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Note 7 - Long-term Debt (Details Textual)
12 Months Ended
Apr. 18, 2020
USD ($)
Apr. 29, 2016
USD ($)
ft²
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Apr. 01, 2016
USD ($)
Proceeds from Issuance of Long-term Debt, Total     $ 1,214,500  
Line of Credit [Member]          
Line of Credit Facility, Maximum Borrowing Capacity     2,050,000    
Long-term Line of Credit, Total     $ 0 0  
Line of Credit [Member] | London Interbank Offered Rate (LIBOR) [Member]          
Debt Instrument, Basis Spread on Variable Rate     1.80%    
Loan [Member]          
Debt Instrument, Periodic Payment, Total   $ 18,000      
Debt Instrument, Interest Rate, Stated Percentage   2.60%      
Debt Issuance Costs, Gross   $ 139,000      
Debt Instrument, Interest Rate, Effective Percentage     3.23%    
Debt Service Coverage Ratio   1.25      
Amortization of Debt Issuance Costs     $ 11,000 11,000  
Debt Issuance Costs, Noncurrent, Net, Total     85,000 $ 85,000  
Amortization of Debt Issuance Costs Annual Expected Amount Over Remainder of Debt Term     $ 85,000    
Loan [Member] | Building [Member]          
Area of Real Estate Property (Square Foot) | ft²   27,300      
SBA CARES Act Paycheck Protection Program [Member]          
Proceeds from Issuance of Long-term Debt, Total $ 1,214,500        
Duluth Economic Development Authority [Member]          
Tax Exempt Industrial Revenue Bonds Amount Sold to Bank         $ 3,415,000