On Alternative Components

If you cannot source some of these listed components as per the parts spreadsheet, then here are some suggested alternatives and notes on selecting alternatives.

Arduino Beetle

If you use a different type of micro-controller than you should ensure that the controller is Arduino-compatible, the code is able to be loaded identically and that all of the necessary input and output pins are available as per the circuit diagram (2 analog input pins, 1 digital input pin and 2 pins capable of outputting digital PWM signals at a minimum).

One possible alternative would be an Arduino nano or clone such as this with a cost of 18.000 COP.

Voltage Converters

The specific voltage converters may be hard to obtain, but any alternatives that meets the following specs below will work fine.

Required Specifications

• Voltages:
  • 5V for the micro-controller, EMG sensors, debug button and lock switch.
  • 4.8-6.6V for the servomotors.
• Amps: ~3A peak current draw for the servomotors, other devices negligible.
• Efficiency: >90% ideally.
• Dimensions: Less than 20mm x 20mm x 10mm

Suggested Alternatives

If you can locate any of the alternatives listed below, they may work for you:

5V line

• LM323 (5 V, 3 A) voltage regulators with a 0.1 μF capacitor in series across the ground and input power pins. Estimated total cost of ~5.200-9.400 COP.
  • Note: These regulators are 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.
  • The motors could be powered under the same 5 V if necessary as this is within the acceptable input voltage range for them. However, the low efficiency of this converter will result in significant power losses for the relatively high current draw of the motors.
• 1x 5V 3A fixed voltage regulator.
  • This would be a better option if also running the motors of the same 5V power line due to it's higher efficiency.

6V line

• LM2596 (3A variable voltage).
• XL4015 (5A variable voltage) as well if 3 A limit is not enough, for example if you choose to add an additional motor or use different motors with a higher current draw.

Circuit board

If you cannot find the specific strip-board/prototyping board layout that we have recommended in the parts list, then you may need to redesign you own circuit board layout to fit within the board that is available to you. But you can follow the basic schematic diagram for the electronics found here.

18650 charger

• One alternative would be this 2-cell AC wall charger
• We also have some cheaper alternatives like this 2-cell AC wall charger, or this Single-cell AC wall charger.

Servo Motors

If you cannot find the specific servos we recommend substituting a different servo shouldn't be a major issue. It must fit these requirements

• Standard servo profile (low profile servos will also work but are significantly more expensive)
• 5-7V operating range. This is to allow the PWM signals from the arduino to be read without additional electronics
• 180 degree operating range
• Internal Metal Gearing, this is strongly recommended as the force going through the servos can sometimes be quite high and plastic gears can strip under repeated stress.

Batteries

Please read this chapter on battery selection. In short, the batteries must have a high maximum discharge rate (for when the servos activate) as well as a high capacity per KG. If you are simply looking for a smaller battery then an 18500 lithium ion cell is likely the best option.

We found lithium polymer batteries (like the ones in RC cars, drones and phones) were unsuitable due to the low maximum discharge rate of the cells.