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Circuits
of the Week - 2008
-- July to Dec 2008
Last Updated on:
06/27/2009 02:55 PM
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| Dec 28, 2008 |
Remote LED Indicator Light --
NEW
There are times when you would like to transmit a signal from one LED indicator light to
second LED at another location. The circuit below works well for this application.
It takes advantage of the fact that the internal infrared LED inside an opto-isolator has a
lower voltage drop than the visible LED being tapped into. Using a darlington type opto-isolator
also means very little current needs to be diverted to the isolator..... |
| Dec 21, 2008 |
Ultra Low Power LED Flasher
The efficiency of some newer LEDs is amazing.
Some of the latest green LEDs can launch blinding light with just one milliamp of current.
I take advantage of one of these newer devices in the circuit below. The flashing circuit
uses a classic multivibrator oscillator, made from a tiny National Semiconductor’s LMC7215 low
power voltage comparator. The circuit produces a short 10ms pulse every two seconds,
drawing power from a 3v supply. I suggest using a surface mounted green LED from
Kingbright..... |
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| Dec 14, 2008 |
DC
CURRENT INDICATOR #4 December 15, 2008
The circuit below uses some common components to turn on an LED whenever DC current above a
certain level is detected. The circuit uses a very popular LM393 dual voltage comparator
from National Semiconductor and a common 1N4148 signal diode. The diode acts as a crude
0.7v voltage reference. Only one of the comparators inside the 8 pin package is used.
A pair of resistors across the diode forms a voltage divider, which produce a reference
voltage of about 0.015v. .... |
| Dec 7, 2008 |
High Intensity Line Powered LED Flasher
There are times when you would like to generate an
intensely bright flash of light using electrical power from the AC line. The circuit
below can drive a one to three watt power LED with about 350ma of peak current. Since
the circuit drives the LED with a constant current, any LED color can be used. I have
also shown how you could use an array of 7 LEDs, wired in parallel. This array may not
produce as much light as the single power LED but the array would be cheaper to build. .... |
| Nov 30, 2008 |
Reduced Power Sequential LED Flasher
The 74HC4017 is a neat decade counter which
has 10 decoded outputs. When supplied with a low frequency clock and with a LED attached
to each of the outputs, an interesting ten step sequential flasher effect can be produced.
The LEDs can be configured in a wide variety of patterns of loops, lines or arrows. But,
these circuits can often be power hogs. To reduce the average current needed to produce
the same effect, you can strobe the individual LEDs with a short 10mS pulse.
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| Nov 23, 2008 |
Circuit Forms Ideal Diode Function
This circuit uses a low power op amp and a p-channel FET to form a diode function with a very
low 0.05 volt voltage drop. With the selected FET, the circuit can handle up to 2 amps
of current. Higher currents are possible with a FET with a lower channel resistance. The
total current drawn by the circuit during operation is a low 20 microamps. The maximum
voltage is 15v. |
| Nov 16, 2008 |
White LED Night Light
As many of you know, I
have a pet peeve with poorly made LED night lights. Often, the light from the LED
quickly fades, so within months, the light is useless. I have posted several versions of
modified night lights using higher quality components. This circuit is yet another
version, which produces much more light than those other designs. The circuit brings together
two high power white LEDs made by Cree with a compact AC to DC power supply from Bias
Power..... |
| Nov 9, 2008 |
Miniature Pushbutton On/Off Switch
In many battery powered devices, you would like to turn the circuit
on and off by pressing a small pushbutton switch. One press turns the device on and
second press turns it back off. The flip/flop circuit below uses a minimum number of
parts to perform this function. It operates from DC supply voltage which can range from
2v to 6v. It draws very little current and takes up little space. Yet, it can
control up to one amp of current. The circuit uses a single flip/flop device made by
Texas Instruments. ... |
| Nov 2, 2008 |
Wireless LED Driver
The circuit solves this problem by sending
power to the LED through the plastic, using a magnetic coupling technique. The circuit below
can route power through plastic enclosures as thick as ¼ inch. The circuit will not work
through metal boxes..... |
| Oct 26, 2008 |
Power Reminder Beeper Circuit
This hobby electronic circuit produces a short beep once each minute. It is powered directly
off the 120vac power line to remind you that a device connected to the circuit is turned on.
It is simple enough to be packaged inside a small plastic box. It might be ideal for
computers, printers or some test equipment that shouldn't be left on all night. |
| Oct 19, 2008 |
White LED Life Tester
There are lots of white LEDs for sale these days. Many eBay and electronic component
surplus sales claim to sell some very bright devices. Over the years I have purchased
some of them to experiment with. I also have purchased some products, which contain
white LEDs. A major disappointment for me was some LED night lights I bought from
Costco. In just three months of operation, the night lights I purchased were too dim to
be of any use. What I have discovered is that many of the LEDs |
| Oct 12, 2008 |
3v Low Battery Voltage Flasher
Many battery powered devices use two AA alkaline cells. Often you will not know when it
is time to replace the batteries until the device powered by them actually stops operating.
The hobby circuit below can be connected to a 3v battery, to give you some warning when the
battery is nearing its end of life. It will flash a LED when the battery voltage drops
to about 2.4 volts. The electronic circuit draws only 1ua of current in standby mode and
jumps to only 20ua when flashing, so it can safely... |
| Oct 5, 2008 |
Very Low Voltage Oscillator
OK, you may not ever need an oscillator to function using a power supply voltage below one
volt, but if you ever do need one, here is one way to do it. Getting any electronic circuit
to operate below one volt is a real challenge. Typical silicon bipolar transistors don’t
work below 0.7 volts. Some old germanium transistors do work down at low voltages but
those are hard to find and are usually offered only in large packages.... |
| Sept 28, 2008 |
Watt-Hour Meter Monitor
This is a simple and safe way to monitor the power and energy use of a typical home. A
battery powered device is attached to the top of a typical spinning wheel type watt-hour
meter. The device contains a visible red LED and a PIN photo diode. Light pulses
are bounced off of the watt-hour meter’s spinning wheel. The circuit detects the black
mark on the wheel and produces a clean TTL level logic signal,.... |
| Sept 21, 2008 |
Solar Powered Lithium Ion Battery
Charger
The circuit below feeds a controlled current and voltage to a 3.6v lithium ion battery. The
current is limited to 300ma and the voltage is limited to 4.2 volts. The circuit uses a
LTC1734 IC from Linear Technology. No diode is needed between the circuit and a 6 volt solar
panel. ... |
| Sept 13, 2008 |
Isolated Power Supply for Digital Panel Meters
-- September 13, 2008
Many inexpensive digital panel
meters, such as the one shown below, are designed to operate from a 9v battery. However,
the nature of the panel meter internal design often requires that the DC supply used to power
the device be isolated from the ground terminal of the panel meter’s input. This often
makes it difficult to use with a common connection between a DC power supply ground and a
sensor’s circuit ground. But, you can generate this floating voltage fairly easily using
the simple circuit below. |
| Sept 7, 2008 |
Small Battery Capacity Tester
The only truly accurate way to measure the
current capacity of a battery is by connecting the battery to a constant current load and
measure how long the battery can sustain that current before its voltage drops below a
recommended “cutoff” point. The circuit below is designed to do this type of test..... |
| Aug 31, 2008 |
Battery Powered Plus-Minus 15v Supply
Many classic operational
amplifier circuits call for a split positive and negative 15 volt supply. The circuit
below makes the job of generating such a supply much easier by using an off-the-shelf DC to DC
converter, powered from a cheap 6v battery made from four AA cells. The converter, part
number VESD1-S5-D15-SIP, is available from Digikey, their catalog number 102-1410, for about
$6.00. To insure clean voltages, I follow the output of the converter with two voltage
regulators, also available from Digikey. The result is a very nice supply rated at 30ma
from each supply voltage. This should be plenty of current for most circuits. |
| Aug 24, 2008 |
Poor Man's Timer
Often during testing of certain equipment and components, you would like to keep track of the
elapsed time in hours, minutes and seconds. There are some nice commercial digital timers and
counters available for just this sort of application. However, although it may not be as
accurate, there is a cheaper way to go. I’m sure many of you have some old wrist watches
or battery powered dial faced alarm clocks collecting dust in some drawer.
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| Aug 17, 2008 |
Poor Man’s Digital Counter Using Pedometer
There are many occasions when you
may want to count something electronically. Perhaps it is car traffic on a street or
items moving down an assembly line. It might be the number of times a machine is
activated or maybe you want to count the number of people entering doorway. Commercial
counting modules do exist but if you want to use something cheaper, you can modify a pedometer
to do much the same thing.... |
| Aug 10, 2008 |
High Power LED Flasher
Power LEDs are becoming more popular these days. The DC input power to these devices
ranges from 1 watt to 5 watts. Normal LEDs only receive about 0.05 watts. At this
higher power, these devices can emit a lot of light. You can buy them in just about any
color in the rainbow but white seems to be the most popular. One application for these
devices is a flashing light. If the flash duration and flash rate is kept low, the
average current can be low enough that even a small battery can last quite a while..... |
| Aug 3, 2008 |
Reduced Power Relay Driver
Relays can handle a lot of power. However, for certain power sensitive designs you would
like to reduce the power needed to hold a relay closed. The circuit below performs such
a task. It uses a single CD4093 quad NAND gate. When the “on” logic input signal
is detected, the relay is first pulsed on for about 500ms. This is sufficient time to
insure the relay is fully closed. After that initial pulse the relay is then driven with
a square wave signal, whose duty cycle can be adjusted. The signal duty cycle can be
adjusted from about 10% to 90%. In most cases a 50% duty cycle will hold the relay
closed. This reduces the average DC current required by the same factor, which means a
4:1 reduction in power. The circuit can operate over a wide 3v to 15v range. |
| July 27, 2008 |
12v Light/Dark Switch
NEW
Often, for certain low voltage lighting systems; you would like to turn off the lights during
the bright light of the day. Most commercial day/night switches are designed for AC
lighting. The circuit below was designed for a 12v DC system. But, it could be
modified for other voltage as well. It uses an inexpensive phototransistor as the light
detector. An n-channel FET is used to switch power to the lights. A transistor
circuit is included to provide some hysteresis. This keeps the circuit from fluttering
the light during the transition from day to night and night to day. It is recommended
that a plastic tube be placed over the transistor to prevent it from being illuminated by the
lights it is controlling. By selecting the appropriate power FET, the circuit could
control over 100 watts worth of 12v lighting. (July 27, 2008) |
| July 20, 2008 |
Medium Power 12v Brush Motor Speed Controller
In many
applications, you would like to hold the speed of a motor constant, even as variations in the
power supply voltage or mechanical load try to change its speed. In other applications,
the average current to the motor needs to be limited, so the initial in-rush current when
starting the motor is not too high.... (July 20, 2008) |
| July 13, 2008 |
Computer Controlled 100ma Current Source
Often in industrial control systems a constant
current source is needed, which is controlled by a computer and referenced to circuit ground.
The circuit below converts a zero to 5v signal from a computer’s analog output into a current,
with a full scale of 100ma. The circuit shown requires a 9v DC supply but any voltage from 9v
to 12v will work. |
| July 6, 2008 |
Precision Low Capacitance Meter
This circuit was originally designed to measure the volume of the fluid inside a 10cc
syringe. It used two copper foil strips attached to the outside barrel of the syringe.
The fluid between the two copper strips increases the capacitance. As the fluid volume
decreased, the capacitance also decreased. With the circuit shown below, it is possible
to calibrate the circuit, so the voltage produced is proportional to the fluid volume inside
the syringe. With the values shown, the circuit produces a 5v DC output,... |
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