+15v ---*
	|
	|
	-
       | | R1
       | |
        -
	|
	|
	| < v = 3.5
	-
       | | R2 = 5K
       | |
        -
	|< v = 3.5
	|
	|
	-
       | | R3
       | |
        -
	|
	|
	*
-15v-----