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Circuits
of the Week - 2009, July to Dec
Last Updated on:
12/30/2009 04:05 PM
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| Dec 27, 2009 |
ON/OFF SWITCH WITH LATCHING RELAY
The circuit below switches AC or DC power to a load using an ultra low power 3v latching relay
with two coils. The relay can handle about 10 amps of current, up to 250vac. Each
time the small pushbutton is pressed, the circuit either latches the relay contacts closed or
unlatch them open. The 3v powered circuit uses a single 74HC14. This device contains six
Schmitt trigger inverters in the one 14 pin package..... |
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| Dec 19, 2009 |
24VAC POWERED WHITE LEDs
24vac is a popular voltage. It is often used in low voltage lighting systems and
industrial controls. It is much less dangerous than 120vac/240vac line voltages.
The circuit below is one method to efficiently power a cluster of 7 white LEDs from 24vac.
It uses a classic bridge rectifier and filter capacitor to convert the AC to DC..... |
| Dec 13, 2009 |
VCR TV Remote Range Extender Circuit
I often get frustrated by how poor some infrared
remotes in many new electronic devices operate. I used to be able to turn on the TV from
30 feet away. Now, it seems that I have to get within 15 feet. To address this
issue, I designed the circuit shown below. The circuit taps into the TV remote’s
existing infrared LED and drives 8 more efficient devices. The result is a
communications range some 4 to 5 times longer..... |
| Dec 6, 2009 |
9v Battery Eliminator
If you use 9v battery powered devices for long periods of time, you may get tired of
constantly changing batteries. The circuit below can be wired into any 9v battery powered
device, drawing power from an external AC to DC power adapter. I designed the circuit
around a very common LM317 adjustable voltage regulator. ... |
| Nov 22, 2009 |
On/Off Latch Circuit
Often you want to turn on and off power to a device with a single pushbutton switch. The
circuit below performs this logic function with just a few parts and can operate from DC
supplies between +3v to about 15v. It uses two inverters within a 74C14 or a CD4069 hex
inverter package. Each press of the button toggles between the two on/off logic states.
The logic output is connected to a PMV65XP p-channel FET from Philips. This component should
be able to handle 2 amps of current. The inputs of any unused devices within the IC
package should be connected to either V+ or ground. |
| Nov 15, 2009 |
Cut Phone Line Alarm --
Burglars, rapists and other criminals will often cut a phone
line before they break into a home. The cut phone line will prevent many alarm systems
from alerting the police. The battery powered circuit below will sound an alarm when the
unit no longer detects the voltage present on an active phone line..... |
| Nov 8, 2009 |
Supercapacitor Voltage Limiting Circuit
Supercapacitors are working their way into more and more applications where electrical energy
needs to be stored. These robust devices can be charged and discharged 1000s of times
and will typically outlast a battery. Many supercap manufacturers claim a life span of
10 years or more. A supercapacitor is often chosen to supply power to low current load
for many hours at a time, recharged by a solar panel. |
| Nov 1, 2009 |
Lead-acid Battery Low Voltage Indicator
Lead-acid batteries should not be discharged below a certain voltage. Discharging a cell
below about 1.8v and can often damage it, which will result in a lower amp-hour capacity.
Large battery banks are critical in off-grid energy storage systems. One bad cell in a
series wired string will weaken the whole bank. The circuits below are designed to be
connected across either a single 2 volt cell or a 6v three cell battery bank. It will
alert the user of a weak cell by turning on a red LED when the cell voltages drop below 1.8v.
The circuit takes advantage of a low voltage LMP2231 op amp from Nation Semiconductor and a
LM385 voltage reference in the first circuit. In that circuit the red LED must have a forward
voltage of about 1.6v at 5ma, in order to operate properly. In the second three cell
bank circuit, a TLV3701 voltage comparator is used. The circuits are simple enough that
they could be placed on a postage stamp size circuit board. |
| Oct 25, 2009 |
White LED is Powered by One NiMH Cell
There are a lot of white LED driver circuits floating around. A popular circuit
called the “Joule Thief” can drive one white LED from a single 1.2v or 1.5v battery cell.
Most of these circuits use one or two transistors to form a voltage boost circuit. Yes,
they those circuit do drive a LED but they are not very efficient and they don’t do a good job
of controlling the current to the LED. The circuit below uses a tiny LT1932 IC made by
Linear Technology. This IC can be configured for a wide variety of LED driver needs.
I have shown one simple application. The circuit takes power from one 1.2v NiMH
rechargeable battery or a 1.5v disposable battery and drives a single white LED with 15ma of
current. With an efficiency of about 70%, the circuit should run for about 40 hours, if
a quality 2500ma-hour battery is used. |
| Oct 18, 2009 |
Model Airplane Wing Tip Flasher
The circuit below is designed to flash two white LEDs mounted at the wing tips of a model
airplane. The flasher unit is powered by a 4.5v supply made of three AA or AAA 1.5v
alkaline battery cells. The average current is about 2ma so a set of batteries should
last about 1000 hours of flashing. The circuit uses one hex Schmitt trigger inverter to
perform all the timing functions. A single n-channel FET drives the two LEDs, each with a peak
current of about 20ma. The circuit flashes the two LEDs in a pattern typical of
commercial airplanes. |
| Oct 11, 2009 |
Star Trek Doorbell
In the Star Trek “Next Generation” TV series, the doorbell outside the private quarters of a
crew member makes a particular “beep-boop” sound. The 3v battery powered circuit below
tries to simulate this sound. The circuit uses one 74HCT74 dual D flip/flop IC, wired as two
one-shot circuits. Both are designed to produce a pulse about one half second long.
The first pulse turns on a 555 timer to form the beep sound. The second flip/flop one
shot .... |
| Sept 27, 2009 |
Pot Controlled Variable LED Intensity Circuit
The circuit below uses power from four rechargeable AA NiMH cells to drive 3 white LEDs.
A potentiometer varies the duty cycle of a pulse width modulator circuit to vary the intensity
of the LEDs from 0% to 100%. The beauty of the circuit is that when the pot is all the
way toward 0%, very little power is drawn from the battery. Likewise, when the pot
slider is moved toward the 100% end, full power is fed to the LEDs. In effect, the pot
becomes the on/off switch..... |
| Aug 30, 2009 |
Do It Yourself Portable Cell Phone Charger
It seems that as cell phones become more sophisticated, their operating times become shorter
and shorter. If you are on the go all the time and have difficultly finding AC or DC
outlets to charge your phone, you might consider this simple scheme.... |
| Aug 23, 2009 |
12v Battery Load Cutout Circuit
Lead-acid batteries should not be discharged below a certain point if they are to last.
Also, you don’t want certain 12v powered devices, which might be plugged into a car’s
cigarette lighter outlet, to drain the car battery down to a point where the car’s engine will
not start. To prevent battery damage or a dead battery, the circuit below disconnects a
load from the battery when the voltage dips below a fixed but adjustable voltage and will not
reconnect the load until a reset button is pressed..... |
| Aug 16, 2009 |
Very Loud 3v Powered Beeper
Getting a high sound intensity from a piezoelectric type beeper is not easy when the available
DC supply is only 3v. The circuit below is not only efficient but produces a very
intense sound. The circuit combines a voltage boost section with a resonant feedback
network. The voltage applied to the piezoelectric wafer is about 40 volts peak to peak.
The result is a circuit that generates an attention getting sound without drawing a lot of
current. |
| Aug 8, 2009 |
24vac Powered Isolated +5v Supply
24vac is a very popular power supply in many industrial systems. It is often used in
systems that involve water. It is also popular with many inside and outside lighting
systems. The lower voltage means the danger of human electrocution is much less.
But, many DC powered sensors and control circuits associated with 24vac systems need to be
fully isolated from the 24vac supply. The circuit below accomplishes this feat.... |
| July 26, 2009 |
12v to 19v Laptop Power Supply (Concept)
I have a big screen HP laptop computer. The
large brick size AC to DC power supply is rated at 19.2v and 9.5 amps, which is about 180
watts. I’d like to be able to use the thing in my car. I looked into a 12v car adapter
for it but never really found one that I liked. Most were underpowered. The other option was
to use the existing AC to DC adapter and run it off power from a 12v to 120vac inverter.... |
| July 19, 2009 |
Miniature 5v Line Powered Isolated Supply
Often a circuit requires a 5v DC supply to power some logic circuits. The conventional
method is to use an AC wall adapter. But, many systems, which bring AC power onto a
circuit board, need a small AC to DC power supply right on the circuit board. The
circuit below provides such a supply. It uses a classic series capacitor charge pump
which acts as a current limiting device. .... |
| July 5, 2009 |
Water Flowing in Pipe Indicator -- July 6,
2009
The vibrations associated with water flowing through a pipe are picked up by an inexpensive
piezoelectric wafer. The signal from the wafer is first boosted by a micropower
transistor amplifier and then fed to an ultra low power voltage comparator. When
the vibration signal has sufficient amplitude, a FET transistor switch is activated.
Drawing only 6uA, the whole circuit is powered by a lithium coin battery, which should power
the circuit for many years.... |
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