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Circuits of the Week - December to July 2011
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


Three-Minute Closet Light


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… [Circuit designed by Dave Johnson P.E., 12/25/11]


Zero-Cross Detector-Wide Band


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… [Hobby Circuit designed by Dave Johnson P.E., 2/14/00]


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… [Circuit designed by David A. Johnson P.E., 12/11/11]

12/04/11 Sweeping Siren Alarm -3 volts
  A while back I was challenged by a visitor to the website.  He needed a very loud sweeping siren type audio sound generator powered by 3v.  He tried some of the commercial sirens but they were not very loud when powered by 3v.  He also said that those… [Hobby Circuit designed by David A. Johnson P.E., 6/29/08]


Capacitance Proximity Switch -3 volts


This circuit was designed to provide a touch activated switch function without an external power supply. It draws so little power that a single 3v battery will operate the circuit for many years. It is discussed in more detail in the section on Capacita… [Hobby Circuit designed by David A. Johnson P.E., 2/12/02]


Corona Monitor


The circuit is powered by +5v and has an on-board voltage regulator as a reference voltage.  The current from a reversed biased photodiode is converted into a voltage using an op amp circuit.  That output voltage is fed to a voltage comparator circuit.  Included is an adjustment in the current to voltage converter, to calibrate the installed circuit… [Circuit designed by Dave Johnson, 2/1/10]


Laser Light Detyector -10Mhz to 20Mhz


This circuit was originally designed to detect laser light pulses for an optical Ethernet communications system. It has good ambient light immunity… [Hobby Circuit designed by Dave Johnson P.E., 6/14/00]


40Khz Laser Burst Detector


This circuit was originally designed to detect weak flashed of laser light bounced off of a fabric video projection screen. It was used as part of a firearm training system. It generates a 100mS output pulse whenever it detects a 3ms to 5ms-laser burst,... [Circuit designed by David A. Johnson P.E., 6/13/00]


Band Drummer’s Microphone Switch


I received 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… [Hobby Circuit designed by Dave Johnson P.E., 10/30/11]


Precision Low Capacitance Meter


This circuit was originally designed to measure the volume of fluid inside a medical syringe. As designed, it produces a zero to 5 volt output, corresponding to a capacitance change of about 10 picofarads… [Hobby Circuit designed by Dave Johnson P.E., 5/1/10]


24v NiMH Battery Charger Using a 12v Solar Panel


A while back I received 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… [Hobby Circuit designed by Dave Johnson P.E., 10/16/11]


Nitrogen Spark Generator


Nitrogen or air sparks are very powerful light sources that produce flashes that last only a few nanoseconds. This line powered circuit generates a continuous series of very small sparks across electrodes with a 0.05 inch gap… [Hobby Circuit designed by David A. Johnson P.E., 1998]


Doorbell sounds in Basement


This circuit will activate a beeper alarm whenever the AC line voltage is less than 100vac… [Hobby Circuit designed by David A. Johnson P.E., 4/2005]


Infrared TV/VCR Remote Booster


This circuit will boost the signal from any infrared TV or VCR remote, extending the range by a factor of 3X… [Hobby Circuit designed by David A. Johnson P.E., 5/1/05]


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… [Hobby Circuit designed by Dave Johnson P.E., 9/18/11]


Pump Motor Monitor


This circuit can turn on a light or sound a beeper, whenever an AC line powered pump motor turns on. It is designed to detect the motor current using a small inductor placed on the outside of the motor’s power cable. No direct wire contact is needed… [Circuit designed by David A. Johnson P.E., 9/11/11]


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… [Hobby Circuit designed by David A. Johnson P.E., 9/4/11]


Temperature Sensor MOSFET
   Recently I was asked by a company to discover why one of their products failed to operate properly in cold temperatures.  Two n-channel MOSFETs and a few resistors are wired to form a flip/flop latch circuit.  Each time the pushbutton switch is pressed, the circuit changes state and remains in that state until another activation of the switch. … [Circuit designed by Dave Johnson P.E. for DC Magazine, 12/1/10]


Camera Shutter Time Measurement


A while back I received 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… [Hobby Circuit designed by David A. Johnson P.E., 8/14/11]


AC Line Powered LED Flasher


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… [Hobby Circuit designed by Dave Johnson P.E., 8/7/11]


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… [Hobby Circuit designed by Dave Johnson P.E., 7/31/11]


TV-VCR Light Source 40Khz Repeater


This circuit is designed to be placed directly in front of a standard TV or VCR remote. The exiting light pulses produced by the circuit match the pulses from the remote but are about 10 times more powerful… [Hobby Circuit designed by David A. Johnson P.E., 07/24/11]


60hz Clock Generator is Line Powered


This circuit is connected to the 120vac power line and transfers 60Hz clock pulses to a logic circuit. The optoisolator used provides 5000 volts of isolation between the power line and the logic side of the circuit… [Hobby Circuit designed by Dave Johnson P.E., 4/2005]


Generate 0.5v Negative Supply


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… [Hobby Circuit designed by David A. Johnson P.E., 7/10/11]


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?… [Hobby Circuit designed by Dave Johnson P.E., 7/3/11]

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