Sonar for the visually impaired - component selection

Projects / Project Swiftlet  "Let's go shopping II" Original post date: 07/20/2015

Got my BOM sorted out with the help of BOM-EX ULP in Eagle (Home, Eagle CAD site). I have looked up the digi-key part# and did some clean up.

What I have seen is that 0402 parts are cheaper than the 0603. Most of the parts on this board uses 0402 for size. There are a couple of parts such as the MOSFET array that is pushing the PCB geometry. I had to alter the recommended footprint to meet the minimum spacing.

So going to put together a digikey order early next week, try a few things and then finalize the PCB.

need to determine the exact spacing for the battery holder as they don't come with recommended layout data.
need to finalize the choice of speakers. As for mounting, probably have to make something up and use connector+wire for this iteration. Speaker acoustic and case designs will have to wait until most of the hardware/firmware design are working.
still have to do a small breakout board for the left side to connect the sensors, speaker and haptic motor.
I have yet to figure out the connectors, speakers, ribbon cable and half a dozen smaller things that I need to order extra for trying things out.

I have placed the digikey order for parts.

• Got settled on Hirose DF11 2mm connectors. These are IDC for AWG#26 loose wires. Hopefully this gives a bit more freedom on the cord connecting the two sides. Their pin numbering scheme is a mirror image of the usual ones!

DF11 2mm connector - IDC for AWG#26 loose wires or ribbon cable

• picked JST 1mm IDC connector for speakers
• C&K slider switch I was hoping to use isn't in stock, but thankfully there is is a more expensive Japanese CL-SB-23C-11 which has exact same dimension.
• Trying out Knowles Nautilus Speakers. It has an integrated side-port exit. Also getting their regular speaker for comparison.
• I saw some speakers on Aliexpress, but as usual the quality on their picture doesn't impress me much and I am out of time for trying stuff on slow boat right now. The interesting part is the speaker parts with their enclosures. That's the hardest thing to get right in a prototype if you don't have a 3D printer.

Forgot all about the Thermometer + EEPROM on the sensor PCB when I was working on the BOM. I guess I'll have to make do with the more expensive and less integrate parts I have on hand.

Ordered wrong resistor size: 0603. Changed design to not use the value. Made some bad choices for inductors. I have to redo the footprint and push things aside for the larger inductors. Going to try out cheap Chinese knock off inductors this time and may be I have better luck? If that fails too else fails, then I'll pay the $8 and order the legit part from digikey.
The contacts in the battery holder are pretty stiff. So it looks like the spacing I had is a bit too short. Going have to drill a few holes on a scrap PCB and try it out.

The JST connector I picked for the speaker is the same the one in my broken camera! So I guess I can try that speak out at some point.

Without an enclosure, that Nautilus speaker works a lot better than the Aliexpress "iphone" speakers. Even their $1.5 speaker was a few times louder. The side port exits works really well and when you point it towards the outer ear. With a properly designed enclosure, it would only get better.

Still need to redo the layout to accommodate the much larger inductor. Thankfully, the spacing of the battery holder has increased.

Looks like I'll need to limit the power usage for AA/NiMH battery mode operation. When running from a Li battery, the power available is much higher as I am using a different buck converter.

I have wired up the boost circuit for testing.

MCP1640 Boost converter test circuit

It seems like the inductor would work fine as long as I keep the current below 100mA and input voltage above 1.1V. A tiny bit over, it saturates, the inductor heats up, efficiency drops and the output loses regulation. I have also tried it with an inductor with a higher saturation current rating. This time the MCP1640 heats up, so either way there is only so much power can be delivered in this mode.

It might not be a huge issue as we are drawing above 400mA from the AA battery. From the battery life point of view, it should be avoided in the first place. The only thing that draws that amount of power is the audio circuit and it would seem that the higher efficiency of new speaker might help to reduce that.

Note: At outdoor because of the ambient noise level, the amount of power might need to be increased by a few times.

I bought some JST 1mm SR series IDC connectors for the speaker connection. They are tiny polarized and do not take much vertical spacing. The beauty is that these connectors requires no $200 crimping tool to make. In my case, I just use my finger nail to push the wires into the slots. I used the dull size of the box cutter blade to push it in a bit more for good measure. The contact blades on both sides cuts through the insulations and make air tight contact with the stranded wires inside. The plastic grippers at the end of the connector grip onto the outside of the wires. It is a good system. I am lucky that the AWG28 wires (from scrap piece of round SCSI cables) would fit into the connector that was designed for AWG30. My only complain is that there is no where to grab onto if you want to remove the connector.

JST 1mm IDC connectors