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Circuits
of the Week - 2009, Jan to Dec
Last Updated on:
11/17/2009 02:51 AM
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| Nov 15, 2009 |
Cut Phone Line Alarm --
Burglars, rapists and other criminals will often cut a phone
line before they break into a home. The cut phone line will prevent many alarm systems
from alerting the police. The battery powered circuit below will sound an alarm when the
unit no longer detects the voltage present on an active phone line..... |
| Nov 8, 2009 |
Supercapacitor Voltage Limiting Circuit
Supercapacitors are working their way into more and more applications where electrical energy
needs to be stored. These robust devices can be charged and discharged 1000s of times
and will typically outlast a battery. Many supercap manufacturers claim a life span of
10 years or more. A supercapacitor is often chosen to supply power to low current load
for many hours at a time, recharged by a solar panel. |
| Nov 1, 2009 |
Lead-acid Battery Low Voltage Indicator
Lead-acid batteries should not be discharged below a certain voltage. Discharging a cell
below about 1.8v and can often damage it, which will result in a lower amp-hour capacity.
Large battery banks are critical in off-grid energy storage systems. One bad cell in a
series wired string will weaken the whole bank. The circuits below are designed to be
connected across either a single 2 volt cell or a 6v three cell battery bank. It will
alert the user of a weak cell by turning on a red LED when the cell voltages drop below 1.8v.
The circuit takes advantage of a low voltage LMP2231 op amp from Nation Semiconductor and a
LM385 voltage reference in the first circuit. In that circuit the red LED must have a forward
voltage of about 1.6v at 5ma, in order to operate properly. In the second three cell
bank circuit, a TLV3701 voltage comparator is used. The circuits are simple enough that
they could be placed on a postage stamp size circuit board. |
| Oct 25, 2009 |
White LED is Powered by One NiMH Cell
There are a lot of white LED driver circuits floating around. A popular circuit
called the “Joule Thief” can drive one white LED from a single 1.2v or 1.5v battery cell.
Most of these circuits use one or two transistors to form a voltage boost circuit. Yes,
they those circuit do drive a LED but they are not very efficient and they don’t do a good job
of controlling the current to the LED. The circuit below uses a tiny LT1932 IC made by
Linear Technology. This IC can be configured for a wide variety of LED driver needs.
I have shown one simple application. The circuit takes power from one 1.2v NiMH
rechargeable battery or a 1.5v disposable battery and drives a single white LED with 15ma of
current. With an efficiency of about 70%, the circuit should run for about 40 hours, if
a quality 2500ma-hour battery is used. |
| Oct 18, 2009 |
Model Airplane Wing Tip Flasher
The circuit below is designed to flash two white LEDs mounted at the wing tips of a model
airplane. The flasher unit is powered by a 4.5v supply made of three AA or AAA 1.5v
alkaline battery cells. The average current is about 2ma so a set of batteries should
last about 1000 hours of flashing. The circuit uses one hex Schmitt trigger inverter to
perform all the timing functions. A single n-channel FET drives the two LEDs, each with a peak
current of about 20ma. The circuit flashes the two LEDs in a pattern typical of
commercial airplanes. |
| Oct 11, 2009 |
Star Trek Doorbell
In the Star Trek “Next Generation” TV series, the doorbell outside the private quarters of a
crew member makes a particular “beep-boop” sound. The 3v battery powered circuit below
tries to simulate this sound. The circuit uses one 74HCT74 dual D flip/flop IC, wired as two
one-shot circuits. Both are designed to produce a pulse about one half second long.
The first pulse turns on a 555 timer to form the beep sound. The second flip/flop one
shot .... |
| Sept 27, 2009 |
Pot Controlled Variable LED Intensity Circuit
The circuit below uses power from four rechargeable AA NiMH cells to drive 3 white LEDs.
A potentiometer varies the duty cycle of a pulse width modulator circuit to vary the intensity
of the LEDs from 0% to 100%. The beauty of the circuit is that when the pot is all the
way toward 0%, very little power is drawn from the battery. Likewise, when the pot
slider is moved toward the 100% end, full power is fed to the LEDs. In effect, the pot
becomes the on/off switch..... |
| Aug 30, 2009 |
Do It Yourself Portable Cell Phone Charger
It seems that as cell phones become more sophisticated, their operating times become shorter
and shorter. If you are on the go all the time and have difficultly finding AC or DC
outlets to charge your phone, you might consider this simple scheme.... |
| Aug 23, 2009 |
12v Battery Load Cutout Circuit
Lead-acid batteries should not be discharged below a certain point if they are to last.
Also, you don’t want certain 12v powered devices, which might be plugged into a car’s
cigarette lighter outlet, to drain the car battery down to a point where the car’s engine will
not start. To prevent battery damage or a dead battery, the circuit below disconnects a
load from the battery when the voltage dips below a fixed but adjustable voltage and will not
reconnect the load until a reset button is pressed..... |
| Aug 16, 2009 |
Very Loud 3v Powered Beeper
Getting a high sound intensity from a piezoelectric type beeper is not easy when the available
DC supply is only 3v. The circuit below is not only efficient but produces a very
intense sound. The circuit combines a voltage boost section with a resonant feedback
network. The voltage applied to the piezoelectric wafer is about 40 volts peak to peak.
The result is a circuit that generates an attention getting sound without drawing a lot of
current. |
| Aug 8, 2009 |
24vac Powered Isolated +5v Supply
24vac is a very popular power supply in many industrial systems. It is often used in
systems that involve water. It is also popular with many inside and outside lighting
systems. The lower voltage means the danger of human electrocution is much less.
But, many DC powered sensors and control circuits associated with 24vac systems need to be
fully isolated from the 24vac supply. The circuit below accomplishes this feat.... |
| July 26, 2009 |
12v to 19v Laptop Power Supply (Concept)
I have a big screen HP laptop computer. The
large brick size AC to DC power supply is rated at 19.2v and 9.5 amps, which is about 180
watts. I’d like to be able to use the thing in my car. I looked into a 12v car adapter
for it but never really found one that I liked. Most were underpowered. The other option was
to use the existing AC to DC adapter and run it off power from a 12v to 120vac inverter.... |
| July 19, 2009 |
Miniature 5v Line Powered Isolated Supply
Often a circuit requires a 5v DC supply to power some logic circuits. The conventional
method is to use an AC wall adapter. But, many systems, which bring AC power onto a
circuit board, need a small AC to DC power supply right on the circuit board. The
circuit below provides such a supply. It uses a classic series capacitor charge pump
which acts as a current limiting device. .... |
| July 5, 2009 |
Water Flowing in Pipe Indicator -- July 6,
2009
The vibrations associated with water flowing through a pipe are picked up by an inexpensive
piezoelectric wafer. The signal from the wafer is first boosted by a micropower
transistor amplifier and then fed to an ultra low power voltage comparator. When
the vibration signal has sufficient amplitude, a FET transistor switch is activated.
Drawing only 6uA, the whole circuit is powered by a lithium coin battery, which should power
the circuit for many years.... |
| June 21, 2009 |
Door Knock Beeper
In some apartment buildings and homes, not equipped
with a door bell, it may be hard to hear someone knocking on the front door. The circuit
below provides a means to activate a loud beeping sound inside, whenever someone knocks on the
door from the outside. The circuit is powered by a 9v battery, which should provide
years of service. An on/off switch allows the device to be turned off if desired...
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| June 14, 2009 |
Water Seepage Alarm
This simple circuit sounds a beeper when its electrodes detect water. It is powered by a
single 1.5v N cell. A small 1.5v button battery will also work. |
| June 7, 2009 |
SELF POWERED AC CURRENT INDICATOR
Often there is a need to monitor when AC current flows into a load. The circuit below
works well for this application. It uses an off-the-shelf toroid type AC-1020 current
transformer from Triad, available from Mouser. A wire carrying the AC current is routed
through the hole in the center of the device. Current flowing through that wire induces
a voltage in the secondary winding of the coil.... |
| May, 2009 |
Invited visitors to
submit ideas for Circuit of the Week. |
| April 26, 2009 |
Super Bright 3v Powered LED Flasher
I have posted several kinds of LED flashers over the
years. This is yet one more flasher, designed to flash any high power LED of any color.
It hits the LED with a 25ms 250ma pulse once every 2 seconds. This works out to an
average current about 3ma. It is powered by any 3v source. I suggest two AA cells
but a single 3v lithium coin type cell will also work. With fresh AA cells, the light
should flash for about two weeks. The light flash is very intense and is perfect for any
attention getting night time application..... |
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| April 19, 2009 |
Very Low Power Astable Multivibrator
This classic circuit draws only 200 nanoamps from a 1.5v supply. |
Apr 12, 2009
Happy Easter |
Bad Light Bulb Indicator
This request came from a Discover Circuits visitor. He wanted to turn on a LED indicator
light whenever a circuit detected a burned out tail/brake light on a car or truck. The
circuit shown below uses a transistor switch, to detect current flowing through a low value
resistor. If current is flowing to a light bulb, the circuit shunts current around a LED
light, keeping the LED turned off. If there is no current detected, the LED turns on.
The circuit should operate from 6v to 24v.... |
| Apr 5, 2009 |
Battery Eliminator
If you use a 1.5v or a 3v battery powered MP3 player for long periods of time, you may get
tired of constantly changing batteries. Some older players do not work will NiMH rechargeable
batteries so changing out batteries can get expensive. The circuit below can be wired
into the player, to replace of one or two AAA or AAA batteries. I designed the circuit
using a very common LM317 adjustable voltage regulator..... |
| Mar 29, 2009 |
AC Line Under/Over Voltage Alarm
Power lines, which deviate much beyond normal voltages can
damage expensive electronic equipment. The circuit below sounds an alarm whenever the
line voltage is higher or lower than normal. I set the alarm limits at about +-15% from
standard levels. The circuit rectifies and filters the power line signal. I set
the resistor values, so the DC voltage produced is close to 1% of the RMS value of the line.
Thus, a 120vac line would yield about 1.2v DC. That voltage is fed to a pair of voltage
comparators. The outputs of the comparators decide if the sampled voltage is above,
below or acceptable. .... |
| Mar 22, 2009 |
Pushbutton Controlled Up/Down Ramping Light
-- March 15, 2009
There are times when you want a lamp to gradually change from dark to light and from light to
dark, with each push of a pushbutton switch. The circuit below performs this control
function using a flip/flop and a classic pulse width modulation or PWM method. The
circuit uses a common flip/flop IC to control the on and off logic states. A simple pushbutton
switch toggles between the two states with each press of the button.... |
| Mar 8, 2009 |
Ultra Low
Current Oscillator February 17, 2009
Here is a challenge. Design an astable oscillator which draws only a few nanoamps of
current from a +3v supply. I gave this some thought and came up with the circuit below.
I used some pretty standard parts except for three surface mounted 1000M resistors I had on
hand. The oscillator frequency measured a low 1Hz frequency and the average current was
a very low 3 nanoamps. If I had some higher resistors values handy, I think I could have
gotten the current down below one nanoamp. |
| Mar 1, 2009 |
Chain Link Fence Alarm
February 22, 2009
Long stretches of steel fence can be monitored using the simple circuit below. An insulated
wire is woven through the links in the fence. Where the fence begins, the circuit is
connected to an earth ground and the security wire. At the end of the fence, the wire is
connected to one of the steel posts, held in the ground. The typical earth ground
resistance will be in the 1000s of ohms range, which is low enough to keep the n-channel FET
turned off in the circuit below. If the wire is cut or broken, ... |
| Feb 15, 2009 |
Classic Linear 5v Supply Using 6.3vac Transformer
A classic method for producing a regulated +5v DC supply is shown below. This
circuit consists of an iron core transformer, a bridge rectifier, a filter capacitor and a
voltage regulator. Many people are tempted to use a very popular 6.3v transformer for
this +5v supply but they will often discover that there just isn’t enough voltage from the
transformer to make the circuit work properly under all but very light load conditions.
Higher transformer voltages will work but at the expense of much more power being dissipated
in the voltage regulator.... |
| Feb 8, 2009 |
DC Current Indicator 6
NEW
The circuit below is a simple way to indicate when DC
current is flowing through a wire. The circuit is designed so it will turn on a LED
indicator light, whenever the voltage drop across a shunt resistor exceeds about 0.1 volts.
The value of the resistor can be selected based on the desired current flow. Although I show a
circuit for 12v operation, the circuit can be used over a wide range from 3v to perhaps
24v.... |
| Feb 1, 2009 |
Ideal Blocking Diode Circuit for Photovoltaic Solar
Panels
Most photovoltaic solar panels are used to charge a battery during the daytime. Nearly all
panels come equipped with a blocking diode. The diode prevents DC current from flowing
backwards from the battery bank into the panel at night. The usual blocking device of
choice is a schottky diode with a typical 0.5v voltage drop. In some large panels, which
can often source over 4 amps of current, the power loses due to this voltage drop can add up.
As an example a 4KW solar power system .... |
| Jan 11, 2009 |
Click Sound Generator
January 10, 2009 NEW
Often in computer controlled systems, you would like to generate a “click” sound whenever a
button is pressed. This provides the user with audible feedback that the pushbutton
press has been acknowledged. The two circuits below generate such a sound. A click
sound is generated each time the logic input swings from a logic low to high condition..... |
| Jan 4, 2009 |
Isolated Remote Switch Circuit
NEW
There are many applications when a remote switch’s on/off status needs to be monitored. Reed
relay sensors, toggle control switches and security alarms contacts are just a few examples.
Routing a switch directly to a computer system hundreds of feet away can often lead to erratic
operation and noise problems. If such a control switch is required, a fully isolated
circuit is strongly recommended. The circuit below provides such isolation. When
properly wired, it will have 5000 volts of isolation and can be positioned 1000 feet away..... |
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