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I had problems with a bluetooth module HC-06 because it didnt transmit the data by UART, but someone told me to add a pullup resistor between the VCC and the TX of the module, it works, we tested with oscilloscope and we saw that when the resistor wasnt there the signal was like 200mV. Can someone explain why it works? and what happend when you have 2 voltage together like that?, the 5 Vcc from the source and the 200mv from the HC-06 TX, didnt the 5V affect the pin Tx from the HC - 06?

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It's probably an open collector output, whose circuit looks something like this:

schematic

simulate this circuit – Schematic created using CircuitLab

You have to add an external pull-up to introduce a high level to the circuit. This is a common way to make a voltage-agnostic output interface, since you're only limited by the output transistor voltage rating or any internal ESD diodes on the pin.

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