INCOME TAXES - Deferred tax assets (Details) - USD ($)	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Bad debt reserves	$ 25,000	$ 17,000
Inventory reserves	762,000	653,000
Warranty and other reserves	74,000	77,000
Stock-based compensation	205,000	212,000
Goodwill	852,000	866,000
Acquisition costs	212,000	223,000
Net operating losses - federal	0	0
Net operating losses - state	168,000	77,000
Other	51,000	20,000
Less valuation allowance	(316,000)	0
Deferred Tax Assets	2,033,000	2,145,000
Deferred tax (liabilities):
Prepaid expenses	(260,000)	(79,000)
Depreciation	(1,113,000)	(1,154,000)
Intangibles	(434,000)	(696,000)
Net deferred tax assets	$ 226,000	$ 216,000