Circuit Board Checker

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:

Parts:

Notes:

The Black clip must be connected to the negative ground of the board under test.The Red clip should be connected to a positive voltage source (not exceeding 30V) available on the same board.Metal probe is suitable for this circuit.Two Miniature Crocodile Clips (Red and Black) are also necessary.

Filed Under: hobby project