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Circuits of the Week - June to January 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


Capacitance Touch Activated Momentary Switch -5v


This circuit is discussed in more detail in the section on Capacitance Proximity Switch Technology. The circuit is powered from a standard +5v supply. It has both a source and sink output that change state whenever a metal button connected to the circuit … [Hobby Circuit designed by Dave Johnson P.E., 1998]


Door Knock Beeper


In some apartment buildings and homes, not equipped with a door bell, it may be hard to hear someone knocking on the front door. The circuit below provides a means to activate a loud beeping sound inside, whenever someone knocks on the door… [Circuit designed by David A. Johnson P.E., 4/2005]


Alarm sounds when Water is Detected


This simple circuit sounds a beeper when its electrodes detect water. It is powered by a single 1.5v N cell. A small 1.5v button battery will also work… [Circuit designed by David A. Johnson P.E., 1998]


Self Powered AC Indicator


Often there is a need to monitor when AC current flows into a load. The circuit below works well for this application. It uses an off-the-shelf torrid type AC-1020 current transformer from Triad, available from Mouser. A wire carrying the AC current is… [Hobby Circuit designed by Dave Johnson P.E., 12/30/07]


Variable AC Power Supply


In electronics, it is often handy to have a fully isolated variable AC power supply. With such a device you can safely test various AC powered circuits and even probe around in the circuit with an oscilloscope. The hobby circuit below was designed to pr… [Hobby Circuit designed by David A. Johnson P.E., 5/1/05]


Switches AC or DC power using a 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 c… [Hobby Circuit designed by David A. Johnson P.E., 12/27/09]


LED-Flasher Needs Only 1.5 Volts


Judging from the numerous references I have received, this electronic circuit, hobby circuit seems to be rather popular. Many published circuits which flash LEDs need 3 or more volts. This electronic circuit uses only a single inexpensive C-MOS IC and… [Hobby Circuit designed by David A. Johnson P.E., 4/2005]


Solid State Relay that is Bidirectional


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 produc… [Hobby Circuit designed by Dave Johnson P.E., 5/1/09]


CMOS Schmitt Trigger IC Makes VCO


By changing the supply voltage fed to a classic 4584 Schmitt trigger type oscillator, the oscillator frequency can be changed over a range of 50:1. A 74HCU04 inverter is used at the output of the 4584 to maintain a constant TTL logic level signal… [Circuit designed by Dave Johnson P.E., 12/1/02]


Super Bright 3v Powered LED Flasher


I have posted several kinds of LED flashers over the years.This is yet one more flasher, designed to flash any high power LED of any color. It hits the LED with a 25ms 250ma pulse once every 2 seconds.This works out to an average current about 3ma. It… [Circuit designed by David A. Johnson P.E., 4/2005]


Very Low Power Astable Multivibrator


This classic circuit draws only 200 nanoamps from a 1.5v supply… [Circuit designed by David A. Johnson P.E., 5/29/00]


Bad Light Bulb Indicator


This request came from a Discover Circuits visitor. He wanted to turn on a LED indicator light whenever a circuit detected a burned out tail/brake light on a car or truck. The circuit shown below uses a transistor switch, to detect current flowing through a low value resistor… [Circuit designed by David A. Johnson P.E., 12/17/06]


Battery Eliminator


If you use a 1.5v or a 3v battery powered MP3 player for long periods of time, you may get tired of constantly changing batteries. Some older players do not work will NiMH rechargeable batteries so changing out batteries can get expensive. The circuit be… [Circuit designed by David A. Johnson P.E., 6/1/05]


Under/Over Voltage Alarm on AC Line


Power lines, which deviate much beyond normal voltages can damage expensive electronic equipment. The circuit below sounds an alarm whenever the line voltage is higher or lower than normal. I set the alarm limits at about +-15% from standard levels… [Circuit designed by David A. Johnson P.E., 5/1/05]


Pushbutton Controlled Up/Down Ramping Light


There are times when you want a lamp to gradually change from dark to light and from light to dark, with each push of a pushbutton switch. The circuit below performs this control function using a flip/flop and a classic pulse width modulation or PWM... [Circuit designed by David A. Johnson P.E., 3/22/09]


Timer for Darkroom Camera Shutter


This circuit was designed to control a film exposure shutter for a darkroom. It has 8 time steps ranging from 0.35 seconds to 4 seconds. It is activated by a foot switch and draws power from an external 12-volt DC supply… [Hobby Circuit designed by Dave Johnson P.E., 6/11/00]


Ultra Low Current Oscillator


Here is a challenge. Design an astable oscillator which draws only a few nanoamps of current from a +3v supply. I gave this some thought and came up with the circuit below. I used some pretty standard parts except for three surface mounted 1000M resistors I had on hand. The oscillator frequency measured a low 1Hz frequency and the average current was a very low 3 nanoamps. If I had some higher resistors values handy, I think I could have gotten the current down below one nanoamp… [Circuit designed by Dave Johnson P.E., 6/1/05]


Alarm for a Chain Link Fence


Long stretches of steel fence can be monitored using the simple circuit below. An insulated wire is woven through the links in the fence. Where the fence begins, the circuit is connected to an earth ground and the security wire. At the end of the fence,… [Hobby Circuit designed by Dave Johnson P.E., 10/16/05]


Cheap Low Power 3v 2hz Oscillator


This is yet another method to generate low frequency output pulses. The circuit uses an inexpensive under-voltage monitor IC and a couple of cheap MOSFETs. It could easily be modified to produce pulse frequencies from about one cycle per minute to a maxim… [Hobby Circuit designed by Dave Johnson P.E., 6/2000]


Ocean Wave Power Generator Demo Unit


A while back, a Discover Circuits visitor wanted a simply way to demonstrate how electricity could be generated from the up and down movement of ocean waves. The system was to be part of a science fair project, showing different renewable energy sources.… [Hobby Circuit designed by David A. Johnson P.E., 1/1/12]


DC Current Indicator 6


The circuit below is a simple way to indicate when DC current is flowing through a wire. The circuit is designed so it will turn on a LED indicator light, whenever the voltage drop across a shunt resistor exceeds about 0.1 volts. The value of the resist… [Circuit designed by David A. Johnson P.E., 10/16/05]


Ideal Blocking Diode Circuit for Photovoltaic Solar Panels


Most photovoltaic solar panels are used to charge a battery during the daytime. Nearly all panels come equipped with a blocking diode. The diode prevents DC current from flowing backwards from the battery bank into the panel at night. The usual blocking… [Circuit designed by David A. Johnson P.E., 10/16/05]


Line Status Indicator for 1 Telephone Line


This circuit monitors one phone line… [Hobby Circuit designed by David A. Johnson P.E., 6/10/00]


Click Sound Generator


Often in computer controlled systems, you would like to generate a “click” sound whenever a button is pressed. This provides the user with audible feedback that the pushbutton press has been acknowledged. The two circuits below generate such a sound. A click sound is generated each time the logic input swings from a logic low to high condition… [Circuit designed by Dave Johnson P.E., 4/2005]


Isolated Remote Switch Circuit


There are many applications when a remote switch’s on/off status needs to be monitored. Reed relay sensors, toggle control switches and security alarms contacts are just a few examples. Routing a switch directly to a computer system hundreds of feet away… [Circuit designed by Dave Johnson P.E., 4/2005]

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