Circuit Board Checker Circuit Diagram
This little circuit indicates the basic integrity of a printed board, detecting 0V, positive supply voltage from less than 3V to 30V and floating parts. If the probe is floating, as it would be in a broken track, then both LEDs barely light up, since there is no current to drive the transistors, but if the probe touches 0V or a positive voltage one or other lights. A digital signal should light them in proportion to the mark-space ratio whereas the output of a circuit oscillating at a frequency rate below about 20Hz will cause the LEDs to flicker alternatively. The LEDs will illuminate always at a constant intensity, no matter the voltage supply used, because they are fed by a very simple FET constant-current generator (Q1). Circuit diagram: Circuit Board Checker Circuit Diagram Parts: R1 = 22K R2 = 22K D1 = Red LED D2 = Green LED Q1 = BF245 Q2 = BC547 Q3 = BC557 Notes: The Black clip must be connected to the negative ground of the board under test. The Red clip should be connected to ...