64 December 2015
READER - TO - READER TECHFORUM
All questions AND answers are submitted by Nuts &
Volts readers and are intended to promote the exchange
of ideas and provide assistance for solving technical
problems. All submissions are subject to editing and will
be published on a space available basis if deemed suitable
by the publisher. Answers are submitted by readers and
NO GUARANTEES WHATSOEVER are made by the
publisher. The implementation of any answer printed in
this column may require varying degrees of technical
experience and should only be attempted by qualified
Always use common sense and good judgment!
A router and modem at a
remote location periodically lock up,
requiring a reset by unplugging to
restart. Is there a simple circuit I could
use to drop a relay out for about 30
seconds, every 24 hours? The relay
contacts would be wired to drop out
the power circuit to the devices. That
way, when it does lock up, at least it
would be reset again within a day.
#12151 John C
I’m looking for a schematic for an
electronic brake for a one phase 120
VAC motor. I had a commercial one,
but it failed. I discovered it was potted
with no way to tell what was inside.
#12152 George Gray
LiPo Smart Charger
I’m looking for a smart charger
for LiPo cells that can run from a
solar panel. I’ve heard that the
combination is incompatible because
of fluctuations in output from the
solar panel. Is this true? If so, is a
workaround a larger panel?
#12153 Michael Gage
[#4153 - April 2015]
Over-Current for PWM Circuit
I have a Marlin P. Jones DC motor
speed controller (Part 31566MD, 6-24
volts, 20 amps max). I need to add an
over-current circuit to it.
I’ve found the 31566 schematic.
The simplest solution that I’ve used in
similar circuits is to use a ZXCT1009;
this is a three-terminal current sensor.
You put a 100 mR shunt in the
positive line, attach the ZXCT1009
across it, and a current proportional
to the motor current (about 1/1000
actually) flows out of the third pin to
ground. Simply attach a resistor, say
1K ohm to ground, and you can read
the motor current off this.
The most important advantage
of this chip is the sense resistor can
be grounded at the Arduino, so you
won’t be measuring any errors due
to voltage drop in the ground wiring.
The datasheet shows how to use it,
and provides calculations on how to
use PCB traces as current shunts.
Salamander Bay, Australia
[#6151 - June 2015]
Transistor as an STDT Switch
In a recent issue of NV, Roger
Secura wrote the article “How to Use
a Transistor as an SPST Switch.” My
question is how can an STDT switch
be made using transistors, FETs, or
other non-mechanical components? If
it is possible, please post a circuit.
It’s pretty simple. You just need
another switch that’s connected
to the output of the first switch.
These switches are commonly called
inverters, meaning that the output is
inverted from the input. This means
that for a high level input, the output
is a low level, and vice versa. So, all
we need to create a bi-level output
from a single input is a second
the first inverter. Bipolar transistors
and FETs act pretty much the same
way; the difference being that bipolar
transistors are current-driven, whereas
FETs are voltage-driven.
If you need more theory on
transistor circuits, I refer you to a
series of articles by Ray Marston in
Nuts&Volts titled “Bipolar Transistor
Cookbook.” This is an eight-part
series that N&V has graciously
made available online at www.
Be sure to have plenty of reading
time available, because it contains a
train-load of information. Figure 1 is a
circuit depicting how to connect two
transistors as an SPDT switch.
Hazel Green, AL
[#10153 - October 2015]
Gold vs. Tin
I need to buy IC sockets in bulk
for an upcoming project, and I’m
debating whether the added cost is
worth it to upgrade from tin to gold
contacts. Am I paying for longevity or
simply slightly lower contact resistance
when I spend double or triple for a
gold IC socket?
n FIGURE 1