Start with your salary or hourly rate at your day job. For salary, divide by 1920 for an approximate hourly rate. If you get benefits at work such as health insurance, increase the hourly rate by 10-20%. This is the total hourly rate for the project.
Keep track of every *minute* you spend doing the tasks necessary to make a resistance box.
Decide on the accuracy (resistance tolerance) and precision (number of switch choices).
Draw the schematic.
Select the resistors, switches, connectors, and enclosure.
Select the best vendor for each.
Place the orders.
Receive the orders.
Inspect each shipment for accuracy.
Pay for the parts.
Create mounting holes in the enclosure for every switch and connector.
Assemble the mechanical components.
Install the resistors.
Assemble the device.
Select a resistance meter that is more accurate than the best-case accuracy of the resistance box. The standard engineering rule for this type of test equipment is 10x; if you need to measure a 1% resistor accurately, a pro would need a 0.1% accurate meter. You can get away with less.
Select the meter vendor.
Purchase the meter.
Receive the meter.
Inspect it to make sure it works.
Pay for the meter.
Use the meter to verify the accuracy of every possible resistance combination.
Add up all of the times. This is the project time.
Multiply the total time by the total hourly rate. This is the labor cost.
Divide the total time by 720. This percentage is used for the overhead.
Add up your rent or mortgage, and utility bills for one month.
Multiply that by the overhead percentage to get the project overhead cost.
Add the parts cost, test equipment cost, labor cost, and the overhead cost. That is what it cost you to build one resistance box. This does not include any profit, workman's compensation insurance, safety agency certification, advertising, packaging and shipping, and all the other stuff it takes for a company to produce a deliver a product.
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