Adding Bluetooth LE

Bluetooth LE 4.0 module test rig

Bluetooth 4.0 LE (BLE) is a low energy, wireless protocol allowing communicating between devices. It only works well over a short distances, but that’s just great for a wearable device to be able to connect and synchronise with its wearers smart phone, tablet, laptop, or BLE enabled desktop computer (USB to BLE dongles are very cheap, just a few UK pounds).

When Lightlog project was first started BLE technology was too expensive to include, but after 6 months or so, lower cost modules started to appear on the market. Prices have now fallen past the point where it’s cheaper to include BLE than it is to use a USB solution (e.g. a USB cable and USB support circuitry needed on the device). There are some drawbacks with BLE as it complicates the software needed for both the Lightlog firmware, and the client app/application. BLE modules are also more power-hungry than using a physical USB connection, so extra attention is needed to make sure the battery life is not unduly affected.

Below is a close-up of the test setup for the HM-11 Bluetooth 4.0 LE part. The module is designed to be connected to a custom PCB board with surface mount pads, but here you see it manually soldered to individual breakout wires for testing on a breadboard. The connection pads on the module are tricky to solder to, any physical strain on the wires will easily rip a pad off. I managed to rip a pad off the first one I tried to test; not an auspicious start as back then they were about £10 each and I’d only managed to source 5 for early testing (now they are close to £5 each).

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If you’re interested in some quick technical details… Apart from the usual power (2.5-3.7V) and ground, you only need two other pins to get it working, transmit and receive (RX and TX). The other two wires in the photo are only for testing and debug; one is a to a pull-up resistor to the reset pin (if you pull the pin down to ground for 100ms you can force a reset); the other connects to a small status LED that indicates if the BLE is in discovery mode (slow blink), or connected (solid on) – very handy while testing!