Electronics Notes

Budget

$300 USD cap on components (~ 900.000 COP)

• Not including fixed costs like rent, utilities etc.
• Also not including 3D printing costs
• But including mechanical components such as screws, cord, glue etc. as well

Final Parts List

• 2x TowerPro MG996R Servomotor. Alternative source = 48.000-72.000 COP total
• 4x 3.7V 18650 Li-ion batteries batteries (two to go in arm, and two as spares for the beneficiary to charge while using the arm). Currently using 2200mAh capacity (14 mil each from Suconel). Also available in 3000mAh, 4000mAh, 4800mAh or 9800mAh (unsure of battery qualities and if they have surge protection etc. at this price, may need to test) = ~40.000-72.000 COP total
• 18650 dual battery holder = 2.800 COP
• 1x 10A automotive fuse
• 2-battery USB charger - Regulated
• Arduino Beetle. Datasheet here. Datasheet here
• 2x MP1584
• 2x "Gravity" Analog EMG sensor by OY Motion - $37.50 USD (~120.000 COP) each = ~240.000 COP total

Misc electronics

• Wire
• 2x 1kOhm resistors
• 1x Push button
• 2x toggle switches
• Breadboard/stripboard = ~5.000 COP? - Current Prototyping Board
• 3x 2-pin uni-directional plugs
• 4x 3-pin uni-directional plugs
• Foam

Tools Needed

• Soldering iron
  • Solder
• Hot glue gun
  • Glue
• Wire cutters

Recommended tools

• Wire strippers
• Long-nosed pliers
• Temperature adjustable soldering iron
• Craft-knife/scalpel
• Soldering station
  • With clips, magnifying glass, light etc.

Alternatives Considered

Battery system

• 2x 1800mAh 3.7 V Li-Po batteries in series = 40.000 COP
  • May need higher capacity alternative?
• 2x these to charge = 6.000 COP
• 2x microUSB cables?
  • Ideally would get a single system that can charge 2 cells at once (talk to Julian about this?)

18650 Chargers

1. 2-battery wall charger
2. Also have 2nd option that is combination of battery holder and FC-75 charging chip (available at DidactasElectronicas). Chargeable via microUSB.) Not recommended to give out to patients.

• Cheap alternatives: Double - 1+D, Single - Suconel.
• = 15.000-28.000 COP

Expected Battery Life

• Battery energy capacity: 2 * 3000 mAh * 3.7 V = 80 kJ
• Worst-case motor power usage: 2 * 6 V * 1.4 A (stall current, will be less than this) = 16.8 Watts
• Voltage regulator efficiency ~90%.
• 70 minute estimated motor run time under peak power usage (both motors constantly under stall current).
• It is expected that this will be sufficient for 12 hours of standard usage throughout the day, but further testing on this is needed.

Micro-controller

• 1x Arduino Uno such as this = 23.000 COP

Power conversion

• Alternative option for 5V line this would be to use:
  • L7805 (5 V, 1.5 A)
    • OR LM323 (5 V, 3 A) voltage regulators = 4.200-8.400 COP
  • AND 1x 0.1 μF capacitor = ~1.000 COP
  • L7805 regulators only about 60-70% efficient in power conversion due to requiring 7.4 V input and only providing 5 V output. But this is okay for our 5V line as they are all low power consumption, so efficiency not as important.

Other options investigated

• 1x 5V 3A fixed voltage regulator if we want to run everything off 5V. More expensive and larger - Maybe more efficient/less heat?
• Using 1x LM2596 (3A variable voltage).
  • Should be enough if only using 2 motors or using 3 motors but full 4.2 A stall current is incredibly rare.
  • Use 2 if want 2 different voltage levels (e.g. 5V for Arduino & 6V for motors)
    • Alternative option for this would be to use L7805 for Arduino (lower power so efficiency not important) and high-efficiency 3 A voltage regulator to provide 6 V for motors.
• OR use 1x XL4015 (5A variable voltage) as well if 3 A limit is not enough.
  • NOTE: If we are regularly using this near the 5A limit then attach the provided heatsink.

Requirements

• Voltages: Arduino 5 V, Sensors 5 V, Motors 4.8 - 6.6 V
• Amps: Motors 2-3x 1.4 A peak current draw, other devices negligible
• Efficiency: > 90% ideally.
• Light and small
• Cheap (< 10.000 COP?)

Sensors

• Sensors from Protesis Avanzadas