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Circuits of the Week
Last Updated on:
04/27/2008 06:18 AM
(Dec 2007 - July 2007)
(June 2007 - Jan 2007)
(Dec 2006 - July
2006)
(June 2006 - Jan 2006)
(Dec 2005 - May 2005) |
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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. |
| 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. |
|
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. |
|
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.... |
|
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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(Dec 2007 - July 2007)
(June 2007 - Jan 2007)
(Dec 2006 - July
2006)
(June 2006 - Jan 2006)
(Dec 2005 - May 2005) |
