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Circuits of the Week -- July to Dec 2007
Last Updated on:
10/07/2009 05:53 AM
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| Dec 30, 2007 |
Security Loop Alarm
A wire is looped through various objects which are to be monitored. The loop wire is
typically an aircraft cable but could also be any insulated stranded wire. The loop is
attached to a table or desk. Should the loop be cut, the alarm is sounded. |
| Dec 23, 2007 |
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. |
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| Dec 16, 2007 |
PWM Circuit for Motor Speed Control
Sometimes you want to slow down a brush type DC motor. The most efficient way to do this is
with a pulse width modulation (PWM) technique. The hobby circuit below can operate from about
3 volts to 15 volts. The frequency is fixed at about 2KHz but the pulse width can be varied
from nearly 100% to 0%. ... |
| Dec 9, 2007 |
Run Switching Type AC Power Adapters on DC
I have received a couple emails from people looking for inexpensive DC to DC converters, which
can convert 40v to 60v DC into say +5 or +12v. Such input voltages are often found in
new automotive and industrial applications with a typical DC voltage of 48 volts. It turns out
that many, not all, but many, standard AC line operated power adapters, which use switch mode
techniques, will indeed work great when supplied with DC instead of AC. The unit below works
down to about 30v DC and delivers 500ma at 5v. If you draw less current, it will operate at
even lower DC input voltages.... |
| Dec 2, 2007 |
Solid State Relay Requires Ultra Low Control Current
Most solid state relays require at least 5ma and often more input control current, to fully
turn on the device. But such current levels often force battery powered circuits to use
excessively large batteries. The relay hobby circuit shown below demands only 50uA of
input current. This about 100 times lower than that needed by a typical optically
isolated solid state relays. The circuit uses a combination of a high current triac and
a very sensitive low current SCR, to control up to 600 watts of power to a load, while
providing full isolation and transient protection. |
| Nov 25, 2007 |
Solar Cell Test Fixture
If you are considering building your own solar panel, you might try building the solar cell
tester shown below. When constructing a solar panel it is important to make sure that all the
solar cells used in the panel be closely matched for their short circuit current. One weak
cell will lower the efficiency of the whole panel. The system shown allows individual solar
cells to be tested for their short circuit current under a constant light intensity
condition.... |
| Nov 18, 2007 |
Variable Isolated AC Voltage
Spans 0VAC To 280VAC
I designed and built this circuit about 25 years ago. It came in
handy for many projects that were powered from 120 VAC, 240 VAC and 277 VAC. It provides
complete isolation from the power line. It uses two 1:1 isolation transformers that are wired
in parallel for the140vac range and in series for the 280vac range. The selector switch also
diverts power to the appropriate output socket to avoid mishaps in sending the wrong voltage
to the load. My home-built unit also included an AC volt and amp meter to monitor the output.
However, this circuit only indicates the AC power connections. |
| Nov 11, 2007 |
Inductor Self-Resonance Tester
If you want to know what the self-resonance frequency is of an inductor, this circuit will
help. It adds only a few picofarads of parallel capacitance to the inductor under test. With
the aid of an oscilloscope and a signal generator this circuit will allow you to measure
resonant frequencies from kilohertz to over 10 megahertz. |
| Nov 4, 2007 |
12 Volt DC Power Supply
This is a simple low-cost 50W off-line switching power
supply, which can be used for home projects or to learn operation of flyback converters. It
can work over a universal AC line input range 90-264 VAC and provides a 12VDC output at more
then 4A load. Line and load regulation is better then 0.5%. (added 11/07) |
| Oct 28, 2007 |
Halloween Flashing-Eyes Badge
Two-LEDs act as eyes following the rhythm of music or speech. 3V Battery-operated device
suitable for pins or badges. (added 3/05) |
| Oct 21, 2007 |
10mhz to 20mhz Laser Light Detector
This circuit was originally designed to detect laser light pulses for an optical Ethernet
communications system. It has good ambient light immunity. |
| Oct 14, 2007 |
Pulsed LED Test Circuit
This circuit is designed to test visible and infrared LEDs in pulsed mode operations. It can
drive the LED with peak currents in excess of 10 amps. A light detector nearby can monitor the
response time and intensity of the LED under test. |
| Oct 7, 2007 |
Electric Field Disturbance Monitor
This schematic is the power supply and front-end sections of the field monitor that is
discussed in more detail at Electric Field Disturbance Monitor. The system can detect human
and animal motion by the electric fields they disturb. |
| Sept 30, 2007 |
Simple Nitrogen Spark Generator
Nitrogen or air sparks are very powerful light sources that produce flashes that last only a
few nanoseconds. This line powered circuit generates a continuous series of very small sparks
across electrodes with a 0.05 inch gap. |
| Sept 23, 2007 |
Piezoelectric Triggered Switch
Two different switch circuits are shown. One sources current and the second sinks
current. Both switches are connected to a piezoelectric wafer. When the wafer is
tapped, the switches are activated. |
| Sept 16, 2007 |
Low Power Oscillators
This page has two unusual two-transistor oscillators. I set the component values for a
low frequency application. Both circuits draw only about 1 microamp of current.
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| Sept 9, 2007 |
Pulse Period to Voltage Converter
This is a test circuit converts a square wave input signal into a voltage. But, the voltage
produced is proportional to the time between edges (period) of the signal, not the frequency.
The range is from 100uS to to 10mS, which produces a voltage from 100mV to 10 volts. Other
scale factors are also possible. The circuit is powered from single 15v supply and uses
inexpensive parts. It is great when a signal's period instead of its frequency needs to be
monitored. |
| Sept 2, 2007 |
Precision AC Peak Detector
This unique circuit uses a very inexpensive voltage comparator to form a peak detector. The DC
voltage produced tracks the positive peak of the input signal. It works from about ten
millivolts to about 10 volts peak to peak. The maximum frequency is about 150KHz. |
| August 26, 2007 |
CMOS Inverter 125khz LC Oscillator
This circuit uses a single CMOS inverter to form a series resonant LD oscillator.
The values shown set the oscillation at about 125KHz buth other frequencies are possible by
changing the main LC values. |
| August 19, 2007 |
D-Flip/Flop One Shot Circuit
Yes you can use cheap D flip/flop logic circuits as nice one-shot pulse generators. This
schematic shows how the popular CD4013 and the CD74HC74 can be used to generate pulses ranging
from nanoseconds to seconds. |
| August 12, 2007 |
Optical RFID Test Circuit
I designed this test the concept of using light techniques to send identification data instead
of RF. A more detailed discussion on this scheme can be found in the Imagineered new products
section. |
| August 5, 2007 |
Micro Power
Over-Temperature Alarm
The circuit is powered for years by a single 3 volt lithium battery. It sounds an alarm when
the temperature exceeds a certain point. With some minor changes the circuit could also be
configured for an under temperature (freeze) alarm. The circuit uses a cheap but accurate
thermistor as the temperature sensor. |
| July 29, 2007 |
CMOS Inverter 125khz LC Oscillator
This circuit uses a single CMOS inverter to form a series resonant LD oscillator.
The values shown set the oscillation at about 125KHz but the other frequencies are possible by
changing the main LC values. |
| July 22, 2007 |
Medium Power 125khz Oscillator
This circuit is similar to MEDIUM POWER 125KHZ OSCILLATOR but adds even more inverters in
parallel to deliver yet more power. The values shown are for 125KHz. |
| July 15, 2007 |
Precision Ultra Lower Power Oscillator
This circuit works much like the classic 555 timer, but draws only about 1.5 microamps from a
3 volt battery. It is highly stable under varying temperature and supply voltages. |
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July 8, 2007 |
Clear Fluid Monitor
An optical technique seemed like the best choice for this application. The simple
circuit below works well but only when the light detector can be shielded from bright ambient
light. A visible red LED launches light pulses through the side of the clear tubing and
emerges at the opposite side. A phototransistor forms a light detector. The
circuit takes advantage of the lens effect from any clear fluid inside the tubing. When
there is only air in the tubing, the light scatters and spreads out quickly before it reaches
the phototransistor.... |
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July 1, 2007 |
UL Ground Resistance Tester
UL requires that an electrical system using a metal chassis earth ground connection have a
resistance of less than 0.1 ohms. This resistance must be tested with actual line
current in excess or 10 amps. The circuit shown uses a couple of LEDs to indicate if in
fact the resistance is less than 0.1 ohms. It uses a modified transformer to produce a
2.5vac output with at least 10 amps of supply current. |
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