in the picture. Specifically, jumper JP6 = VCC and the top
end of R33 = digital ground.
Figure 5 shows the N size battery holder epoxy-mounted into the top of the back cover. Note the
orientation and fit.
The Circuit Board
The new power supply board is only slightly larger
than the latching pushbutton. The exact size will depend
on the chosen button. Any latching plastic insulated tip
single-pole single-throw (SPST) equivalent will do. Repeat:
Don’t use a metal switch! Don’t drill the button hole until
the board is completed.
Then, cut the hole to match the fitted
board. Note the output diode (D2) on the
right and the offset diode (D1) under the
78L05 regulator (Figures 6a/6b). This board is
assembled with push-pins, but any hand-wired
method will work.
Figure 7 shows the circuit board in
position in the upper right corner of the
plastic back. I held the board in position with a dab of hot
There is sufficient room in the case for all these
components, but upon reassembly, insure that the loose
wires are positioned above the controller PCB and do not
loop down into the AC board area. Final layout for the
project is shown in Figure 8.
The switch specified in the BOM is a latching type
push-on/push-off button. The K-A-W internal accuracy fails
around 40 VAC, so the circuit should be engaged when
the incoming line voltage drops below about 50 VAC.
Since I am usually working in the 10-50 VAC range, I leave
the battery engaged for the duration of my measurements.
But do not forget to disengage the battery when done.
Since the response of the K-A-W to the battery is
almost instantaneous, one could use a
momentary button instead, and inherently
save on battery life. However, one may also
need their fingers elsewhere when making
The K-A-W draws 7 mA at 5 VDC (from
LM78L05) at the digital PCB, and 9. 7 mA
from the 12V battery. A 2-3 mA bias is
typical for the LM78L05. Note that on my K-A-W units,
the internal power supply sits at 4.95V even at 120 VAC
in, so the LM78L05 circuit at 5V is always supplying the
PCB power if engaged. Some readers may be tempted to
24 July 2015
■ FIGURE 5. Battery holder mount. ■ FIGURE 7. Installed battery and PCB.
■ FIGURE 6a. Assembled switch PCB. ■ FIGURE 6b. Underside of the assembled circuit board.
Tip: In assembling the
PCB, I found it easier
to glue in the PCB
before attaching the
connecting wires to
the rest of the K-A-W.