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Circuits of the Week - December to July 2010
Last Updated on:  08/23/2015 07:05 AM

2015:  Jan-June   July-Dec     2014:  Jan-June    July-Dec     2013:  Jan-June    July - Dec   
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 2006:  Jan-June     July-Dec         2005:  May-Dec


24vac Powered Day/Night Switch


Many low voltage outdoor lighting systems use 24vac to power the devices. The low voltage is much safer to use outside than standard 120vac or 240vac power systems. The circuit below is designed to turn off power to outdoor lights during daylight and turn them back on at night. The circuit uses a phototransistor to detect daylight. I would suggest you use a phototransistor which has a built in daylight filter. These are colored a deep purple. The filter helps insure the circuit reacts only to sunlight… [Circuit designed by Dave Johnson P.E., 12/26/10]


LED Driver using Constant Current


LEDs are current driven devices. The light they produce is proportional to the current flowing though them. One way to maintain a fairly steady light output is to keep the current through the LED fixed, even as the supply voltage may change… [Circuit designed by Dave Johnson P.E., 4/2005]


Battery Free LED Flasher


LED flashers have been used for a wide variety of applications. Some flasher units have been used in food and hardware stores, to draw attention to sale items and are often attached to boxes which dispense product discount coupons. Typically, as the illu… [Circuit designed by David A. Johnson P.E., 4/2005]


Pushbutton Oneshot & Latch


This circuit uses a single IC to convert a noisy pushbutton switch signal into a clean pulse or a sustained push on-push off signal. It can operate from 3v to 18v… [Circuit designed by Dave Johnson P.E., 3/26/00]


Active AC Power Line Indicator


The circuit below is designed to detect when a power cable is active (hot) . When the flat sensor end of the device is held near an active AC line, the bright green LED on the top of the housing will glow. It is powered by a 3v CR2032 lithium coin cell. I… [Circuit designed by Dave Johnson P.E., 5/2/05]


Closet Light with Automatic 3-Minute Timeout


The circuit below is powered by three 1.5v alkaline AA cells. With a finger tap to the pushbutton trigger switch, a cluster of 6 wide angle white LEDs is turned on. The lights remain on for about 3 minutes, then will turn off. The circuit’s standby current is only a few microamps. A set of fresh batteries should last at least 200 light applications. The circuit uses a Schmitt trigger inverter and two transistors. When the pushbutton switch S1 is pressed, the 10uF capacitor C1 is discharged. When the switch is released, the capacitor is charged through R1. With the capacitor C1 discharged, the Schmitt trigger output swings to 4.5v, which turns on transistor Q2. The combination of R4 and Q1 form a constant current control circuit. The current is set about 15ma per LED. When the capacitor charges up to about 2.3v, the Schmitt trigger changes state, turning off the LEDs… [Circuit designed by Dave Johnson P.E., 11/21/10]


Improve Response Time of a Photo Transistor Circuit


This page shows different ways to improve the response time of a photo transistor circuit… [Hobby Circuit designed by David A. Johnson P.E., 11/1/10]


Piezoelectric Tap Switch


This circuit uses a flat piezoelectric wafer, glued inside a plastic box, as a finger tap sensor. With each tap of a finger to the box holding the wafer, the circuit turns on and off AC or DC power to an external device. The circuit is powered by a 9v... [Circuit designed by David A. Johnson P.E., 11/7/10]


Computer Controlled PWM Circuit


There are times when you wish to control a pulse width modulation (PWM) signal with a DC voltage, ranging from 0v to 5v. This voltage is often created by a computer interface. The circuit below performs this voltage to PWM conversion. PWM signals are ofte… [Circuit designed by Dave Johnson P.E., 10/31/10]


Very Loud Pulsating Beeper


Many battery powered alarms draw excessive current when operating. Some of the most attention-getting noise makers, such as the unit shown below, use piezoelectric sound generators with a frequency sweeping tone. When operating with a 9v battery, these units can often draw 100ma or more. In many applications, a continuous alarm sound is not needed. Locating a balloon or a rocket are examples where you want a loud sound but a low average power drain. To lower the average current, the device can be pulsed with short 0.05 second chirps, once every two or three seconds. This lowers the average current by a factor of 60. So, instead of drawing 100ma, by chirping the alarm, the current is lowered to just 2ma. The circuit below generates the pulse train needed for these short but very loud chirp sounds… [Circuit designed by David A. Johnson P.E., 10/24/10]


Motion Alarm


Using a piezoelectric device, this circuit will activate a beeper whenever the circuit is moved. It could be used as an earthquake alarm… [Circuit designed by Dave Johnson P.E., 9/4/05]


Indicator for Bad Light Bulb


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… [Hobby Circuit designed by Dave Johnson P.E., 12/17/06]


Solar-Powered Pump Motor Controller


A Discover Circuits visitor had a problem. He needed a simple on/off controller for his solar powered water pump. His system used two 12v 50 watt solar panels wired in series. The power from the solar panels drove a submerged water pump. However, duri… [Circuit designed by David A. Johnson P.E., 10/3/10]


Ramping Lamp Driver


This circuit drives any 12v lamp, incandescent or LED type, in a ramping fashion. Each press of the button toggles the lamp from a fully off mode to a fully on mode. But, instead of an instantaneous lamp intensity change, the power is slowly ramped up a… [Hobby Circuit designed by Dave Johnson P.E., 9/26/10]


Alarm Discriminator + Battery Monitor of the Electric Field Disturbance Monitor


This schematic is the alarm discriminator plus the battery monitor of my "Electric Field Disturbance Monitor". The system can detect human and animal motion by the electric fields they disturb… [Hobby Circuit designed by Dave Johnson P.E., 3/26/02]


Accurate One Shot Pulse Generator


This timer circuit draws only 3ua of current, and can be powered by DC supplies ranging from 3v to 15v. The circuit produces accurate pulses whose time can easily be adjusted, based on the R1 x C1 time constant. The time should not change,… [Circuit designed by Dave Johnson P.E., 5/1/05]


Alternating CW & CCW Motor Driver


A while back a company needed a motor controller for a custom door latching system. The system used a small brush motor with a speed reduction gear to both latch and unlatch the door. When pulsed for about 250ms in the clockwise direction, the motor would unlatch the door. When spun in the opposite direction for 250ms, it would latch the door. The circuit below performs this function. A single low power pushbutton is used to alternate between the two directions. The circuit is powered by 4.5v using a battery made from three alkaline AA cells. The motor draws about 250ma. The battery should provide many years of service… [Circuit designed by Dave Johnson P.E., 9/5/10]


Low Power Amp Monitors Light


The circuit below takes advantage of a low power op amp with a “rail to rail” feature. The single resistor R1 defines the light sensitivity of the circuit. The output voltage is directly proportional to the intensity of light striking the small photodiode… [Circuit designed by Dave Johnson P.E., 8/29/10]


LM317 adjustable voltage regulator used in Battery Eliminator Circuit


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… [Hobby Circuit designed by David A. Johnson P.E., 6/1/05]


3v Battery Powered White LED Driver


The circuit below will drive any LED color, including white LEDs, using a DC power supply ranging from 2v to 3.3v. White and #000080 LEDs normally require over 3v, so the circuit below uses a charge pump scheme to deliver a constant LED light intensity… [Circuit designed by Dave Johnson P.E., 8/15/10]


Water Seepage Alarm


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… [Hobby Circuit designed by David A. Johnson P.E., 1998]


Momentary Operation Touch Switch


This simple circuit uses a single IC to form a nice touch switch circuit. A single transistor forms the remote active switch sensor. Multiple switches can be wired in parallel. The switch circuit can be located about 500 feet from the control circuit… [Circuit designed by David A. Johnson P.E., 2003]


5Mhz Broad Band Optical Fiber Receiver


This circuit is a simple broad band light detector that uses a very inexpensive IC and a PIN photodiode that is packaged for use with plastic optical fibers. It has a bandwidth from 1KHz to over 5MHz. It is great for experimenting with various modulated … [Hobby Circuit designed by Dave Johnson P.E., 1998]


LED Flasher-Wig/Wag


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


Through Glass Pushbutton Switch


This simple technique allows a mechanical pushbutton switch assembly to activate a reed type relay through a pane of thick glass. Pushing the button moves a magnet near the reed relay, closing the relay contacts… [Hobby Circuit designed by Dave Johnson P.E., 2001]


Water Level Monitor


This system operates much like a classic float switch but without any moving parts. The illustration shows how a system using two copper coils sealed inside a plastic pipe, can detect the level of water outside the pipe.[Hobby Circuit designed by David A. Johnson P.E., 10/16/05]

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Imagineering Ezine    Discover Solar Energy Dave Johnson & Associates Faraday Touch Switches

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