Q&A
Figure 5
+28V
LED10
78L05
+
3V
LED Chaser
.01
OUT IN
COM
0.33
Train
Trac k
25k
+
47uF
4093
4093
4093
Ferrite
Bead
4093
IPS031
4093
.0001 22k
1M
.01
Throttle
100k
50k
Sensitivity
1M
4017
Q5-9
Q9
Q8
Q7
Q6
Q5
CP1 Q4
CP0 Q3
Q2
MR Q1
Q0
1uF Rate
Garden Train Speed Control
LED1
270
especially motors.
Figure 6
Follow the Sun
Q. I’d like to incorporate a photocell to the attached 4017 LED
chaser circuit in order to turn it on during daylight and off at night. A variable
pot would be nice to control the turn-on threshold, and I would like to power
it up from a single three-volt button
battery (like a CR2032). I’ve built this
circuit under a microscope and used
tweezers to solder all of the SMD parts,
so you can see that a relay would be
prohibitive. Can this be done?
J. Smith
that goes
low when
light falls on
the photocell. This, in turn, drives a
third gate that applies power to the
4017. Because of the low voltage,
some LEDs may fail to light in this
design. Red, green, and orange should
work okay, but the voltage is too low
for amber, white, or blue LEDs.
A. The circuit you sent uses a 555
oscillator, which won’t work down
to three volts. You could use a
ZSCT155, which operates down to 0.9
volts, but it draws a hefty 100 uA at
idle. I would use a 4093 Schmitt trigger that draws a mere 0.25 uA at three
volts (Figure 6). One gate generates
an adjustable-frequency square wave
to clock the 4017 decade counter.
The photocell controls a second gate
LCD Power Line
Monitor
Q. Would it be difficult to change
the power line monitor as shown
to have a digital readout rather than
an alarm? I have measured our
power line output with a good quality Fluke meter and it always shows
129 VAC. This seems high, but I
don’t know what to do about it.
— Frank Lemon
A. A recent Chinese import has
made this an easy project. I’m
talking about the PM-128E digital
panel meter that can be jumper-wire
programmed for DC ranges from
+ 17.8V
+
~12VDC
AC Wall-
~ Wart _
78L09
T
200 mV to 500 volts and an AC
range of 200 and 500 volts. The
meter is available from Circuit
Specialists ( www.webtronics.com/
panelmeter.html), and can be used
as-is by simply jumpering the 200
VAC solder pads and plugging it into
the wall.
However, it is safer to use a small
12-volt wall-wart to isolate the AC line
and provide power for the meter.
Either an AC or DC wall-wart
will work, but using a DC output sim-plifies the circuit (Figure 7); an AC
output requires a bridge rectifier and
a 470 uF filter capacitor. Look for an
adapter in the 50 mA to 300 mA
range with a two-prong input.
Construction is straight-forward.
Calibration is done using a DMM or
your Fluke — adjust the CAL pot to
display the reading on your meter.
ALERT! The circuit uses a common
ground for the power supply and
voltage in. Don’t be lured into substituting the one-buck cheaper PM-128
— which requires an isolated power
supply — for the PM-128E.
PM128E
F
o
r
E
l
e
c
t
r
o
n
i
c
s
NUTS & VOLTS
E
v
e
r
y
t
h
i
n
g
+
470uF 47k
IN OU
COM
+9V
IN
0.1 1k GND
9V
MAILBAG
CAL
10k
120k
AC Line Monitor
Figure 7
20
Dear TJ,
Referring to “Relay Contact
Life” in the April 2005 issue: As you
know, opening a direct current
circuit with a lot of inductance can
be notoriously difficult. Forty-seven
JULY 2005
