This circuit converts the RS232 and TTL signal. Some OBD fans has a similiar design but using a small cap and diode in case TXD is transmitting.

This circuit still didn't provide positive power for other circuit, say a small micro, etc. For a fully no-power device, you need "get" the power from the RS232 Serial port signals.
 
Funny N.
New Bedford, MA
http://www.AuElectronics.selfip.com



----- Original Message ----
From: "enkitec@gmail.com" <enkitec@gmail.com>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
Sent: Saturday, November 17, 2007 9:20:45 PM
Subject: Re: [PIC]: RS232 to 5V interface without supply voltage


    See the attached circuit.
    It uses TXD for -V. RTS or DTR could be used instead.

    Mark Jordan


On 17 Nov 2007 at 16:31, Funny NYPD wrote:

> I remember the old mouse use a 555 to convert negative voltage to a positive voltage power supply, this maybe what you are looking for. NS may still have the application sheet available online.
>  
> Funny N.
> New Bedford, MA
> http://www.AuElectronics.selfip.com
> 
> 
> 
> ----- Original Message ----
> From: Peter Feucht <p.feucht@palmed-medizintechnik.de>
> To: Microcontroller discussion list - Public. <piclist@mit.edu>
> Sent: Friday, November 16, 2007 7:38:41 AM
> Subject: [PIC]: RS232 to 5V interface without supply voltage
> 
> Dear Piccers,
> 
> I remember, that a long time ago I saw an application note for a
> RS232-to-PIC(5V) interface which worked without any external power supply,
> but it took the needed voltages from some pins of the serial port. It had
> just a RS232 9 pole Dsub and 3 Pins (Rx,Tx,GND) on the PIC side.
> 
> I just googled but wasn't successful.
> 
> Does anybody remember such an application?
> 
> Thanks for any help and best regards
> 
> Peter
> 
> 
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