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Circuits of the Week - December to July 2009
Last Updated on:  08/23/2015 07:06 AM

2015:  Jan-June   July-Dec     2014:  Jan-June    July-Dec     2013:  Jan-June    July - Dec   
2012:  Jan-June    July-Dec      2011:  Jan-June    July-Dec     2010:  Jan-June     July-Dec      
2009:  Jan-June    July-Dec      2008: Jan-June    July-Dec      2007:  Jan-June     July-Dec       
 2006:  Jan-June     July-Dec         2005:  May-Dec

12/27/09

Touch Switch Exciter

 

If you have hundreds of touch switches that need an excitation signal, then this circuit is what you need.  Its 20KHz 20v peak to peak output signal can supply up to 3 watts of touch switch excitation power… [Hobby Circuit designed by David A. Johnson P.E., 4/2005]

12/20/09

24 Volt AC 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… [Hobby Circuit designed by David A. Johnson P.E., 7/30/06]

12/20/09

LED Array powered by 24 VAC

 

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… [Hobby Circuit designed by Dave Johnson P.E., 7/30/06]

12/13/09

TV /VCR 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… [Circuit designed by Dave Johnson P.E., 12/13/09]

12/06/09

CT 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… [Hobby Circuit designed by Dave Johnson P.E., 4/2005]

11/29/09

Flashing Light to simulate Car Alarm

 

Whenever the car’s ignition is turned off, this circuit activates a flashing LED, which can be positioned to appear as an active alarm system… [Hobby Circuit designed by Dave Johnson P.E., 10/16/05]

11/22/09

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 c… [Circuit designed by Dave Johnson P.E., 12/6/09]

11/15/09

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… [Circuit designed by Dave Johnson P.E., 10/16/05]

11/08/09

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… [Circuit designed by David A. Johnson P.E., 4/2005]

10/25/09

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… [Circuit designed by Dave Johnson P.E., 10/19/08]

10/18/09

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 … [Hobby Circuit designed by Dave Johnson P.E., 10/18/09]

10/11/09

Measure Button Battery Cell's Internal Resistance

 

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… [Hobby Circuit designed by David A. Johnson P.E., 4/2005]

10/04/09

Turn D-Flip/Flop into a 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… [Hobby Circuit designed by David A. Johnson P.E., 4/2005]

09/27/09

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 t… [Circuit designed by Dave Johnson P.E., 9/27/09]

09/20/09

Flashing LED Power Indicator

 

This circuit flashes a power indicator LED to keep the average current low… [Circuit designed by David A Johnson P.E., 1998]

09/13/09

Micropower Pulse Generator

 

It is hard to design a pulse generator that produces clean logic pulses while drawing very low current. This circuit is designed to produce short 2mS pulses at a rate of one per second while drawing only 1 microamp from a 9 volt battery… [Circuit designed by David A Johnson P.E., 1998]

09/06/09

High Ambient Light Immunity with a 40Khz Light Detector

 

The circuit below was designed to turn on an external 12v relay, whenever it detects light from a nearby LED light source, modulated at 40KHz to 50KHz.This circuit was originally designed to operate from a fast moving vehicle… [Hobby Circuit designed by David A. Johnson P.E., 10/1/05]

08/30/09

DIY 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… [Hobby Circuit designed by David A. Johnson P.E., 4/2005]

08/23/09

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… [Circuit designed by Dave Johnson P.E., 7/30/06]

08/16/09

Indicator for Acceptable Voltage

 

I have used this circuit many times in custom test fixtures where a simple go-no go indication was needed.  The circuit can also be used to adjust a particular voltage be within specific high or low limits.  The three LEDs will indicate if the voltage is… [Hobby Circuit designed by Dave Johnson P.E., 12/1/02]

08/09/09

125Khz Ultra Pure Sine Wave Signal Source

 

For some RFID systems operating at 125KHz, a very low distortion signal source reference is needed.  The circuit shown on this page produces a 10-volt peak-to-peak signal into a 50-ohm load, with a distortion of only 0.01%… [Hobby Circuit designed by Dave Johnson P.E., 2/11/02]

08/02/09

40KHz Liight Receiver is Immune to Ambient Light

 

If you want even more sensitivity than the above circuit, try this design. When used with a one centimeter square photodiode, you can achieve a range of several hundred feet with a standard TV or VCR remote control module… [Circuit designed by David A. Johnson P.E., 1998]

07/26/09

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… [Circuit designed by David A. Johnson P.E., 7/26/09]

07/19/09

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 designed by Dave Johnson P.E., 4/2005]

07/12/09

Fullwave Rectifier

 

I have used this handy circuit many times.  It accurately converts an AC signal into pulsing DC, which can be filtered to provide an average of the input voltage.  It works from millivolts to volts.  The circuit shown requires a stable +5v reference if a… [Hobby Circuit designed by Dave Johnson P.E., 2001]

07/05/09

Water Flowing Indicator

 

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… [Circuit designed by David A. Johnson P.E., 7/1/09]


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