Provision for income taxes (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
provision for income taxes Details
Provision at U.S. statutory rate	$ 1,050,000	$ 240,000
Alternative minimum tax	51,000	7,500
Depreciation and amortization	(37,000)	(29,000)
Accrued liabilities and sales allowances	38,000	(31,000)
Non-deductible stock based compensation	23,000	32,000
Other differences	(102,000)	51,000
Decrease in valuation allowance	(756,000)	(263,000)
Income tax provision	$ 267,000	$ 7,500