6 Silicon chip Australia’s electronics magazine siliconchip.com.au
Also, I want my UHF radio to turn
off after a delay, when the ignition
switch is turned off. Nicholas’ circuit
is good for this – again, omit vibration
switch S1. Run an ignition-switched
power line (via a diode) to the bottom
of S2. The load is the UHF radio.
Now the radio receives power when-
ever the ignition is on and stays on
for a while after ignition is turned off.
Further, pressing S2 powers the radio
for a while (or extends the time) even
with the ignition off.
I made these circuits using a
CD4093B Schmitt Trigger device, but
Nicholas’ circuit would give a simi-
lar result in a more compact package.
Peter Manins,
Black Rock, Vic.
USB Port Protector gotcha
Thank you for the excellent kit and
well-written constructional article on
the USB Port Protector from May 2018
(siliconchip.com.au/Article/11065). I
built up the kit, tested it according to
the instructions then plugged in a USB
drive. It didn’t work.
After some disappointment, I fig-
ured the fault must be on the data lines
(quite a simple bit of the circuit) and
started testing with the diode test on
my multimeter. I found a dead short
across TVS2 both ways. I decided to
remove it and test it again. It worked
perfectly once TVS2 was removed.
I tested the remaining circuit and it
was fine.
Looking closely, I noticed that the
anode extends all the way under TVS
and I figured I had installed the item
just like all the other surface mount
components assuming the middle of
the device is insulated. In fact, TVS
must be placed with a gap between the
device and the landing for the cathode
with the little lead spanning the gap.
Once I had re-installed it this way,
there was no problem. I usually work
with through-hole circuitry, so it was
challenging and informative to create
this little device.
Grant Muir,
Christchurch, NZ.
Nicholas responds: you are right; I
hadn’t noticed that the anode runs un-
der the device body. Since I provided
‘normal’ pads for an SMD diode, you
are right that it will have to be placed
carefully to avoid shorting them out.
This device’s construction is similar
to an SMD transistor (eg, SOT-223)
that uses a large tab to transfer heat
into the board, but with one lead in-
stead of three.Dual-gate Mosfets can be used for
crystal sets
I was somewhat surprised that you
published a crystal set design in the
Circuit Notebook section of the Febru-
ary 2019 issue (page 44; siliconchip.
com.au/Article/11408).
I have also experimented with a du-
al-gate Mosfet, the 3SK143. These are
also known in the online radio com-
munity as a “3DQ” device, although
that is actually a batch code. You can
order them online in packs of five, at
a reasonable cost, but they can takeup to two weeks to get here from Asia.
These devices have a high input
impedance and a rather low output
impedance (500-800W) which means
that you can drive an efficient pair
of earbuds without a matching trans-
former.
This makes the set compact and also
lowers the cost as these transformers
can be quite hefty and expensive. I’ve
even built a few into small Tic Tac box-
es and the odd matchbox.
These 3DQ Mosfets are small sur-
face-mount devices and are tricky to
solder. The best way I found is to use
a standard piece of ‘doughnut board’,
which has no tracks on its copper
side, only solder lands. The spacing
between each land is near perfect for
these devices, and you can use some
solid core jumper wire to extend your
connections to other parts of the set.
The device acts as a synchronous
detector, with both gates wired to the
top of the antenna coil. The output for
the earbuds is taken from the drain.
The source is usually connected to a
separate coil on the ferrite rod which
then connects to the bottom of the an-
tenna coil. This is necessary for output
impedance matching. This extra coil
needs to be movable along the rod for
best adjustment.
You can find suitable circuits by
searching online for “3dq circuit”.
The sets I’ve built perform really well,
bringing in all 12 local AM stations in
Brisbane, with only a 10-metre length
of multi-stranded CAT4 data cable as
the antenna and no ground wire.