This is only a preview of the May 2002 issue of Silicon Chip. You can view 28 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Articles in this series:
Items relevant to "PIC-Controlled 32-LED Knightrider":
Items relevant to "The Battery Guardian":
Items relevant to "Build A Stereo Headphone Amplifier":
Items relevant to "Automatic Single-Channel Light Dimmer; Pt.2":
Purchase a printed copy of this issue for $10.00. |
Don’t get caught with a flat battery
BUILD THE
By JOHN CLARKE
BATTERY GUARDIAN
Got a big stereo system in your car? Got a fridge in your van or
4WD? Ever had the battery discharged to the point where you
couldn’t start the motor? Not a good feeling, is it? You need our
Battery Guardian. It monitors the battery voltage and will switch
off the current to your fridge or stereo (or whatever) when the
battery voltage falls below a preset level to allow you to still start
your engine.
32 Silicon Chip
www.siliconchip.com.au
E
LECTRIC FRIDGES IN VANS
and 4WDs are a great idea but if
you are not careful, they can severely discharge the battery and leave
you stranded. Maybe the battery will
end up with severe damage as well.
The same problem applies if you have
a big stereo system and you like to play
it without the motor running.
Operation on 12V is fine when the
motor is running and battery charge is
maintained but if the fridge is allowed
to run for too long when the motor is
stopped, it can flatten the battery in
a relatively short time. This is where
the Battery Guardian comes into play.
It monitors the battery voltage and
disconnects power to the fridge before
the battery becomes too flat to allow
the engine to be started again.
Note that some fridges already have
a low battery cut-out that prevents
operation if the battery voltage goes
below 10.5V. The cut-out is included
for two reasons. One is to prevent the
battery from being discharged to the
point beyond which the battery life
is reduced. The second is to prevent
the fridge motor from stalling since it
would not be able to drive the fridge
pump at such a low voltage.
However, at 10.5V, no vehicle battery could start the motor and therefore
you could easily be stranded way out
in Woop Woop.
By contrast, the SILICON CHIP Battery Guardian disconnects the power
when the battery voltage drops to
about 11.5V. At this voltage, the battery
should still have sufficient reserves
to start the engine but you can set the
cut-out voltage higher or lower to suit
your vehicle.
The SILICON CHIP Battery Guardian
has to operate without causing any significant additional current drain from
the battery. If it did have a significant
current drain, it would become part of
the problem rather than being the solution. This fact means we cannot use a
relay to control the power switching.
A suitable automotive relay would
draw some 120mA continuously when
activated so clearly we had to rule this
option out.
Instead of a relay, the Battery Guardian uses a power Mosfet and this cuts
power consumption dramatically. In
fact, the whole circuit draws an average current of less than 2.5mA.
Fig.1 shows the circuit. It uses three
www.siliconchip.com.au
MAIN FEATURES
•
•
•
•
•
•
Cuts power to load (eg, fridge)
when battery voltage drops
below a preset level.
10A rating.
Low power drain.
Chirping sound during cut-out.
Flashing LED indication during
cut-out.
Automatically reconnects power
when battery recharged.
low-cost ICs, the power Mosfet and
not much else.
Mosfet Q1 provides the switching
for the 12V rail (ie, between the 12V
IN and the 12V OUT). This rail is
fused using fuse F1 (10A), to protect
against short circuits on the 12V
output.
High side switching
One of the problems with using a
Mosfet to switch +12V rail is that its
Source and Gate electrodes cannot be
connected to the 0V side of the supply.
Instead, we are using the Mosfet as a
“high side switch” (ie, switching the
positive supply rail). This means that
the gate voltage must be referenced
to the source electrode of the Mosfet
which rises to almost the full positive
supply when the Mosfet is switched
on.
Hence, we need to generate a gate
voltage for the Mosfet which is tied
to its source electrode and isolated
from the 0V line. And Q1 needs a gate
voltage which is at least 10V above
its source in order to switch fully on.
This voltage is provided using an
oscillator circuit (IC1) which drives
transistor Q2 and a small step-up
transformer, T1, wound on a ferrite
toroid. The output of T1 is rectified
using D1 and a 0.1µF capacitor (for
filtering) to derive a signal which is
fed to Q1’s gate.
IC1 is a CMOS 7555 timer which
is connected to operate in astable
(continuous) mode. Its frequency
of oscillation is set by the .0015µF
timing capacitor on pins 6 & 2 and by
the associated series 1MΩ and 1kΩ
resistors on pin 7.
Using the timing components
shown, IC1 runs at about 1kHz, with
the charging time (.0015µF x (1MΩ +
1kΩ) x 0.693 = 1.04ms. By comparison, the discharge time is very short,
around 1µs, since the 1MΩ resistor is
not involved.
The pin 3 output of IC1 is high while
the timing capacitor is charging and
low when discharging (ie, the output
is a pulse waveform with a high duty
cycle). This pulse signal is inverted
using NAND gate IC3a and inverted
All the parts fit on a single PC
board, so the circuit is easy to
build. Note that the corners of
the PC board must be removed
to clear the corner mounting
pillars inside the case.
May 2002 33
Fig.1: the circuit uses IC1 to provide a 1kHz signal which pulses Q2 on and off.
Q2 in turn drives transformer T1, the output of which is rectified and filtered
to provide a DC voltage to turn on Mosfet Q1. REF1, VR1 & IC2a set the cutoff
voltage and provide the gating signal to IC3b.
again using IC3b (assuming that pin
6 of IC3b is high).
IC3b drives the base of transistor
Q2 via a 1kΩ resistor. As a result, Q2
switches on for about 1µs every 1ms
and pulses the primary of transformer
T1. The secondary of transformer T1
drives diode D1 and its associated
0.1µF filter capacitor and the resulting
DC voltage turns on Mosfet Q1.
Zener diode ZD1 limits the gate voltage applied to Q1 to a safe value – ie,
to no more than 15V between gate and
source (or 27V above ground).
Q1’s “on resistance” is typically
.02Ω and this means that it will dissipate about 0.5W when supplying 5A
to the load (eg, fridge or whatever).
In addition, as the Mosfet turns off,
it dissipates power as its gate voltage
34 Silicon Chip
falls. In fact, the dissipation will be
higher during this turn-off period
(about 50ms), as its “on resistance”
increases. For this reason, a heatsink
has been used to ensure that the device
runs cool.
Mosfet Q1 is switched off (to cut
the power to the load) when pin 6 of
IC3b is pulled low. This sets pin 4 of
IC3b high and so transistor Q2 turns
off (and remains off). As a result, the
470kΩ resistor between Q1’s gate and
source terminals discharges the 0.1µF
capacitor over a 47ms period and the
Mosfet switches off.
Voltage sensing
Pin 6 of IC3b is controlled by a voltage sensing circuit consisting of REF1
and comparator IC2a.
REF1 is a 2.5V voltage reference
and is supplied with current via a
10kΩ resistor from the 12V rail. Its
2.5V output is attenuated by trimpot
VR1 (which sets the cut-out voltage)
and applied to the inverting input
(pin 2) of IC2a. At the same time, the
non-inverting input (pin 3) monitors
the supply voltage via a voltage divider consisting of 47kΩ and 10kΩ
resistors.
Normally, with a fully charged
battery, the voltage on pin 3 is greater
than that on pin 2 and so pin 1 is high.
As a result, pin 6 of IC3b is also high
and so IC3b gates through the pulses
from IC1 to Q2, as described previously. The 220kΩ feedback resistor
between pins 1 & 3 of IC2a provides
a small amount of hysteresis, so that
IC2a switches cleanly at the cut-out
setting.
The resistors connected to pin 3
set the voltage at this pin to about
www.siliconchip.com.au
1/5Vcc (ie, one fifth of the supply
voltage). This means that if we want
Q1 to switch off at 11V, we have to set
VR1 so that pin 2 is at 2.2V. When pin
3 falls below this voltage (ie, as the
battery voltages falls below 11V), pin
1 of IC2a goes low and so IC3b blocks
any further pulses from IC1 and IC3a.
As a result, Q2 remains off and so Q1
also turns off and disconnects power
to the load (ie, the fridge).
At the same time, the low output at
pin 1 of IC2a pulls pin 3 down to 1.86V,
since the 220kΩ feedback resistor and
10kΩ resistor are now effectively in
parallel. The voltage on pin 3 is now
effectively 0.169 x Vcc, which means
that the battery voltage must now go
above 13V before the voltage at pin
3 equals the 2.2V at pin 2 and pin 1
switches high again.
Without the hysteresis provided
by the 2.2MΩ feedback resistor, pin 1
would simply cycle rapidly between
high and low as the battery voltage
recovered each time the fridge load
was removed.
Note that REF1 has a 10µF capacitor
across it. That’s there to ensure that
REF1’s output is initially low when
power is first applied, so that pin 1 of
IC2a is high. Pin 1 of IC2a will then
go low again if the supply voltage is
below the cut-out value set by VR1
but only when REF1’s output has
settled to its correct value – ie, after
the capacitor has charged via the
10kΩ resistor.
Indicator circuitry
As well as controlling pin 6 of IC3b,
IC2a also drives the inverting input
(pin 6) of comparator IC2b. As shown,
IC2b’s non-inverting input is connected to VR1’s wiper, which means that
it is nominally at 2.2V.
When pin 1 of IC2a is high (ie, Q1
on), IC2b’s output at pin 7 is low and
so NAND gate IC3d and any following
circuitry is disabled. However, when
pin 1 of IC2a goes low (ie, to turn
Q1 off), pin 7 of IC2b goes high and
allows NAND gate oscillator IC3d to
operate.
The feedback components between
pins 11 & 12 and the associated 10µF
timing capacitor set the frequency of
the NAND gate oscillator. To understand how this works, just remember
that the output of a NAND gate only
goes low when both inputs are high.
Assume initially that Mosfet Q1 is
on. This means that pin 7 of IC2b and
www.siliconchip.com.au
thus pin 13 of IC3d are low and so pin
11 of IC3d will be held high.
Pin 12 of IC3d will also be high
during this time, since the 10µF timing capacitor will charge via the 1MΩ
feedback resistor. At the same time,
PNP transistor Q1 will be off (since
its base is held high by pin 11) and
so both LED1 and the piezo siren will
also be off.
OK, now let’s see what happens
when the Mosfet (Q1) switches off.
When that happens, pin 7 of IC2b
goes high and pin 11 of IC3d switches
low. The 10µF timing capacitor now
discharges into this low output via
D2 and a series 4.7kΩ resistor until it
reaches the logic low threshold of pin
12. When that point is reached, pin
11 of IC2b switches high again and
recharges the 10µF capacitor via the
1MΩ feedback resistor, whereupon
pin 11 switches low again.
This cycle continues while ever
pin 13 of IC3d is high, with the 10µF
timing capacitor charging via the 1MΩ
resistor and discharging via D2 and
the 4.7kΩ resistor (ie the capacitor
discharges far more quickly than it
charges). As a result, pin 11 of IC3d is
high for about 10s and low for about
10ms during each complete cycle.
Each time pin 11 pulses low, Q3
turns on and briefly flashes LED1. It
also briefly enables NAND oscillator
IC3c (by pulling pin 9 high). This
oscillator runs at around 1-2kHz (depending on the setting of VR2) and
briefly drives the piezo siren.
As a result the piezo siren briefly
“chirps” and the LED flashes once
every 10 seconds to let you know that
the power to the load (fridge) is “off”.
Unlike NAND gate oscillator IC3d,
IC3c runs with an even duty cycle,
since its .01µF timing capacitor both
charges and discharges via VR2 and its
series 10kΩ resistor. In practice, VR2
is adjusted so that the frequency is the
optimum for the piezo to produce its
loudest output.
Power for IC1, IC2 and IC3 is derived
from the incoming 12V supply rail via
fuse F1 and a 10Ω resistor. Further
supply decoupling is provided by several 10µF and 0.1µF capacitors, while
zener diode ZD1 protects the circuit
from voltage transients by clamping
any spike voltages to 16V.
Construction
All the parts for the Battery Guardian are mounted on a single PC board,
Parts List
1 PC board, code 05105021,
122 x 60mm
1 plastic case, 130 x 67 x 44mm
1 front panel label, 129 x 67mm
1 piezo transducer (Jaycar AB3440 or equivalent.)
1 4-way PC mount terminal strip
(Altronics P-2103)
1 mini-U heatsink, 19 x 19 x
10mm
1 ferrite toroid, 17m OD x 10mm
ID x 6mm (Jaycar LO-1230 or
equivalent.)
1 10A 3AG fuse
2 PC-mount 3AG fuse clips
1 1m length of 0.25mm enamelled copper wire
1 200mm length of 0.8mm tinned
copper wire
1 M3 x 6mm screw
1 M3 nut
2 M2.5 x 9mm screws
2 PC stakes
2 100kΩ horizontal mount
trimpots (code 104)
(VR1,VR2)
Semiconductors
1 7555 CMOS timer (IC1)
1 LM393 dual comparator (IC2)
1 4093 quad Schmitt NAND gate
(IC3)
1 STP60NE06 60A 60V N
channel Mosfet (Q1)
1 BC640 NPN transistor (Q2)
1 BC327 PNP transistor (Q3)
1 LM336-2.5 reference (REF1)
1 15V 1W zener diode (ZD1)
1 16V 1W zener diode (ZD2)
2 1N914, 1N4148 switching
diodes (D1,D2)
1 5mm high-brightness red LED
(LED1)
Capacitors
3 10µF 16VW PC electrolytic
7 0.1µF MKT polyester
1 .01µF MKT polyester
1 .0015µF MKT polyester
Resistors (0.25W, 1%)
2 1MΩ
5 10kΩ
1 470kΩ
1 4.7kΩ
1 220kΩ
1 2.2kΩ
1 100kΩ
3 1kΩ
2 47kΩ
1 10Ω
so it’s a snack to build. This board is
coded 05105021 and measures just
122 x 60mm. The completed assembly
May 2002 35
Fig.2: follow this diagram
when installing the parts
on the PC board. Take care
to ensure that transformer
T1 is correctly oriented – it
is secured to the PC board
using a couple of wire loops.
then fits neatly inside a standard plastic case measuring 130 x 67 x 44mm
(see photo).
Start by inspecting the PC board for
shorted tracks or breaks in the copper
by comparing it with the published
pattern. While you’re at it, check that
the holes are large enough for the
component leads, particularly for the
screw terminals.
Note also that the corners of the PC
board must be shaped as shown on
the PC layout diagram (Fig.2), so that
it can be fitted into the box – ie, the
corners have to be removed to clear
the integral mounting pillars. You can
remove the corners by first cutting
out a rectangular piece using a small
hacksaw and then carefully filing to
shape using a round file.
Alternatively, you can use a mini-drill fitted with a small grinding
disk (eg, a Dremel tool, or similar).
Fig.2 shows how the parts are fitted
to the PC board. Begin by installing the
three wire links plus two PC stakes to
terminate the wiring from the piezo
transducer. This done, install the resistors in the positions shown.
Table 1 shows the resistor colour
codes but we recommend that you
also check each value using a digital
multimeter as some of the colours can
be difficult to decipher.
Diodes D1 & D2 can go in next, followed by zener diodes ZD1 and ZD1.
Take care to ensure that these are all
installed the right way around and
don’t get ZD1 & ZD2 mixed up (their
voltages are different).
Now for the three ICs. These are
all soldered directly to the PC board,
again making sure that they are oriented correctly. It’s easy to identify pin
1 on each IC – it will be adjacent to a
notch or dot at one end of the body.
Next, install the capacitors, taking
care to ensure that the electrolytics
are oriented as shown. That done,
the transistors can go in but don’t get
Q2 and Q3 mixed up – Q2 must be a
BC640, while Q3 is the BC327.
The Mosfet transistor (Q1) is mount
ed horizontally on a small heatsink
and is secured using a 10mm M3 screw
and nut. This means that you have to
bend Q1’s leads down by 90° before
installing it on the board.
This is best done by first slipping
an M3 screw through the device tab,
positioning it on the board and then
gripping one of the leads with a pair of
needle-nose pliers just before it reach
es its mounting hole. The device is
then lifted clear of the board, the lead
bent at right-angles and the procedure
then repeated for the remaining two
leads.
Once all the leads have been bent,
Table 2: Capacitor Codes
Value
IEC Code EIA Code
0.1µF 104 100n
.01µF 103 10n
.0015 152 1n5
Table 1: Resistor Colour Codes
No.
2
1
1
1
2
5
1
1
3
1
36 Silicon Chip
Value
1MΩ
470kΩ
220kΩ
100kΩ
47kΩ
10kΩ
4.7kΩ
2.2kΩ
1kΩ
10Ω
4-Band Code (1%)
brown black green brown
yellow violet yellow brown
red red yellow brown
brown black yellow brown
yellow violet orange brown
brown black orange brown
yellow violet red brown
red red red brown
brown black red brown
brown black black brown
5-Band Code (1%)
brown black black yellow brown
yellow violet black orange brown
red red black orange brown
brown black black orange brown
yellow violet black red brown
brown black black red brown
yellow violet black brown brown
red red black brown brown
brown black black brown brown
brown black black gold brown
www.siliconchip.com.au
The Tiger
comes to
Australia
The BASIC, Tiny and Economy
Tigers are sold in Australia by
JED, with W98/NT software and
local single board systems.
The piezo transducer is secured to the lid of the case using two M2.5 x 10mm
screws. Before mounting it, drill a small hole directly in front of the element, to
let the sound escape.
the device can be secured to the PC
board with its heatsink and the leads
soldered. Note that it’s not necessary
to isolate the tab from the heatsink,
since the heatsink doesn’t touch any
other parts. However, because Q1’s
tab is connected to its drain terminal
(which is connected to the +12V rail),
this means that the heatsink will be at
+12V when the circuit is operating.
The PC board assembly can now
be completed (except for transformer
T1) by installing the 4-way screw terminal block, the fuse clips (make sure
these go in with the retaining flanges
towards the outside), pots VR1 & VR2
and the LED. The latter should be
mounted so that the top of its plastic
body is 30mm above the PC board.
By the way, we’ve provided two sets
of mounting holes for the righthand
fuse clip, so that you can use either a
3AG fuse or the shorter M205 type (the
position shown on Fig.2 is for a 3AG
fuse). Unless you have a good reason
to do otherwise, stick with a 3AG fuse
as these are more commonly available
from service stations (note: M205 fuse
clips are smaller).
We’ve also designed the board to
accept the two commonly available
trimpot sizes for VR1 and VR2. It’s up
to you which type you use.
Winding the transformer
The primary and secondary of
transformer T1 are wound on a ferrite
toroid as shown in Fig.3. Wind on 14
turns of 0.25mm enamelled copper
wire for the primary, in the direction
shown. Similarly, wind on 19 turns for
the secondary, in the direction shown.
Once the coils have been wound,
scrape away the enamel from the ends
of the leads and install the unit on the
PC board. Make sure that you get the
Fig.3: here are the
winding details for
transformer T1. Be
sure to wind the
turns on in the
directions shown.
Tigers are modules running true compiled multitasking BASIC in a 16/32 bit core, with typically
512K bytes of FLASH (program and data)
memory and 32/128/512 K bytes of RAM. The
Tiny Tiger has four, 10 bit analog ins, lots of
2
digital I/O, two UARTs, SPI, I C, 1-wire, RTC and
has low cost W98/NT compile, debug and
download software.
JED makes four Australian boards with up to 64
screw-terminal I/O, more UARTs & LCD/keyboard support. See JED's www site for data.
TIG505 Single Board
Computer
The TIG505 is
an Australian
SBC using the
TCN1/4 or
TCN4/4 Tiger
processor with
512K FLASH
and 128/512K RAM. It has 50 I/O lines, 2
RS232/485 ports, SPI, RTC, LCD, 4 ADC, 4 (opt.)
DAC, and DataFLASH memory expansion.
Various Xilinx FPGAs can add 3x 32bit quad shaft
encoder, X10 or counter/timer functions. See
www site for data.
$330 PC-PROM Programmer
This programmer plugs into a PC printer port and
reads, writes and edits any 28 or 32-pin PROM.
Comes with plug-pack, cable and software.
Also available is a multi-PROM UV eraser with
timer, and a 32/32 PLCC converter.
JED Microprocessors Pty Ltd
173 Boronia Rd, Boronia, Victoria, 3155
Ph. 03 9762 3588, Fax 03 9762 5499
www.jedmicro.com.au
www.siliconchip.com.au
May 2002 37
positions, then removing the board
and drilling the four holes to accept
the power cables.
Once all the holes have been drilled,
clip the board back into the case and
mount the piezo transducer on the
lid using two M2.5 x 10mm screws.
The transducer’s leads can then be
soldered to the two PC stakes on the
PC board.
Finally, install the fuse and you’re
ready for the smoke test.
Testing
Fig.4: this full-size artwork can be used as a drilling template for the front
panel.
Fig5: this is the full-size etching pattern for the PC board. Check your board
carefully before installing any of the parts.
windings the right way around, with
the primary towards IC1 and the secondary towards Q1. The toroid is then
secured using short lengths of tinned
copper wire which loop over either
side of the core and solder to the PC
board (see Fig.2).
Final assembly
There’s not much to the final assembly, apart from drilling a few holes in
the case and clipping the PC board
into position.
The first step is to affix the front
panel label to the lid. The label can
then be used as a template for drilling
the mounting holes for the LED (3mm)
and the piezo transducer (2.5mm). You
will also have to drill a small hole in
front of the piezo transducer to let the
sound out.
You also have to drill four holes in
one end of the case, in line with the
screw terminal block. This is best done
by clipping the board into the integral
slots in the case, marking out the hole
TABLE 3: SETTING THE CUTOUT VOLTAGE
Remaining Battery
Capacity
0%
Battery Cutout Voltage
(Typical)
10.5V
10%
11.0V
13.0V
2.2V
15%
11.2V
13.25V
2.24V
20%
11.5V
13.6V
2.3V
25%
11.6V
13.7V
2.32V
30%
11.7V
13.8V
2.34V
38 Silicon Chip
Voltage Required
To Reapply Power
12.4V
VR1 Setting (Voltage
At Pin 2 of IC2)
2.1V
Before applying power, check the
assembly carefully to make sure that
all parts are installed correctly. That
done, apply power from the battery
and use your multimeter to check for
+12V on pin 8 of IC1, pin 8 of IC2 and
pin 14 of IC3.
Assuming these are correct, check
the voltage between Q1’s gate and the
“+12V IN+” terminal (positive lead to
Q1’s gate). You should get a reading of
either 15V or 0V, depending on VR1’s
setting. If you get a reading of 0V, rotate
VR1 anticlockwise until the reading
jumps to 15V.
If the voltage only reaches a volt or
two when you rotate VR1, check that
you have wound T1 correctly. If you
wind one of the windings in the wrong
direction, the windings will operate
in anti-phase.
Now adjust VR1 clockwise until
you get a reading of 0V. LED1 should
now flash once every 10 seconds or so
and the piezo transducer should chirp
when the LED flashes. Adjust VR2 for
the best sound from the transducer.
Setting the cut-out voltage
Assuming it’s all working correctly,
VR1 can now be used to set the cut-out
voltage (ie, the battery voltage at which
the fridge is disconnected).
Table 3 shows how to set VR1 for
various cut-out voltages from 10.5V
to 11.7V. It also indicates the remaining battery capacity for each of these
voltages but note that these are typical
figures only and are not precise.
Basically, it’s just a matter of selecting the desired cut-out voltage
and adjusting VR1 to get the correct
reading on the wiper. So, if you want
to set the cut-out voltage to 11.5V,
for example, adjust VR1 for 2.3V on
its wiper (ie, 2.3V between the wiper
and ground).
Generally, a cut-out voltage of about
11.5V or 11.6V is the way to go, since
www.siliconchip.com.au
Scope 1: the top trace shows the low-going output from
pin 4 of IC3b (it is low for 1.47µs), while the middle trace
shows the collector of Q2 which is pulled to the 12V
supply when switched on via a low-going signal from
IC3b. When IC3b goes high, the collector goes below
ground due to the back EMF produced by the primary of
T1. The lower trace is the voltage on Q1’s gate which is
about 26V above ground (14V above the 12V supply rail).
Scope 2: this scope shot shows Q1’s gate rise time
following the first low-going signal from NAND gate
IC3b. The top trace shows IC3b’s output at pin 4, while
the lower trace shows Q1’s gate voltage. Notice how
the gate voltage reaches 20V (8V above 12V) the instant
IC3b’s output goes low and high again. The full gate
voltage is reached after about four pulses from IC3b – a
period of around 4ms.
Scope 3: this shot shows how the gate voltage (bottom
trace) falls when pin 1 of IC2a switches low (middle
trace). As shown, the gate voltage on Q1 falls slowly (via
the associated 470kΩ resistor) over a period of about
100ms.
Scope 4: this expanded scope shot shows the outputs from
IC3d (top trace) and NAND oscillator IC3c (bottom trace).
The output from IC3d is 30ms wide and drives LED1,
while IC3c drives the piezo transducer. Its frequency here
is 746Hz (as set by VR2).
this leaves about 20% battery capacity
in reserve for starting the car’s engine.
However, you can set the cut-out voltage higher or lower than this to suit
your own particular requirements.
Installation
The Battery Guardian simply
connects in-line between your car’s
cigarette lighter socket and the fridge
(or load). Your fridge will already be
fitted with a cigarette lighter plug and
this can be removed and transferred
across to the Fridge Cutout’s input
power leads. The fridge itself is then
www.siliconchip.com.au
connected to the top two terminals of
the screw terminal block.
Be sure to use automotive power
cable for all supply connections to and
from the Battery Guardian.
Do not connect the Battery Guardian directly to the battery. If you don’t
wish to use the cigarette light socket,
the +12V supply should be taken from
a fused (but unswitched) terminal on
the fusebox.
Note that if you wish to use the
Battery Guardian with a large car
stereo system, you cannot power all
the amplifiers via the circuit because
they are likely to draw more than 10A,
which would exceed the fuse rating.
Instead, the Battery Guardian would
be connected in line with the supply
to the head-end unit; ie, the CD/tape/
tuner unit. That way, if the battery
drops below the threshold, the headend unit will be cut off and so the
current drain from the amplifiers in
the system will drop to a low value.
Finally, the Battery Guardian could
also be used to protect the batteries
in a 12V lighting system, with the
overall current limit again set by the
10A fuse.
SC
May 2002 39
|