Condensed Consolidated Statements of Operations and Comprehensive Income (Loss) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jan. 31, 2021	Jan. 31, 2020	Jan. 31, 2021	Jan. 31, 2020
Income Statement [Abstract]
Revenues	$ 11,672	$ 9,628	$ 38,612	$ 31,270
Cost of revenues	8,868	6,488	26,398	25,358
Gross margin	2,804	3,140	12,214	5,912
Selling and administrative expenses	2,454	3,619	9,805	8,362
Research and development expenses	1,289	1,082	3,466	4,813
Operating loss	(939)	(1,561)	(1,057)	(7,263)
Other income (expense):
Investment income	160	191	316	415
Interest expense	(9)	(25)	(84)	(75)
Other income (expense), net	0	(132)	129	(75)
Loss before provision for income taxes	(788)	(1,527)	(696)	(6,998)
Provision for income taxes	12	19	37	48
Net loss	$ (800)	$ (1,546)	$ (733)	$ (7,046)
Net loss per common share:
Basic and diluted loss per share (in Dollars per share)	$ (0.09)	$ (0.17)	$ (0.08)	$ (0.78)
Weighted average shares outstanding:
Basic and Diluted (in Shares)	9,185,235	9,104,228	9,164,713	9,059,251
Condensed Consolidated Statements of Comprehensive Loss
Net loss	$ (800)	$ (1,546)	$ (733)	$ (7,046)
Unrealized (loss) gain on marketable securities:
Change in market value of marketable securities before reclassification, net of tax	(22)	50	(18)	258
Reclassification adjustment for realized gains included in net income, net of tax	(12)	2	(21)	1
Total unrealized (loss) gain on marketable securities, net of tax	(34)	52	(39)	259
Comprehensive loss	$ (834)	$ (1,494)	$ (772)	$ (6,787)