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).
Simple Circuit Board Checker 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 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.
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