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Measuring a Bluetooth modules output? Answered

There is a serial Bluetooth module that I have used before on an Arduino project but now I'm trying to use in a project without the Arduino. Basically I need it to be an electronic trigger, I don't care really what the signal is just how long and the voltage and current.

There are 4 pins on the module: Vcc, GND, Tx, and Rx. There are 2 open pads: Key and State. When I send a command from my phone the module sends a signal out the Tx pin, and it is this signal I would like to measure so I can pick a command with an appropriate signal and tailor the rest of the circuit accordingly.

I tried a voltmeter between Tx and GND but saw no signal on any level measuring DC. Is there something I'm missing? Is the signal more complex or different, or is that test setup flawed? I tried other combinations, Rx-GND, Tx-Rx, RX-Tx, GND-Tx, GND-Rx but saw nothing.


If there's anything, it'll be WAAAAY too fast for a voltmeter to see. It WILL consist of at least 10 pulses, probably +/- around 3V.

What are you aiming to do ? To make the module "transmit", its going to have to be sent a little string of something surely ?

That said, the RN41 module from Roving Networks CAN use I/O on its own AIUI.

I just want to trigger a 555. Basically I want to trigger a 555 from my phone but an Arduino for that seems like overkill.

Ah, that might be easier, but, as I said, its going to be a fast serial string from the pin. Have you tried piping it straight onto the 555 as it is now ? Or adding a little mosfet, driving that from the Tx pin on the BT module ?

What's the 555 doing ?

It's triggering a transistor which needs to stay open for several seconds otherwise I would skip the 555 as well. I will dig up some sutible components and try with the 555.

You shouldn't have too much difficulty if you use a high gain switch - your timing isn't critical, and the burst of serial data will be short enough not to affect your timing.