SparkFun Forums

[ysc]

Please help me to find whether my RN-41 works or not.

I am new to RN-41. I powered up the RN-41 DIP module with 3.3V, I can not see the device of RN-41 from the My Bluetooth Places on my desktop, although the bluetooth of desktop found lots of cellphones. The RN-41 DIP module is power from PIC-USB-4550 develop with a voltage regulator of 250mA output. I check the power supply to RN-41, the voltage is right. I measured the TX, it is 3.3V, the RX is about 0.4V.

What I should to do to find out this RN-41 works or not?

Thanks for your help,

Chen.

[lyndon]

Connect an LED to pin 21. It should blink on power up.

[ysc]

Thanks for your reply.

I connected PIN 21 to a LED, when powered it up, nothing happens to the LED.

Anything suggestion?

Thanks,


Chen.

[phishguy]

Do you have your reset line hooked up? If so, disconnect it. You may have the wrong reset polarity. It should work with it being disconnected. Also, the LED should be connected in series with an approximately 470 ohm (not too critical) resistor between pin 21 and VCC (+3.3V).

[lyndon]

This is how my RN41 is configured:
All GND pins to ground (I think there are 6 of them)
Vdd to 3.3V
CTS looped back to RTS (pins 15, 16)
LED on pin 21 with 330ohm resistor to Vdd.

Tx & RX to a microcontroller.

All other pins N/C

At power up the LED blinks quickly then slows down after 1 minute when the setup period ends. LED is on continuously when a Bluetooth connection is made.
HTH

[ysc]

Do you mean to connect LED on pin21 back to Vdd (+3.3V) with a resistor 330 Ohm? Will the pin21 be connected to the negative end of the LED?
I connected the LED on pin21 to GND directly.

I will try to configure it as yours. But I doubt that my RN-41 from sparkfun is bad. Because when powered up, the desktop can not find the device from My Bluetooth Places. I tried to reset from PIO4 too, but nothing happens.

Thanks lot for your helps,


Chen.

[phishguy]

Do you mean to connect LED on pin21 back to Vdd (+3.3V) with a resistor 330 Ohm? Will the pin21 be connected to the negative end of the LED?

Yes. An LED requires a resistor to limit the current. If you don't use a resistor, the LED will burn out and the output driver can also burn out. Outputs of IC's can sink more cuurent than they can source. So when the ouput goes low, you will be lighting the LED. With the connections as stated, your module should be working. Make sure that you leave the reset line disconnected.

To do a default reset of the module, you need to have PIO4 high at power up, and then toggle PIO3 three times. Again, it is recommended to use a resistor to pull up these pins. A 4.7K resistor should work fine.

If it still doesn't work, you may indeed have a bad module. You should only need to connect the power and ground pins in order to be able to see the module from another Bluetooth device.

[ysc]

Thanks for your reply. I have a question about the LED. I measured the resistance of the LED, one way is about 1.5Mohm, another way is infinite. Why the LED will burn out without a serial resistor of 330 ohm? Will its resistance of 1.5Mohm drops down once it is powered up?

Thanks,

Chen.

[phishguy]

It is a diode with a forward voltage drop of approximately 1.5V (depends on the LED). When the voltage exceeds this, the current will inrease dramatically unless you have a resistor in series to limit the current. Always use a resistor in series with a LED or use a constant current source such as a LED driver IC.

[Philba]

yes and using a DMM to measure the resistance of an LED isn't going to give you meaningful results.

[phishguy]

Also, to figure out the proper resistor to use, you need to look up the data of the LED to find the typical voltage drop and current rating. If you have a typical voltage drop of 1.5V (Vfd), current of 10mA, and the VCC is 3.3V you can calculate the resistor by the following formula.

R = (VCC-Vfd)/I
R = (3.3-1.5)/.01 (10 mA)
R = 1.8 /.01 = 180 ohms

These are examples and you will need to plug in the correct values for your LED and VCC.

[ysc]

I see.

Thank you very much,


Chen.