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Circuits
of the Week - 2009, Jan to June
Last Updated on:
11/23/2009 04:57 AM
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| 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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