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Circuits of the Week - June to January 2008
Last Updated on:  08/23/2015 07:05 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

06/29/08

Night Light from White LED

 

This simple circuit is designed to plug into a standard AC electrical outlet. It uses four super bright white light emitting diodes (LED) in conjunction with a capacitor coupled full wave rectifier circuit… [Circuit designed by David A. Johnson P.E., 10/1/03]

06/22/08

Universal LED Universial 3V Flasher

 

The circuit below will flash any LED color with fixed current pulses powered by a 3v battery. It uses a charge pump approach, which routes a constant current pulse through the LED, regardless of the LED voltage requirement. Using this approach, white and… [Circuit designed by Dave Johnson P.E., 6/1/08]

06/08/08

Dvd Recorder + TV Camera Make Cheap Data Logger

 

Sometimes recording the results of a test requires as much ingenuity as designing the product. Consider a situation which requires the collection of many different variables over a long period of time… [Hobby Circuit designed by David A. Johnson P.E., 4/2005]

06/01/08

Four Line Telephone Line Status Indicator

 

Many home businesses use multiple phone lines. This circuit gives you a visual indication when a line is in operation. The two AA battery cells should provide enough power for about one year of operation… [Circuit designed by Dave Johnson P.E., 2/11/07]

06/01/08

Wireless RFID Smart Key Detector

 

Many cars now use a key with an imbedded RFID microchip. The microchip is read when the key is inserted into the car’s ignition. The keys containing a microchip often look the same as most other keys. People duplicating car keys will often not know of the microchip. This circuit will light a LED if it detects a key containing a 125KHz RFID chip… [Hobby Circuit designed by Dave Johnson P.E., 6/1/08]

05/25/08

Classic Plus & Minus DC Power Supply

 

This is a classic example of a regulated DC power supply that produces both a positive 15v and a negative 15v from a 20vac wall adapter… [Circuit designed by Dave Johnson P.E., 10/16/05]

05/18/08

Video Signal Edge Enhancements

 

I designed this circuit many years ago, based on the claims that the technique would improve the quality of standard TV images. The circuit adds information to the edges of the objects and was reported to bring out more detail. After building and testing … [Circuit designed by David A. Johnson P.E., 1998]

05/11/08

Incandescent Lamp Inrush Current Limiter

 

This circuit limits the large inrush current often associated with large incandescent lamps. With the components shown the current is limited to 1 amp, but it could be scaled to any desired current… [Hobby Circuit designed by David A Johnson P.E., 7/9/06]

05/04/08

Light to Frequency Converter

 

This circuit uses a CMOS version of the classic 555 timer, to form a light intensity to frequency converter. A small PIN photo diode is used as the light detector. The pulses produced are short, so in some applications you may want to stretch them or fe… [Hobby Circuit designed by Dave Johnson P.E., 5/4/08]

04/27/08

Universal Flasher Using A Latching Relay

 

Latching relays are nifty devices. Most contain two separate coils. When a voltage is applied to one coil the relay latches in one state and stays in that state until voltage is applied to the unlatch coil. Since the latching and unlatching pulses only… [Hobby Circuit designed by Dave Johnson P.E., 10/16/05]

04/20/08

Harvest Earthworms with this Electrical Device

 

When I was a kid my engineer uncle built a circuit similar to the one below. My cousin and I used it to force earth worms out of the ground to be used for fishing. It worked like a champ. Please be careful since there is high voltage at the probe tips… [Hobby Circuit designed by David A. Johnson P.E., 4/2005]

04/13/08

Shadow Activated Motion Detector

 

The hobby circuit below works quite well in typical indoor room lighting. I would not recommend its use in direct sunlight. Two small PIN photodiodes positioned about one inch apart form a shadow detector. With no shadow cast on the devices, both devic… [Hobby Circuit designed by David A. Johnson P.E., 4/13/08]

04/05/08

N-Ch & P-Ch Transistor Form Push-Pull Driver

 

This circuit can produce high speed output signals with fast rise and full times. The unique change pump action allows the voltage of the upper P-ch device to range from millivolts to hundreds of volts. The output current is only limited by the rating of … [Circuit designed by Dave Johnson P.E., 1998]

03/30/08

0.5v to 6v Voltage Converter

 

Conventional silicon transistors just can't operate at voltages less than about 0.7v. Old germanium transistors could be used, but those are hard to find these days and most are rather large in size. Some new n-channel MOSFET devices with very… [Hobby Circuit designed by Dave Johnson P.E., 4/2005]

03/23/08

40Khz Voltage to Frequency Converter

 

This circuit was designed to frequency modulate a 40KHz carrier, using human voice frequencies. A common flip/flop is used at the core of the circuit… [Circuit designed by Dave Johnson P.E., 12/12/04]

03/16/08

32Khz Ultra Low Power Crystal Oscillator

 

I have used this circuit many times when I needed a low frequency reference, which did not draw much power. With the components show, the current from a 3v battery is less than 1.2 microamps… [Circuit designed by Dave Johnson P.E., 5/1/05]

03/16/08

Ultra Low Power 32Khz Crystal Oscillator

 

I have used this circuit many times when I needed a low frequency reference, which did not draw much power. With the components show, the current from a 3v battery is less than 1.2 microamps… [Hobby Circuit designed by David A. Johnson P.E., 5/1/05]

03/09/08

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… [Circuit designed by Dave Johnson P.E., 8/16/09]

03/02/08

1us Light Pulse Receiver

 

This circuit is designed to detect very weak light pulses lasting 1uS. It uses a tuned LC feedback network to provide high sensitivity while giving high ambient light immunity. A post voltage amplifier is included with a gain of about X20. The circuit is … [Hobby Circuit designed by Dave Johnson P.E., 1998]

02/24/08

Count Cars with Laser

 

This is an illustration how a laser could be used to count traffic and measure the speed of each car passing through the sensor area… [Hobby Circuit designed by David A. Johnson P.E., 1/7/07]

02/17/08

Wig/Wag LED Flasher

 

This simple circuit will flash two LEDs in an alternating fashion… [Circuit designed by Dave Johnson P.E., 4/2005]

02/03/08

Long Period Computer Watch Dog Timer

 

This circuit uses a simple 4060 IC oscillator/timer that is reset periodically by a computer. Should the computer fail to send a pulse, the output changes state. The time can easily be set from seconds to hours… [Circuit designed by David A. Johnson P.E., 12/26/98]

01/27/08

Ten Turn Pot Forms Frequency Readout

 

Let’s say you are designing a signal generator for a test fixture. To adjust the frequency you would like to use a ten turn pot with a nice dial mechanism attached as a frequency display. However, a display range of 0 to 9.99 would be impractical since th… [Hobby Circuit designed by David A. Johnson P.E., 1/27/08]

01/20/08

Xenon Lamp Flash Detector

 

This circuit uses a small 2.5mm square photo diode in conjunction with a 100mH coil to detect the short light flashes from a xenon lamp. The coil makes the circuit immune to normal room lights. Its 10mv sensitivity can detect light flashes from a range … [Circuit designed by Dave Johnson P.E., 2/15/02]

01/13/08

Wide Band Zero Cross Detector

 

This circuit was designed to convert a low amplitude 40KHz signal into a clean square wave signal. It will work with inputs as small as 5mv peak-to-peak or as large as 3 volts peak to peak. The input frequency can range from a few kilohertz to about 150… [Circuit designed by Dave Johnson P.E., 2/14/00]

05/08/05

Audio Signal Detector/Switch

 

For some audio systems it is desirable to have an automatic switch circuit turn on other devices, such as an amplifier, if an audio signal is detected on one line. The hobby circuit below will activate a transistor switch when it detects at least 50mv... [Hobby Circuit designed by David A. Johnson P.E., 5/1/05]

05/01/05

Convert AC to DC signal with is Full Wave Rectifier

 

This is a classic circuit that can accurately convert an AC signal to DC. 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. At 40KHz the input signal can be … [Hobby Circuit designed by David A. Johnson P.E., 5/1/05]


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