shown in the photo on page 48.
Everything was installed in the chassis
shown in the Parts List. It was a tight
fit, and I might opt for the next larger
size in this series were I to construct
another one. I did not detail a power
supply in the schematic, since most
constructors use what they have on
hand from their junk box.
The properly bypassed, 5V regulator (7805) shown in the print can be
driven from any DC source of 7. 5-12
VDC. I show the power supply components that I used in the photo on
page 48. This was a small 120/7.5 VAC
transformer, full-wave bridged into a
1,000 μF ‘lytic to provide an 8V source
for the 7805. Power requirements are 5
VDC at 160 mA with all digits lit to 8.
To conserve space, you could use
a wallwart type of transformer. Or, if
you prefer battery operation, use four
AA cells in series with an IN4001
diode. This will give a supply voltage
of about 5. 5 VDC; no 7805 used here.
Also, for battery operation, you
can replace RP1 with a 330 Ω resistor
and RP2 with 2,000 Ω. This will cut
the current consumption in half
( 50 mA average) and still give a
decent readout. This sort of readout-brightness/current-draw tradeoff
comes down to preference, and is
thus best determined by the user.
One last item about construction —
as mentioned earlier in this text, you
can simplify this project (with some loss
of overall accuracy) by eliminating U11,
U12, RP2, display No. 2, Q2, and Q3.
You will end up with a four-digit counter that has no decimal point. The clock
input U11A P12 then goes directly to
U13 P12. Change the 10 MHz XO to 1
MHz XO, which is shown in the Parts
List, and eliminate all front panel LEDs.
The ranges will now be:
Totalize to 9,999 counts
This will require a little more care in
use. Without an overflow light, you may
have to work down from a slower range
to eliminate ambiguity. This modification will also reduce current consumption for battery use. You will also need
to change the clock output connections
to accommodate these ranges:
• Microseconds — U8 P1 (same)
• Milliseconds — U9 P7
• Seconds — U10 P9
• Totalize — Same
In addition to being a very useful
piece of test equipment, this is an
interesting and educational project,
since it involves many types of logic
applications — display circuitry, high-speed sequential timing, etc.
Be sure to ground all unused IC
inputs. Take your time wiring it, and
double-check your connections (the
most common source of errors) as
you proceed, and you should have
no problems. NV