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Circuits of the Week - December to July 2007
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


Hour Meter triggered by Machine Vibration


This hobby circuit allows an off-the-shelf battery powered digital hour meter to be turned on and off, according to a machine's operation, without requiring a direct electrical connection to the machine… [Hobby Circuit designed by Dave Johnson P.E., 1998]


Ramping LED Driver


The circuit below was designed to drive a LED with an intensity ramping mode. Two 555 timers generate two different triangle waveforms. The upper device generates a 10KHz signal while the lower unit produces a 1Hz signal. The two signals are fed to a v… [Circuit designed by David A. Johnson P.E., 12/23/07]


PWM Circuit for Motor Speed Control


Sometimes you want to slow down a brush type DC motor. The most efficient way to do this is with a pulse width modulation (PWM) technique. The hobby circuit below can operate from about 3 volts to 15 volts. The frequency is fixed at about 2KHz but the puls… [Hobby Circuit designed by Dave Johnson P.E., 12/16/07]


RS-232 Serial Interface Status Indicator Circuit


I designed this circuit many years ago and have used it several times since. It uses a single logic IC to indicate the TXD and RXD line activity of a typical RS-232 serial data communications link. The thresholds are set at plus 3 volts and minus 3 volts. Two dual color LEDs are used as indicator lights… [Hobby Circuit designed by Dave Johnson P.E., 12/9/07]


Audio Frequency Digital Noise Generator


When you need to test an audio circuit with broadband noise, this circuit works great. It uses just three inexpensive C-MOS ICs that generate a series of output pulses whose widths vary randomly. I included a level control pot… [Circuit designed by David A. Johnson P.E., 1998]


Solar Cell Test Fixture


If you are considering building your own solar panel, you might try building the solar cell tester shown below. When constructing a solar panel it is important to make sure that all the solar cells used in the panel be closely matched for their short circ… [Hobby Circuit designed by David A. Johnson P.E., 11/25/07]


Variable Isolated AC Voltage Spans 0Vac to 280Vac


I designed and built this circuit about 25 years ago. It came in handy for many projects that were powered from 120 VAC, 240 VAC and 277 VAC. It provides complete isolation from the power line. It uses two 1:1 isolation transformers that are wired in parallel… [Circuit designed by Dave Johnson P.E., 1998]


Inductor Self-Resonance Tester


If you want to know what the self-resonance frequency is of an inductor, this circuit will help. It adds only a few picofarads of parallel capacitance to the inductor under test. With the aid of an oscilloscope and a signal generator this circuit will allow you to measure resonant frequencies from kilohertz to over 10 megahertz… [Circuit designed by David A. Johnson P.E., 1998]


Very Loud Beeper Circuit


This circuit generates a continuous tone instead of a pulsing one. The circuit delivers about 110db (12 inches away) from a 9v battery using a single inexpensive C-MOS IC. An off-the-shelf piezoelectric beeping device is driven at resonance to insure maxim… [Hobby Circuit designed by Dave Johnson P.E., 2/26/06]


Voltage to Frequency Converter -40Khz


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… [Hobby Circuit designed by David A. Johnson P.E., 12/12/04]


10mhz to 20mhz Laser Light Detector


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


Pulsed LED Test Circuit


This circuit is designed to test visible and infrared LEDs in pulsed mode operations. It can drive the LED with peak currents in excess of 10 amps. A light detector nearby can monitor the response time and intensity of the LED under test… [Circuit designed by Dave Johnson P.E., 12/17/06]


Electric Field Disturbance Monitor -Front End + Power Supply Circuit


This schematic is the power supply and front-end sections of my field monitor, "Electric Field Disturbance Monitor". The system can detect human and animal motion by the electric fields they disturb… [Circuit designed by Dave Johnson P.E., 3/26/02]


Simple 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… [Circuit designed by Dave Johnson P.E., 1998]


Two Transistor Oscillators


This page has two unusual two-transistor oscillators. I set the component values for a low frequency application. Both circuits draw only about 1 microamp of current… [Circuit designed by David A. Johnson P.E., 9/16/07]


Pulse Period to Voltage Converter


This is a test circuit converts a square wave input signal into a voltage. But, the voltage produced is proportional to the time between edges (period) of the signal, not the frequency. The range is from 100uS to to 10mS, which produces a voltage from 100mV to 10 volts. Other scale factors are also possible… [Circuit designed by Dave Johnson P.E., 1998]


Precision AC Peak Detector


This unique circuit uses a very inexpensive voltage comparator to form a peak detector. The DC voltage produced tracks the positive peak of the input signal. It works from about ten millivolts to about 10 volts peak to peak. The maximum frequency is abo… [Circuit designed by Dave Johnson P.E., 2001]


CMOS Inverter Parallel LC Oscillator


I have used this parallel resonant LC oscillator circuit countless times. The oscillator frequency is determined by the inductor and capacitor values. I have shown an adjustable inductor to make it easy to set the frequency to a specific value… [Circuit designed by David A. Johnson P.E., 2001]


D-Flip/Flop One Shot Circuit


Yes you can turn flip/flop ICs into low current oscillators. This schematic shows you how… [Circuit designed by David A. Johnson P.E., 4/2005]


Optical RFID Test Circuit


I designed this test the concept of using light techniques to send identification data instead of RF. A more detailed discussion on this scheme can be found in the Imagineered new products section… [Circuit designed by David A. Johnson P.E., 1998]


Micro Power Over-Temperature Alarm


The circuit is powered for years by a single 3 volt lithium battery. It sounds an alarm when the temperature exceeds a certain point. With some minor changes the circuit could also be configured for an under temperature (freeze) alarm. The circuit uses a c… [Circuit designed by Dave Johnson P.E., 1998]


CMOS Inverter 125khz LC Oscillator


This circuit uses a single CMOS inverter to form a series resonant LC oscillator. The values shown set the oscillation at about 125KHz but the other frequencies are possible by changing the main LC values… [Circuit designed by David A. Johnson P.E., 3/12/02]


125KHZ Medium Power Ocsillator


This circuit is similar to MEDIUM POWER 125KHZ OSCILLATOR but adds even more inverters in parallel to deliver yet more power. The values shown are for 125KHz… [Hobby Circuit designed by David A. Johnson P.E., 3/12/02]


Precision Ultra Lower Power Oscillator


This circuit works much like the classic 555 timer, but draws only about 1.5 microamps from a 3 volt battery. It is highly stable under varying temperature and supply voltages… [Circuit designed by Dave Johnson P.E., 5/29/00]


Clear Fluid Monitor


An optical technique seemed like the best choice for this application. The simple circuit below works well but only when the light detector can be shielded from bright ambient light. A visible red LED launches light pulses through the side of the clear tubing and emerges at the opposite side. A phototransistor forms a light detector. The circuit takes advantage of the lens effect from any clear fluid inside the tubing. When there is only air in the tubing, the light scatters and spreads out quickly before it reaches the phototransistor… [Circuit Solution designed by David A. Johnson P.E., 5/1/05]


UL Ground Resistance Tester


UL requires that an electrical system using a metal chassis earth ground connection have a resistance of less than 0.1 ohms. This resistance must be tested with actual line current in excess or 10 amps. The circuit shown uses a couple of LEDs to indicate if in fact the resistance is less than 0.1 ohms… [Circuit designed by Dave Johnson P.E., 7/10/06]

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