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Circuits
of the Week - 2011, July - Dec
Last Updated on:
03/23/2012 01:18 PM
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Dec 25, 2011
Merry Christmas |
3 Minute Closet Light #2
I have a coat closet by my
front door. The thing is very dark at night and really needs some kind of illumination
inside. I could hire an electrician to install a light in the closet ceiling and then
wire it to a toggle switch outside the door but that would cost me quite a bit of money.
That doesn’t sound like a good investment for something which does not get much use. I
looked around for some battery powered LED lights which would emit enough light for the small
space and I found one made by Sylvania.... |
| Dec 11, 2011 |
Low Power AC to DC Power Supply 2
Last month I showed how to use
a small common mode choke as a fully isolated transformer and a 130v sidac to generate about
100mw of DC power from the AC line. This second circuit still uses the same inexpensive
tiny off-the-shelf 5mH common mode filter coil with a split bobbin as a high frequency
transformer but uses two SCRs instead of a sidac to discharge a capacitor. This method
increases the available power to about 500mw. The circuit routes short 300v pulses to
the primary winding of the coil every half cycle of the 60Hz line frequency..... |
Happy TDay
Nov 20, 2011 |
Low Power AC to DC Power Supply
When you need some DC power fully isolated from the AC power line, this circuit may be just
the ticket. The circuit uses an inexpensive tiny off-the-shelf 6.8mH common mode filter
coil from with a split bobbin as a high frequency transformer. The circuit routes short
130v pulses to the primary winding of the coil every half cycle of the 60Hz line frequency.
It uses a silicon bilateral voltage triggered switch called a “sidac” to discharge a 0.1uF
high voltage capacitor...... |
Happy
Halloween
Oct 31, 2011 |
Band Drummer’s Microphone Switch
I got an email from guy who plays drums for a small band. He wanted a simple way to turn
on and off his microphone with just a tap of his drum stick onto a small box. The
circuit below performs this task with the aid of a piezoelectric device as a shock sensor and
a small dual coil latching relay with two sets of contacts. The contacts can be wired
any number of ways to a microphone circuit. I show one circuit which should work on both
dynamic and condenser mics. It should mute the mic without producing a pop sound...... |
| Oct 16, 2011 |
24v NiMH Battery Charger Using a 12v Solar
Panel A while back I got
an email from a guy in the UK. He wanted to use a small solar panel, designed to charge
12v lead acid batteries, but use it to charge a 24v battery pack. His 24v battery pack
used 20 AA size NiMH cells. He went on to explain that his 20 cell NiMH battery pack was
used very infrequently. He used the pack to open and close window blinds powered by a
brush motor and controlled by a rocker type toggle switch. He said that the motor would
draw about 2 amps from the 24v pack for 20 seconds for each operation. He also said that
typically the blinds were operated only 4 times each day; two times to open and two to close.
He wanted to know how to charge the higher voltage battery using the lower voltage solar
panel..... |
| Sept 18, 2011 |
Triangle Signal Generator
A Discover Circuits visitor needed a variable low frequency triangle signal generator, which
would produce a full 5 volts peak to peak signal using a 5v supply. I suggested the
circuit below. It uses three rail to rail op amps (2 LMC6484 dual op amp packages).
The first circuit produces a clean 2.5v reference voltage. The second is configured as a
classic multivibrator oscillator, whose frequency can be changed using a variable resistor.
The waveform generated across the oscillator timing capacitor is fairly linear and is
restricted to only 1 volt peak to peak. It swings a half volt above and below the 2.5v
reference. The voltage across the capacitor is then amplified by the third op amp by a
factor of 5. The result is a pretty clean triangle signal, which approaches to within a
few tens of millivolts of the supply rails. |
| Sept 4, 2011 |
Motorized Duck Decoy
Timer
I received a nice email from a duck hunter. He uses some motorized decoys to lure ducks
into his line of fire. However, the battery powered decoys only have an on/off switch so
once turned on; the decoy flaps its wings or paddles around continuously. The hunter
wanted a more realistic motion, which would power the motor for say 10 seconds, then stop for
another 10 seconds before starting up again. The simple circuit below should work well for his
application... |
| Aug 14, 2011 |
Camera Shutter Time Measurement while back
I got an email from someone who needed a simple way to measure the speed of a photographic
camera. I thought about this and suggested the method below. It relies on an
oscilloscope and a cheap photodiode to measure the speed..... |
| Aug 7, 2011 |
AC LINE
POWERED LED FLASHER NUMBER 3
Some of the latest LEDs are amazing. They can produce a
surprising amount of light with very little current. In the light flasher application
shown below I take advantage of a nice surface mounted LED from King Bright. This
flasher circuit draws only about 3ua of average current from the AC power line to flash this
green LED. This flasher might be used as a power indicator light.... |
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| July 31, 2011 |
Computer Controlled Power
Relay
Often there is a need to turn on and off a 120vac device using a low power +5v logic signal
from a computer. The circuit below shows how this could be done. The circuit uses
a power relay with a 120vac coil and 10 amp contacts. A small solid state relay turns on
and off the relay. The solid state relay provides 5KV isolation from the computer. |
| July 10, 2011 |
0.5v Negative Supply 4
A while back someone needed a
simple way to generate a negative 0.5v supply from a conventional transformer type AC to DC
power supply. The circuit below uses a couple schottky diodes and a filter capacitor to
perform the feat. The circuit is handy when using “rail to rail” op amp circuits which
get close to zero volts at their output from a single DC supply but can’t quite deliver
without a negative supply. With the aid of the 0.5v supply circuit shown, those devices
can produce a true zero output voltage. |
July 4, 2011
Happy
Birthday USA |
Remote TV Camera Power
A while back I received an
email from a company with a problem. They needed a security camera installed at some remote
location. The camera was a high quality color camera sending out standard NTSC video
through a long 500 foot 50 ohm coax cable. With some difficulty they got the coax
strung. They were told that there would be AC power near the camera location to power
that device. But, they then learned that in fact, there was no power available.
They thought about using a solar panel and a battery but they decided to call me for help
instead. Was there some way to power the camera without stringing another cable?... |
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