circuit board tester

Circuit Board Tester

• Indicates the basic integrity of a printed board
• Self-powered - 3 to 30V range

Circuit diagram:

Parts:

R1,R2___________22K 1/4W Resistors

D1______________LED (Any dimension and shape, preferably red)

D2______________LED (Any dimension and shape, preferably green)

Q1____________BF245 or 2N3819 General-purpose N-Channel FET

Q2____________BC547 45V 100mA NPN Transistor

Q3____________BC557 45V 100mA PNP Transistor

Probe_________Metal Probe 3 to 5 cm. long

Two Miniature Crocodile Clips (Red and Black)

Comments:

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).

Note:

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.

REF: http://www.redcircuits.com/Page130.htm