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Circuits
of the Week - 2010, Jan - June
Last Updated on:
08/24/2011 05:03 AM
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| May 2, 2010 |
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. |
| April 25, 2010 |
Unplugged Appliance Alarm
I got a call from an administrator at a large prison. He told me that he was having
problems with inmates who keep unplugging refrigerators, vending machines and microwave ovens
in a commons area. He wanted a battery-powered device, which could be mounted to the
outside of the power cord of an appliance. The device would produce a loud alarm
whenever the power plug of the appliance was unplugged from a power outlet....... |
| Apr 11, 2010 |
36 Hour Watchdog Timer
A watchdog timer is often used in computer systems to detect when a computer is hung up and is
no longer running properly. If it does become locked up and is not able to supply a reset
pulse to the circuit within a defined time frame the timer times out and activates some kind
of alarm. Watchdog timers can also be used for humans...... |
| Mar 28, 2010 |
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. A supercapacitor is often chosen to supply power
to low current loads for many hours at a time, recharged by a solar panel. They are also
handy when infrequent heavy power demands are needed.... |
| Mar 14, 2010 |
AC Power Line Hi/Lo Voltage Tester
If you wish to test a line-powered device under both a 15% high and a 15% low voltage
condition, you can use the circuit below. The circuit uses an 18v transformer with a 3
amp rating. A double pole, double throw toggle switch then switches in the transformer
voltage in either a buck or a boost mode. In the buck mode, the transformer
voltage..... |
| Feb 28, 2010 |
Line Powered White LED Array
There has been a lot of excitement lately in the lighting industry. Some companies are
beginning to manufacture general lighting fixtures, which replace incandescent and fluorescent
lamps, using LED lights. When proper heat management is designed into the light
fixtures, these LED lights can produce more light with less electricity and last much longer
than other devices. They can also operate well in cold temperatures. Some of these
new lighting fixture designs use a large number of white LEDs, wired into long series wired
string arrays. The circuit below is designed to provide a regulated DC voltage for these
strings. |
| Feb 21, 2010 |
Logic Controlled 12v DC Motor with Automatic Brake
There are times when you would like to control a DC motor using a simple +5v logic swing
signal. The circuit below performs this feat and also includes an automatic motor
braking circuit. When the input logic signal swings from 0v to +5v, the n-channel MOSFET
Q2 turns on, which turns on the p-channel MOSFET Q1. Current flows into the motor
through the schottky diode D2. When the control logic swings back 0v, Q1 turns off.
This activates the braking action of Q4, which applies a short across the motor. The
sudden stop is often desired in some motor controlled systems. |
| Valentine's Day |
AC Current Controls Hour Meter
Many systems require routine maintenance based on machine operation time. The circuit
below is a simple way to turn on a hour meter whenever AC power is supplied to the machine.
An inexpensive snap-on current transformer from Magnetics Inc, is used to detect the AC
current. The output of the transformer is fed to a voltage doubler circuit. When 0.5 AC
amps or more of current is flowing, the circuit generates sufficient voltage to turn on the
n-channel MOSFET. Once tuned on, the transistor routes voltage to the hour meter.
Some hour meters contain their own battery, so no external power is needed. In those
cases, the transistor switch alone will turn on the hour meter. |
| Feb 7, 2010 |
+5v Powered Charge Pump Battery Charger
The circuit below will trickle charge a four cell
pack of AA or AAA NiMH batteries. The circuit draws current from the +5v available a USB
connection and pumps about 70ma of current into the battery. This should be enough current to
fully charge a pack of 2500ma-hour cells in about 36 hours. The circuit uses a single
74HC14 hex Schmitt trigger inverter in conjunction with a voltage doubler charge pump circuit.
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| Jan 31, 2010 |
Ultra Low Current Oscillator #4
The circuit below was inspired by a programmable unjunction transistor (PUJT) circuit sent to
me by Karl Isbrecht. In Karl’s circuit, the unijunction circuit made click sounds in a
speaker and consumed very little power. His circuit reminded me that a PUJT circuit
could be made using a couple common transistors wired in a configuration like a SCR. In
the circuit below, the output of the two transistor pulse generator is connected to a CD4013
dual flip/flop. One flip/flop is configured as a non-inverting buffer. The second
is wired as conventional divide by two counter. The result is a nice low frequency
square wave generator, which draws only 2ua from a 9v supply. The circuit will also work
with a DC supply voltage ranging from 3v to 15v.... |
| Jan 17, 2010 |
Energy Harvesting Using a Current Transformer
Energy harvesting is all the rage these days. With some modern electronics, information
from low power sensors can be sent to a distant data collection point using a low power RF
transmitter. To power these remote sensors, various energy sources can be tapped into.
Machine vibration, temperature differences, ambient light and stray RF have all been used as
low power energy sources. Sometimes, the sensors are located near AC power cables.
Rather than making a direct connection to those cables, an AC current transformer, such as the
one shown below, can be used to capture a bit of power..... |
| Jan 10, 2010 |
On/Off Latch Circuit
update
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 tiny parts and can operate from DC
supplies between +2v and 5v. It uses two inverters within a 6 pin SN74LVC2G14 device
from Texas Instruments. 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. |
| Jan 3, 2010 |
Bidirectional Solid State Relay
The circuit below can switch up to 4 amps of current with a peak voltage up to 20v. It
is fully isolated using two inexpensive red LEDs. One LED forms a red light source, which
illuminates a second red LED. The circuit takes advantage of the 1.5v produced by super
bright red LEDs when illuminated with red light. The voltage produced by one red is
routed to two n-channel FETs, configured in a bi-directional switch circuit. The two
FETs selected have a low gate-source threshold voltage, which allows them to turn on hard from
the 1.5v from the red LED. The bi-directional configuration allows the circuit to switch
both AC and DC power. Using quality super bright red LEDs, the circuit will switch on
fully with only 5ma of LED current. |
| 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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