Income tax (Details 2) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Current deferred tax asset (liabilities):
Accounts payable and accrued expenses	$ 73,000	$ 82,000
Accounts receivable	(1,156,000)	(1,026,000)
Allowance for doubtful accounts	43,000	46,000
Prepaid expenses	(88,000)	(75,000)
Deferred revenue	551,000	376,000
Net current deferred tax liability	(577,000)	(597,000)
Long-term deferred tax asset (liabilities):
NOL - federal
NOL - State	7,000	4,000
Foreign tax credit	40,000	38,000
Book - Tax depreciation	(13,000)	0
Net long-term deferred tax asset	34,000	42,000
Net deferred tax liability	$ (543,000)	$ (555,000)