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Circuits of the
Week -- Jan to June 2008
Last Updated on:
06/27/2009 02:55 PM
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| June 29, 2008 |
3v Sweeping Siren Alarm
The circuit uses a LTC1799 precision frequency generator from
Linear Technology. A 74HC14 hex Schmitt trigger from Texas Instruments is also used to
perform several other functions. One section is wired as a simple 7Hz square wave
oscillator.... |
| June 22, 2008 |
Universal-3v-LED-Flasher
The circuit below will flash any LED color with fixed current pulses powered by a 3v
battery. It uses a charge pump approach, which routes a constant current pulse through the
LED, regardless of the LED voltage requirement. Using this approach, white and blue LEDs,
which normally require about 3.6v can be flashed with a 3v battery. With the component values
selected, ... |
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June 15, 2008
HAPPY
DAD'S DAY |
Fully Isolated 50/60Hz Sync Generator
This circuit will produce a single pulse at the zero voltage cross points of the power line
voltage. An opto-coupler provides a very safe 5KV isolation. |
| June 8, 2008 |
DVD Recorder + TV Camera Make Cheap Data
Logger
Sometimes recording the results of a test requires as much ingenuity as designing the product.
Consider a situation which requires the collection of many different variables over a long
period of time. Perhaps you are conducting temperature cycle tests on an electronic circuit
and you want to know what changes occur in circuit performance over a 24 hour period. You
could stay up all night and collect the data yourself.... |
| June 1, 2008 |
Wireless RFID Smart Key Detector
Many cars now use a key with an imbedded RFID microchip. The microchip is read when the
key is inserted into the car’s ignition. The keys containing a microchip often look the
same as most other keys. People duplicating car keys will often not know of the microchip.
This circuit will light a LED if it detects a key containing a 125KHz RFID chip. |
May 25, 2008
Memorial
Weekend |
Classic Plus And Minus DC Power Supply
This is a classic example of a regulated DC power supply that produces both a positive 15v and
a negative 15v from a 20vac wall adapter. |
| May 18, 2008 |
Video Signal Edge Enhancements
I designed this circuit many years ago, based on the claims that the technique would improve
the quality of standard TV images. The circuit adds information to the edges of the objects
and was reported to bring out more detail. After building and testing the circuit, I could
definitely see a difference between it and a regular TV display but I don’t think most people
would go to the trouble of installing the circuit for only a marginal improvement. Still, it
is an interesting circuit with which someone might experiment. |
May 11, 2008
HAPPY
MOM'S DAY |
Ramping LED Driver
The circuit below was designed to drive a LED with an intensity ramping mode. Two 555
timers generate two different triangle waveforms. The upper device generates a 10KHz
signal while the lower unit produces a 1Hz signal. The two signals are fed to a voltage
comparator. The result is a pulse width modulation (PWM) signal, which with the aid of
the FET, drives the LED in such a way that its average light output slowly ramps from about
zero light to maximum and then slowly dims back down. The circuit should operate over a
supply voltage ranging from 3v to 12v. You can easily vary the ramping time by changing the
value of the 1M resistor. For an interesting effect, you can place a 1N4148 diode in
parallel with the 1M resistor, with the cathode (banded end) side connected to pin 3. |
| May 4, 2008 |
Light to Frequency Converter
This circuit uses a CMOS version of the classic 555 timer, to form a light intensity to
frequency converter. A small PIN photo diode is used as the light detector. The
pulses produced are short, so in some applications you may want to stretch them or feed them
through a flip/flop to produce a square wave signal. Although the circuit shown is
designed for a 5v supply, it could operate from almost any voltage from 3v to 15v. |
| Apr 27, 2008 |
Universal Flasher Using a Latching Relay
Latching relays are nifty devices. Most contain two separate coils. When a voltage
is applied to one coil the relay latches in one state and stays in that state until voltage is
applied to the unlatch coil. Since the latching and unlatching pulses only need to last
about 25 milliseconds, it is possible to control a sizeable amount of power using little
energy. |
| Apr 20, 2008 |
Worm Harvester
When I was a kid my engineer uncle built a circuit similar to the one below. My cousin and I
used it to force earth worms out of the ground to be used for fishing. It worked like a champ.
Please be careful since there is high voltage at the probe tips. |
| Apr 13, 2008 |
Motion Shadow Alarm
The hobby circuit below works quite well in typical indoor room lighting. I would not
recommend its use in direct sunlight. Two small PIN photodiodes positioned about one
inch apart form a shadow detector. With no shadow cast on the devices, both devices
produce nearly identical current levels. The current is converted to a voltage with a
load resistor in parallel with the photodiode. One resistor is adjustable so the two
voltages can be carefully balanced under uniform lighting. When a hand or an arm is
moved over the sensors, casting a shadow, |
| Apr 5, 2008 |
N-CH And P-CH Transistors Form Push-Pull
Driver
This circuit can produce high speed output signals with fast rise and full times. The unique
change pump action allows the voltage of the upper P-ch device to range from millivolts to
hundreds of volts. The output current is only limited by the rating of the transistors. I have
used this circuit beyond 2MHz. |
| Mar 30, 2008 |
0.5v to 6v Voltage Converter
Conventional silicon transistors just can't operate at voltages
less than about 0.7v. Old germanium transistors could be used, but those are hard to find
these days and most are rather large in size. Some new n-channel MOSFET devices with very low
gate-source threshold voltage can operate at quite low voltages. I've been experimenting with
various devices and came up with one circuit (shown below), which demonstrates how to boost
the low voltage from a single solar cell to a higher voltage.... |
Mar 23, 2008
Happy Easter |
40KHz Voltage to Frequency Converter
This circuit was designed to frequency modulate a 40KHz carrier, using human voice
frequencies. A common flip/flop is used at the core of the circuit. |
| Mar 16, 2008 |
Ultra Low Power 32KHz Crystal Oscillator
I have used this circuit many times when I needed a low frequency reference, which did not
draw much power. With the components show, the current from a 3v battery is less than
1.2 microamps. |
| Mar 9, 2008 |
AC Line Powered LED Strings
This shows 4 different ways to string white LEDs that are powered by a 120vac or 240vac power
line. The circuit was designed by Ken Schultz. |
| Mar 2, 2008 |
Flashing LED Advertising Badge #1
I have seen numerous flashing light badges at trade shows and conventions. They are often
handed out as gifts to promote some business. The devices often use inefficient circuits,
which cause the battery power source to be quickly depleted. My circuit is simple but
efficient enough to provide months of continuous LED flashing. It also has a tiny push-button
switch to turn on and off the light flashing, extending battery power. |
| Feb 24, 2008 |
Long Period Computer Watch Dog Timer
This circuit uses a simple 4060 IC oscillator/timer that is reset periodically by a computer.
Should the computer fail to send a pulse, the output changes state. The time can easily be set
from seconds to hours. |
| Feb 17, 2008 |
Wig/Wag LED Flasher
This simple circuit will flash two LEDs in an alternating fashion. |
| Feb 10, 2008 |
Low
Battery Voltage Flasher
This circuit is designed to monitor two alkaline cells (3v) that from the battery often used
in portable electronic equipment. It use an inexpensive IC from Panasonic that is connected to
an efficient LED flashing circuit. When the battery voltage drops below a certain point the
circuit flashes the LED. In the off state the circuit draws only 1uA, while in the active
flashing state it draws 20uA. |
| Feb 3, 2008 |
Long Period Computer Watch Dog Timer
This circuit uses a simple 4060 IC oscillator/timer that is reset periodically by a computer.
Should the computer fail to send a pulse, the output changes state. The time can easily be set
from seconds to hours. |
| Jan 27, 2008 |
Ten Turn Pot Forms Frequency Readout
Let’s say you are designing a signal generator for a test fixture. To adjust the frequency you
would like to use a ten turn pot with a nice dial mechanism attached as a frequency display.
However, a display range of 0 to 9.99 would be impractical since there is no such frequency as
0. But since many pot dials have a display.... |
| Jan 20, 2008 |
Xenon Lamp Flash Detector
This circuit uses a small 2.5mm square photo diode in conjunction with a 100mH coil to detect
the short light flashes from a xenon lamp. The coil makes the circuit immune to normal
room lights. Its 10mv sensitivity can detect light flashes from a range of over 100
feet. Reflections from a room’s walls and ceiling is usually enough to trigger the
circuit. The entire circuit draws only 3 microamps from a 6 to 9 volt battery. |
| Jan 13, 2008 |
Wide Band Zero Cross Detector
This circuit was designed to convert a low amplitude 40KHz signal into a clean square wave
signal. It will work with inputs as small as 5mv peak to peak or as large as 3 volts
peak to peak. The input frequency can range from a few kilohertz to about 150KHz. |
| Jan 6, 2008 |
Simple Security Wire Loop Alarm
A wire loop is used to protect valuable objects in this simple alarm circuit. The
electronic hobby circuit is powered by a 9v battery. The alarm beeper is activated if
the wire loop is severed. The standby current is so low that the 9v battery should last
for many years. |
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