■ FIGURE 6
Auto Cigarette Switch
goodies they used to sell. I
wonder: Are there any old
schematics left to build one?
LR8 can only supply 10 mA of current.
Which is why the external pass transistor
is needed. This transistor is configured
as an emitter follower; it has very low
voltage drop and will pass as much current as your transformer can provide.
That is, it’s not short-circuit protected as
is the LR8 chip by itself — so be careful.
Mouser Electronics (800-346-6873;
www.mouser.com) has the LR8 in stock.
QAs an owner of a ‘ 68 Mustang
for 32 years, I followed the T-Bird light series with interest.
When the high-power circuit
was presented, however, it took me but
a moment to realize that it could not be
made to work in an early Mustang. Like
most cars of that era, the lights are rigidly grounded through the metal housings
and metal sockets, and must be switched
on the high side. Also, the 1157 lamp
has two filaments sharing a common
ground, so, if floating all the grounds was
somehow accomplished, the parking
lights would no longer function due to
all the grounds being lifted by the sequencer. I hope this helps in some way.
— Tim Young
POWER ON DEMAND
QRadioShack used to sell a
gizmo which plugged into a
car cigarette lighter and had a
12-volt accessory outlet. The
electronics inside turned the outlet ON
when the engine was running, and turned
it OFF when the ignition key was turned
off. There was no connection to the ignition wiring and it turned ON a couple of
seconds after the starter cranking, to save
all power to start the car first and save the
appliance from fluctuating voltage. There
was even a manual on/off switch. It was
quite a clever design (plug and play, no
wiring) — great for a cell phone charger,
CD player, etc. But RadioShack has since
discontinued it along with many other
ANot that I can find —
but I have an idea of
how they did it.
When the engine is
not running, the battery voltage
is 12. 6 volts. Crank to start, and
the voltage drops to as low as
four volts. Once the engine is running,
the alternator engages at 14.2 volts to
replace the energy used to start the car.
And there it stays until you turn the car
off, after which the battery goes back
to its 12. 6 volts “wait” state.
What this gizmo does is monitor
the battery voltage and turn the outlet
ON when it exceeds 12. 6 volts. I would
do this using the circuit in Figure 6. The
switch consists of just three parts: transistor, resistor, and a zener diode. When
the voltage is below 13 volts — the
breakdown voltage of the 1N5243B
zener — the zener doesn’t pass current,
which means the PNP transistor is
turned OFF. After the alternator is on-line, the lighter’s voltage exceeds 13
volts and forward biases the 2N2222
transistor, which now turns ON the PNP.
The ZTX1151A transistor was
selected for its low voltage drop (
typically 230 mV) and high current (3A).
Any PNP transistor can be substituted
— so long as the collector current (IC)
equals or betters your load requirement.
What you have to look for is saturation
(VCE (SAT)) voltage. For each volt that’s
dropped across the transistor, that’s one
less volt delivered to your load.
Your explanation of LS impedance
is not quite correct. A loudspeaker is a
complex electromechanical transformer
coupling electrical power to sound
pressure level power. It has a "primary"
impedance (seen by the amplifier)
defined by cone mass, cone suspension
compliance, magnetic flux density,
voice coil length and diameter, and air
loading — to name the main ones. Even
with a voice coil of zero resistance, the
speaker would reflect a load impedance
to the amplifier and take power.
But real voice coils have resistance and it turns out that the DC
resistance is not that far off the rated
AC impedance. One final point, you
are right in saying that speaker
impedance is nominal. An eight-ohm
speaker typically exhibits an impedance varying from below 6 ohms to
30 ohms, depending on frequency.
— Roger Jones, P.Eng.
Vice President Engineering
George KELK Corporation
Toronto M3B 2T5
AAs far as I know, the Mustang
light fixture has to be modified to accept three #1157
bulbs. How do I know? I lived
down the street from Shelby’s Cobra
Mustang modification facility in Torrance,
CA in 1964. I saw those awesome 428
engines he was dropping under the
hood — and the new electronics. Hence,
my April ‘06 design. But, as you say, not
everybody is into that much rewiring. So
here’s a circuit (Figure 7) for those car
buffs who want to use grounded lamps.
If you refer back to the May ‘06
issue, you will see where I describe in
detail switching a load on the high side.
Here is a case in point. For the load to be
grounded, the B+ side has to be turned
on and off. Normally, this requires a
high-side driver with off-set electronics.
Fortunately, SCRs aren’t so fussy. A
single resistor from anode to gate is all
you need to trigger the SCR on. The SCR
doesn’t care about either the anode or
cathode relationship to ground — just the
difference between them. Which is why
I’m able to use a simple optoisolator —
an NEC PS2501. This device comes in
packages of one, two, and four isolators
(PS2501-1, PS2501-2, and PS2501-4,
respectively). This IC is also comfortable
blinking on/off in Ontario winters, and is
available from Digi-Key (800-344-4539;
As before, a 4017 is sequenced by