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Circuits of the Week -- May
to Dec 2005
Last Updated on:
12/30/2009 03:57 PM
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| Dec 25, 2005 |
Short Range Reflective Object Sensor
Drawing only 30uA from a 3v supply, this circuit will detect a human finger with a range of
about 1 inch. The sensor uses an inexpensive infrared LED and a matching photo diode. |
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Dec 18, 2005 |
TIME TO DUST INDICATOR
I thought about this circuit when I heard that a lot cleaning personal in hotels were either
dusting rooms more often than necessary or not enough. I have not yet built and tested this
circuit completely but in concept it should work. The circuit draws very low current from a
+3v battery and could be housed in a package similar to a small ashtray. The assembly might be
placed in a suitable out of the way area to collect dust. It would alert a maid when it was
time to dust the room. The circuit detects dust with an infrared LED that is pulsed so its
light shines onto a smooth flat plate. Any dust settling onto the surface is detected by a
phototransistor, mounted at a 90 degree angle from the LED. When the dust reaches a particular
level, sufficient light is reflected into the phototransistor to change the logic state of the
circuit to an alarm condition. The alarm output could be connected to a beeper, a flashing LED
or to one of the LED flashing circuits in this hobby circuit section |
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Dec 11, 2005 |
TEMPERATURE SENSOR WITH 4 TO 20mA CURRENT LOOP
I designed a circuit similar to this one years ago to accurately measure the air temperature
inside a building 1000s of feet from a control room. The circuit uses the very robust
current loop method. It uses a highly accurate semiconductor temperature sensor and an
equally accurate voltage reference. The circuit includes a diode bridge, so it is polarity
independent. By using the component values indicated, the circuit should not require
calibration. It has a range from –40F to +120F and an accuracy of plus or minus one
degree F. |
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Dec 4, 2005 |
CIRCUIT PROTECTS FROM BATTERY POLARITY REVERSAL
This simple circuit can protect a sensitive electronic circuit from an accidental connection
of a battery with a reversed polarity. The N-channel FET connects the electronic device
to the battery only when the polarity is correct. The circuit shown was designed for a
device powered from a single 1.5 volts button cell battery. However, the circuit will
operate with higher voltages as well. |
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Nov 27, 2005 |
20MHz VCSEL 3mW LASER TEST CIRCUIT
This circuit takes advantage of some new vertical cavity surface emitting lasers (VCSEL) that
don’t require light output control circuits. The circuit shows how to drive the device from a
single high speed CMOS IC. The circuit can easily be modified to transmit signals from
kilohertz to about 50MHz. |
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Nov 20, 2005 |
AUDIO FREQUENCY DIGITAL NOISE GENERATOR
When you need to test an audio circuit with broadband noise, this circuit works great. It uses
just three inexpensive C-MOS ICs that generate a series of output pulses whose widths vary
randomly. I included a level control pot. |
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Nov 13, 2005 |
DOORKNOB ALARM
Many companies offer simple alarm devices for personal use in bedrooms or hotel rooms. A metal
chain attached to a box holding the electronics is placed around the inside doorknob of a wood
door. Anyone grabbing the knob from the outside is detected by the electrical capacitance
change that occurs from the human hand contact between the knob and the box. Almost all of the
commercial devices sold use a more expensive and power consuming radio frequency circuit
approach to detect the capacitance change. But, a very inexpensive and micro power technique
can also work. This circuit schematic should dramatically reduce the cost of the device and
allows it to operate for many years from one set of batteries.
Also see
Completed Project by Matthew C. Ruschmann |
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Nov 6, 2005 |
LOW POWER 12,000 VOLT
POWER SUPPLY
f you need about 12,000 volts DC for an ion generator this circuit might be the ticket. It
draws power from the 120vac power line but it uses a small 6KV camera flash trigger coil. The
output signal is isolated from the power line. Although the circuit can only deliver about 5uA
of current it can produce dangerous shocks, so be careful. |
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Oct 30, 2005 |
SOLID STATE RELAY REQUIRES ONLY 50uA DRIVE CURRENT
This circuit demands a control current that is 100 times smaller than that needed by a typical
optically isolated solid state relays. It is ideal for battery powered systems. Using a
combination of a high current triac and a very sensitive low current SCR, the circuit can
control about 600 watts of power to load while providing full isolation and transient
protection. |
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Oct 23, 2005 |
AUDIO
FREQUENCY DIGITAL NOISE GENERATOR
When you need to test an audio circuit with broadband noise, this circuit works great. It uses
just three inexpensive C-MOS ICs that generate a series of output pulses whose widths vary
randomly. I included a level control pot. |
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Oct 16, 2005 |
Charge
Coupled MOSFET Relay
This circuit acts as an AC/DC relay with a rating up to 50 volts peak and up to 10 amps of
current. The differential oscillator supplies voltage to the gates of the two FETs with
good isolation will drawing only 1.5ma of current. |
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Oct 9, 2005 |
Very
Low Power Gated Crystal Oscillator
The circuit gates the output of a continuously operating 32KHz crystal oscillator to the input
of a C-MOS buffer when clock pulses are needed. The technique gets around the problem of a
slow starting crystal oscillator by keeping the oscillator going and switching on a transistor
power stage only as needed. The method keeps the standby power consumption to a very low 1uA
when used with a 3v supply. |
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Oct 2, 2005 |
Low
Power 40KHz Light Receiver
Drawing only 100uA, this circuit provides high sensitivity with excellent ambient light
immunity. |
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Sep 25, 2005 |
Shadow Activate Motion Detector
This circuit can sound an alarm when the shadow of a hand or arm moves over two
small photo diodes. |
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Sep 18, 2005 |
Infrared Safety Switch
Using a slotted optical sensor, this circuit can be used as a replacement for a large mushroom
pushbutton safety switch. |
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Sep 11, 2005 |
Two Pushbutton Motor Controller
Two small pushbutton switches, a few diodes and two relays form a method to control on/off
power to a brush motor as well as the motor direction. The circuit was originally
designed for a motorized lifting platform. |
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Sep 4, 2005 |
Motion Alarm
Using a piezoelectric device, this circuit will activate a beeper whenever the circuit is
moved. It could be used as an
earthquake alarm. |
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Aug 28, 2005 |
Motor Speed Regulator
This circuit regulates the speed of a small drill motor I use for PC board work. It is much
handier for delicate work than a larger AC powered tool. The tool I have came with a useless 9
volt AC adapter. The motor would whine at high RPMs unloaded, but would slow to a crawl when
loaded. This circuit solves the problem.
Designed by Andrew R.
Morris
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Aug 21, 2005 |
Table
Lamp with auxiliary 12v Automotive Style LED Light
This circuit provides about one watt of non-isolated DC power for an automotive type
12v LED array lamp in addition to a standard incandescent table lamp. |
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Aug 14, 2005 |
Universal LED Flasher
This low power LED flasher circuit draws about 100ua from any
DC power supply ranging from 3 volts to 24 volts. The 1.0Hz
10ms LED pulsed current is held to a constant 10ma. |
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Aug 7, 2005 |
Non-isolated Off-line AC to DC Power Supply
This compact efficient circuit can provide up to 100ma of a regulated 5 volts from an AC power
source ranging from 20vac to 120vac. |
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July 31, 2005 |
Oscillator Draws 2 Nanoamps
This page shows two low frequency oscillators that draw ultra low current. |
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July 24, 2005 |
Button Battery Cell Internal Resistance Measurement Circuit
The open cell voltage of a small silver oxide button cell battery remains very close to 1.5
volts over most of the life of the cell. However, there is a relationship between
battery health and the internal resistance of the cell. This circuit when used in
conjunction with an oscilloscope will measure the internal resistance. You will have to
use a known good cell as a reference. |
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July 17, 2005 |
120vac
Under Voltage Tester
Using a cheap 24vac transformer, this circuit can test a product under low 95vac conditions.
With the components show, it has a rating of 250 watts max. |
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July 10, 2005 |
Low Voltage 120vac Power Line Alarm
This circuit will turn on a beeper when the line voltage drops below 100 volt AC. |
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July 3, 2005 |
Low Power H Bridge Motor Controller
This circuit uses a few logic gates and a 74AC240 to control power to a small gear motor. |
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June 26, 2005 |
Electrodeless Water Level Monitor
This is an illustration how two coils inside a PVC pipe can be used to monitor the level of
water in a tank. Current induced into the water from the bottom coil is only detected by
the upper coil, when it is under water. |
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June 19, 2005 |
1.5V
Battery Powers Capacitance Switch
Using a unique differential time comparison technique this capacitance switch can detect a
change of just one picofarad. |
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June 12, 2005 |
Negative 0.5 Volt Supply
With the component values shown, this simple circuit will provide a negative 0.5 volts at
about 2 milliamps. It is perfect for rail to rail op amp circuits, where a true 0 volt
output is needed. |
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June 5, 2005 |
Low power 100KHz Light Receiver
By starving a high speed logic inverter for current, this circuit can produce a sensitive
100KHz light receiver circuit, which is immune to ambient light, but only drawing 100
microamps from a 3 volt supply. |
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May 29, 2005 |
Automotive Electrical Voltage Indicator
This circuit is used for troubleshooting the voltages of standard 12 volt DC car circuits.
A single LED indicates if the voltage is high, low or open. |
| May 22, 2005 |
TV/VCR Infrared Remote Booster
This circuit will boost the signal from any infrared TV or VCR remote, extending the range by
a factor of 3X. |
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May 15, 2005 |
Basement Doorbell
This circuit will activate a beeper in the basement, whenever the front doorbell is pressed. |
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May 8, 2005 |
Audio Signal Activated Switch
Drawing only 2uA from a 3v lithium battery, this circuit will turn on a transistor switch,
whenever it detects an audio signal greater than 0.035v RMS. |
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May 1, 2005 |
Full
wave Rectifier
Using a LF412 high speed dual op amp, this circuit will rectify any signal up to about 100KHz
with a minimum amplitude of about 50mv peak to peak.
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