อยากได้เหมียนกัน..สงสัยต้องทำเอง..
แต่ต้องหา ic เบอร์ MAX232 ก่อน 555
ถ้าหาไม่ได้นี่เลย...ถูกกว่าเยอะแต่ต้องทำเองนะครับ
หรือ
Now, to adapt TTL levels to RS232 we use a PNP transistor. This transistor will turn on when it’s base voltage is below it’s emitter voltage. The transistor turns on when the TTL transmitter outputs a logic 0 (0V). This pulls up the collector toward the emitter wich is tied to +5V, feeding +5V to the RS232 receiver (RxD). With a logic 1 (5V) the transistor is turned off and the 4.7K pulls the signal down to the negative voltage on the capacitor, feeding -12V to the RS232 receiver. The voltage levels at the RS232 side of the adapter, +5V and -12V, may not the conventional RS232 levels for 0 and 1 logic but are valid signal levels.
Transistor Q1 is connected to TxD, and the goal is to borrow a negative voltage connection for Q1. Conditions make this converter to work only half-duplex, so that a negative voltage is always present when transmitting data from TTL level to RS232 level.
