This can be powered by voltages in the range of 3.3 to 12 V or more but component values below are for a 5 V power supply (not included).  The circuit board design is on OSH Park at  This does not use a microcontroller so no program downloading is required.  The main circuit element is a ULN2003A Darlington transistor array (16-pin DIP).  The board is small to minimize board cost.  Board mounting is intended to be done via the two panel-mount SPDT switches.

One switch (S1) is used to select between the continuity and discontinuity functions.  The other switch (S2) is used to enable or disable the beeper.

The tester's positive test input terminal is the power supply and it's negative input test terminal is a ULN2003A input as described below.  There is no extra circuitry for current-limiting, ESD protection, etc. (KISS principle used).

Build Options:

Key specifications of the ULN2003A Darlington transistor array are:

If a different power supply voltage or different LEDs are used, the LED current-limiting resistors (R1, R4) may need to be changed.  The most accurate way to determine the resistor value is to use an adjustable power supply, a DMM, and a 100 to 1000 Ohm resistor.  Hook the resistor in series with the LED and the DMM across the LED. and set to measure voltage  Then change the power supply voltage until the desired brightness is reached.  Then measure the LED voltage.  Then set the DMM to measure current and put it in series to get the LED current.  Then use Ohm's law:  R = (Vs - 0.6 - Vled) / Iled, where Vs is the power supply voltage that will be used, 0.6 is approximately the ULN2003A Darlington transistor voltage drop, Vled is the LED voltage drop, and Iled is the desired LED current.  Use the nearest available resistor value; precision is not required.

The PCB side with IC U1 ULN2003A is called the top side.  The IC must be mounted on the top side for the PCB interconnections to be correct.  All the other components can be put on either side or mounted remotely instead of on the board.  What works well is as shown below: IC, beeper, R2 & R3 on the top side; switches, LEDs, R1 & R4 on the bottom side.  To make it easier to solder, the order of building is important, see Build Instructions below.