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Circuits of the Week
- 2008
Last Updated on:
05/11/2008 04:58 AM
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(Dec 2005 - May 2005) |
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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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((Dec 2007 - July 2007)
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2006)
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