Hack Notes CVA 090421
Working on our Schematics today - making decisions about parts, and discovering that Eagle is hard to use.
We are trying to use Eagle software as it's common in the community, free for non-commercial use (and only $49 if you sell stuff that has small & simple boards).
Here is the basic physical form factor we discussed:
- On the back side of the PCB mounts a 3xAAA battery holder
- We can include a mount-point on the back as well for a JST connector, allowing builders to optionally use a LiPo instead of AAA's. (This way we don't have to inclue LiPo + charger in kit, which would be a further $20-$30 but it's still an easy option.)
- On the front goes most of the components.
- This allows everything except the motor array to be in one case meaning simpler construction and less wires.
- One downside is increased weight in one place on the armature, rather than distributed in two.
- The case will have thin grooves down the sides to fit the long edges of the PCB, holding it in place.
- The width of the PCB+components+battery(pack) is sufficient that we can mount an HM55B compass chip at a right angle across the top without needing to add any kind of bulge for it.
- We can achieve this by having two right angle headers, one on each side of the PCB. By using different size headers, the through-hole pins can meet the board at different heights, but the sockets will be at the same height. The width can be adjusted to match the width of the pins on the Parallax breakout board.
- Some drawings or pictures of a mock-up would make this easier to understand. Maybe later!
568-1708-5-ND = 12 bit shift register, 20-SOIC, $2.17. Same thing 20DIP: 568-3142-5-ND.
Sparkfun Arduino Pro Mini - 3.3V/8MHz http://www.sparkfun.com/commerce/product_info.php?products_id=8824 There is a download link for Eagle schematics. Circuit includes the ATMEGA168, oscillator, passives, and serial programmer pin out. Also includes a reset button which we will eliminate.
The Pro Mini does not have the bootloader flash (ISP) pinout organized, we could copy that from the Ardiuno Pro circuit: http://www.sparkfun.com/commerce/product_info.php?products_id=8783
We want an on/off switch. EG1943-ND $0.92 - lower profile. CKN9564-ND, $0.53 - winner for now
3xAAA battery holder, 2479K-ND. Plus, pin out and part for JST jack (but no LiPo included in kit). (Amazingly least terrible) schematic here.
Provide a pin out for talking to other circuits - I2C, SPI. Plus VCC and GND.
Socket for pager motor array - double layer right angle header. 14 pins total: 7x2. Digikey # S5521-ND.
Pin out for connections to compass IC. 6 holes in PCB, right angle female headers connect into male headers from HM55B. S4338-ND, plus S5517-ND on battery side - must clip bottom row leads.
64KB EEPROM I2C 8 SOIC footprint: AT24C64CN-SH-B-ND. Useful for logging data from device during use e.g. motor activity. Same thing 8DIP: AT24C64C-PU-ND.
Voltage Regulator. Specs: 3.3V output, >250mA. Two choices: TKS71733CT-ND accept from 1.8-14V, 280mA output, $1.52. MCP1824T-3302E/OTCT-ND accepts from 2.1-6V, 300mA, $0.79. Both require 2 external caps (typically 1uf and 2.2 or 4.7uf). These will add a few dimes more in cost.
What shape do we want the board to have? Match shape of battery holder
JST connectors: 455-1734-1-ND (top entry), 455-1749-1-ND (side entry), $0.43/0.41 each. Females.
Bill Of Materials (BOM): we need to make a list of all parts once we have the circuit down.
Do we need mounting holes? NO
They sell GPS chips for $40 now - we could put a footprint down and I bet some hackers would go nuts for that...
People who want some boards: Ayer - 1 Eric - 1 Skory - 1