This project is based on the Pic-A-STAR continuity checker developed by Harold E. Johnson, W4ZCB and posted on the PIC-A-STAR Forum. It has been modified only by the inclusion of a circuit board (not really necessary) and probes - one of which has a tactile switch to prevent the possibility of leaving the unit turned on and discharging the battery. The circuit board will be made available to Club members as required for either Pic-A-STAR construction or as a handy test instrument.

The checker only applies some 96mV to the circuit being checked and is much safer for checking low voltage digital circuits than a garden variety multi-meter. A recent measurement of the probe voltage of a 10M sensitivity multimeter showed 1.47V. Using this checker, circuit connections above about 60 ohms are viewed as open-circuit whereas those below are viewed as short-circuit. A buzzer indicates short-circuit or continuity status.
The circuit board is quite small, measuring some 56mm x 20mm. The battery will fit in the Jaycar HB6015 jiffy box with plenty of room for the battery. The buzzer is attached externally at one end and the LED protrudes through the top. Probe leads exit the remaining end of the box. See image elsewhere on this page. Empty space in the box can be filled with a scrap of foam rubber.

Parts List:
Resistors - 1 x 4k7 (R1), 1 x 10k (R2), 2 x 100R (R3 and R5), 1 x 22k (R4) and 1 x 1k (R6).

Capacitors - 1 x 100n capacitor (C1).

Semiconductors - 1 x LM339 IC (IC1), LED, 1 x PNP transistor (TR1). The actual type of transistor is unimportant as long as it is a PNP. I used a BC558 as I had a few on hand.

Miscellaneous - buzzer, 9V battery and connector, tactile switch, small jiffy box (Jaycar HB6015) and 2 ballpoint pens or similar for the probes.

All parts except the circuit board and probe pens/pencils are available from Jaycar for around $10.00. The board will be available through the Club for Club members.
This image shows mounting of the buzzer on the end of the case. There is nothing very challenging or special about this. Just make sure that you drill the holes in such a position as to clear the internal mounting posts. Otherwise you will end up with a couple of unnecessary ventilation holes in the case.

This image shows the completed board before being mounted in the case. Unlike in this shot, the board is tipped upside down so that the LED protrudes through the top of the case. The transistor must be mounted well down on the board so that there is room for the battery.

The wires that are visible at the bottom right hand corner of the image are on the end of the 3-wire cable that go to the probe that isn't finished yet. They are, in fact, the free end of the grey cable at the left.

The board isn't glued in place. It is simply placed in position with a piece of foam rubber between it and the battery. The case lid then provides enough compression to hold everything in place.
Final design/construction of the two probes is still being done. Here you can see a Pentel Techniclick pencil that I am using for one of the probes. The image shows a complete pencil (at the bottom) and another one completely disassembled with the tactile switch to be included. The switch is pictured adjacent to the slot where the pencil's lead ejector fits.

The pencil is largely easy to disassemble. The white lead gripper, or clutch, is pulled straight out with a pair of pliers after the writing end is removed. The remaining pieces are then emptied out.

So far, I have fitted the switch (not glued in position yet) and am now working on the pin point. The pin fits through the writing end where the lead used to be and is a nice, solid fit. It only needs soldering in position and it's done.

Email: General Information: vk3cmz@marc.org.au.
Created 2007 by Kevin Crockett. Comments and suggestions may be e-mailed to