Figure 23: Level Converting By Resistor - SimCom SIM808 Hardware Design

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change the resistors in the following figure from 5.6K to 14K.

Figure 23: Level converting by resistor

If the voltage of UART is 3V or3.3V, user also can use following reference circuits:
Figure 24: Isolation circuit by diodes
No
te: when a diode used to isolate voltage cross, customer should notice that there's voltage drop on the diode. And the
signal's voltage level should meet the customer's electrical character. The recommend diode is Schottky diode e.g.
RB551V-30TE-17 and SDM20U40.
If the voltage of UART is 5V on customer side, customer can use the following reference circuits :
Figure 25: TX level converting by transistor
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SIM808_Hardware Design_V1.00 2014.03.27