yours cant support true rs232. The problem with mine is that i have to deal with negative voltage thats why that diode is in place. Veb of bc847 is 6v and thats absolute max. In your case it will stress it to 8v or 12v depending which rs232 i will connect. I have 2 devices one has voltage level of +-8 the other one +-12.

4

Easily fixed with a diode. :sunglasses:

5

Thanks guys then i will proceed with 1 transistor and eliminate max232.

6

If you look at an early design of the Arduino... specifically, the SEVERINO, you can see how it was accomplished without using a MAX232.

Severino Design Page

7

Very useful schematic pwillard!

What the purpose of choosing r6 and r7 like that? 10k and 1k on collector.

I calculated it to be 750ohm for 3.3v.