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Circuits of the Week - June to January 2014
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

06/29/14

High Impedance JFET Buffers

I have used these circuits many times. They are great when you need a low gain AC signal amplifier with a very high input impedance. It is good to beyond 50MHz… [Hobby Circuit designed by Dave Johnson P.E., 4/2005]

06/22/14

Solar Powered Infrared 1W Security Camera Illuminator

Many security cameras are equipped with an array of infrared LEDs to illuminate an area in front of the camera with invisible light for night time surveillance. However, there are times when a remote area, well away from the camera, needs additional infrared light. The circuit below uses a modified solar powered path light to bathe a fairly large area with 850nm infrared light… [Circuit designed by David A. Johnson P.E., 6/22/14]

06/15/14

LED Test Signal Generator -40Khz

This 40KHz crystal controlled oscillator circuit drives an infrared LED with powerful 40ma pulses. The circuit can be used to test optical communications circuits, designed to receive 40KHz modulated light signals… [Hobby Circuit designed by David A. Johnson P.E., 2001]

06/08/14

5+V Classic Linear Supply Using 6.3Vac Transformer

A classic method for producing a regulated +5v DC supply is shown below. This circuit consists of an iron core transformer, a bridge rectifier, a filter capacitor and a voltage regulator. Many people are tempted to use a very popular 6.3v transformer… [Hobby Circuit designed by David A. Johnson P.E., 10/16/05]

06/01/14

Advertising Badge with Flashing LED

I have seen numerous flashing light badges at trade shows and conventions. They are often handed out as gifts to promote some business. The devices often use inefficient circuits, which cause the battery power source to be quickly depleted… [Circuit designed by David A. Johnson P.E., 1998]

05/25/14

3v Powered Super Bright 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… [Hobby Circuit designed by Dave Johnson P.E., 4/2005]

05/18/14

Scare Off Pigeons

I have an area on my roof where a large flock of pigeons like to roost. Their nesting materials and droppings really make a mess. Often there is enough junk from them that it clogs my roof gutters. Perhaps one way to scare them away… [Hobby Circuit designed by Dave Johnson P.E., 4/2005]

05/11/14

Touch Alarm for Door Knob

I designed this hobby circuit many years ago and it seems be a very popular construction project. Many companies offer simple alarm devices for personal use in bedrooms or hotel rooms. A metal chain attached to a box holding the electronics is place… [Hobby Circuit designed by Dave Johnson P.E., 5/29/00]

05/04/14

Improved Solar Path Light

 

The circuit below was designed to replace the guts of an existing path light assembly with one which has a much better performance.  I suggest using a quality 10 cell solar panel with at least a 4 square inch area. Buy a panel with solar panels that fills… [Hobby Circuit designed by David A. Johnson P.E., 3/13/11]

04/27/14

Water Cistern Level Alarm

I received an unusual request from a Discover Circuits visitor a while back. The guy had a deep water cistern which has to be filled when nearly empty. The cistern was described as a deep cylindrical tube, holding 1000s of gallons of drinking water. Periodically, the cistern is filled from a tanker truck, when the water inside reached a critical level. The guy wanted a simple circuit he could build which would flash an LED when the water level reached a certain depth. That flasher would tell him it was time to order more water… [Circuit designed by David A. Johnson P.E., 4/27/14]

04/13/14

LED Flasher-Wireless Circuit

When you would like to flash a LED through a thick plastic case without wires, use this hobby circuit. It performs this feat. It draws about 300ua of average current at 5v. It can be powered from a supply ranging from 3v to 6v… [Hobby Circuit designed by Dave Johnson P.E., 4/2005]

04/06/14

Remote Relay Driver

I received a call from a Discover Circuits visitor who wanted a simple way to turn on an outside lighting system using a low voltage control line. I suggested the simple circuit below. The circuit is powered by a 9v battery and uses a very efficient latching relay. Two low voltage wires lead from the a toggle switch to the remote relay. A toggle switch selects either the on or off condition of the relay. The relay contacts are rated at 10 amps, so the small relay can handle a sizeable amount of power… [Circuit designed by Dave Johnson P.E., 4/6/14]

03/30/14

Voltage Converters -0.5 to 6 Volts

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

03/23/14

High DC Current Indicator

A while back, someone requested a simple method to indicate when 20 to 50 amps of DC current was flowing through a wire. The circuit below works well and is about as simple as you can get. A sensitive reed relay… [Hobby Circuit designed by Dave Johnson P.E., 4/2005]

03/16/14

Battery-Powered Freezer Alarm

A typical freezer can contain hundreds of dollars with of food. If the device becomes unplugged or if the door is left open too long, the food inside can quickly become spoiled. The hobby circuit below is a battery powered alarm system, which is attached… [Hobby Circuit designed by Dave Johnson P.E., 7/20/06]

03/09/14

Battery-Powered LED Flashing Advertising

A single lithium battery will provide months of continuous LED flashing. It also has a tiny push-button switch to turn on and off the light flashing… [Hobby Circuit designed by Dave Johnson P.E., 4/2005]

03/02/14

Touch Activated Switch Needs 1.5 Volts

A single 1.5v silver oxide button cell powers this complete touch activated switch circuit for 5 years. It features both a normally open and a normally closed set of solid state switch thermals. It also has an adjustable sensitivity, which can be set… [Hobby Circuit designed by Dave Johnson P.E., 1/18/02/]

02/23/14

Home Made Geophone Detects Foot Steps

A home made geophone is made from a strong magnet, a coil of wire and a rubber band. The circuit is sensitive enough to detect the vibrations of a nearby foot stomp. It could be used as an earthquake detector… [Hobby Circuit designed by David A. Johnson P.E., 4/2005]

02/16/14

CMOS Logic Inverter Amplifier Characteristics

This chart shows some of the measured characteristics of three different unbuffered CMOS logic gates, used as voltage amplifiers. … [Circuit designed by David A. Johnson P.E., 10/16/05]

02/09/14

Candle Power

A single lit candle in conjunction with an inexpensive thermopile voltage generator provides enough power to charge a small battery when used with a small low voltage DC to DC converter… [Hobby Circuit designed by Dave Johnson P.E., 2012]

02/02/14

Characteristics of CMOS Logic Inverter Amplifiers

This chart shows some of the measured characteristics of three different unbuffered CMOS logic gates, used as voltage amplifiers. … [Hobby Circuit designed by Dave Johnson P.E., 10/16/05]

01/26/14

Charge Coupled MOSFET Relay #2

This circuit acts as an AC/DC relay with a rating up to 50 volts peak and up to 10 amps of current. The differential oscillator supplies voltage to the gates of the two FETs with good isolation will drawing only 1.5ma of current… [Circuit designed by Dave Johnson P.E., 10/16/05]

01/19/14

Service Request Beeper

A single press of a pushbutton switch turns on a beeper for one second but can’t be activated again for 60 seconds… [Hobby Circuit designed by David A. Johnson P.E., 9/25/05]

01/12/14

Watchdog Timer set for 36 Hrs

A watchdog timer is often used in computer systems to detect when a computer is hung up and is no longer running properly. If it does become locked up and is not able to supply a reset pulse to the circuit within a defined time frame the timer times out a… [Hobby Circuit designed by David A. Johnson P.E., 4/11/10]

01/05/14

Voltage Boost to Series Regulator provided by Capacitor

This circuit adds some capacitors and diodes to a traditional transformer type series regulator circuit to extend the normal operating range. It can insure regulation during low line voltage conditions or it can squeeze a few more watts out of a plug-in-t… [Hobby Circuit designed by Dave Johnson P.E., 1998]


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