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Circuits of the Week - December to July 2008
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


Remote LED Indicator Light


There are times when you would like to transmit a signal from one LED indicator light to second LED at another location. The circuit below works well for this application. It takes advantage of the fact that the internal infrared LED inside an opto-isol… [Circuit designed by Dave Johnson P.E., 12/28/08]


Ultra Low Power LED Flasher


The efficiency of some newer LEDs is amazing. Some of the latest green LEDs can launch blinding light with just one milliamp of current. I take advantage of one of these newer devices in the circuit below. The flashing circuit uses a classic multivibra… [Hobby Circuit designed by David A Johnson P.E., 12/22/08]


DC Current Indicator #4


The circuit below uses some common components to turn on an LED whenever DC current above a certain level is detected. The circuit uses a very popular LM393 dual voltage comparator from National Semiconductor and a common 1N4148 signal diode. The diode … [Circuit designed by David A. Johnson P.E., 10/16/05]


High Intensity Line Powered LED Flasher


There are times when you would like to generate an intensely bright flash of light using electrical power from the AC line. The circuit below can drive a one to three watt power LED with about 350ma of peak current. Since the circuit drives the LED with… [Circuit designed by Dave Johnson P.E., 4/2005]


Reduced Power Sequential LED Flasher


The 74HC4017 is a neat decade counter which has 10 decoded outputs. When supplied with a low frequency clock and with a LED attached to each of the outputs, an interesting ten step sequential flasher effect can be produced. The LEDs can be configured in… [Hobby Circuit designed by Dave Johnson P.E., 11/30/08]


Ideal Diode Function


This circuit uses a low power op amp and a p-channel FET to form a diode function with a very low 0.05 volt voltage drop. With the selected FET, the circuit can handle up to 2 amps of current. Higher currents are possible with a FET with a lower channel… [Circuit designed by David A. Johnson P.E., 1/22/08]


White LED Night Light


As many of you know, I have a pet peeve with poorly made LED night lights. Often, the light from the LED quickly fades, so within months, the light is useless. I have posted several versions of modified night lights using higher quality components… [Circuit designed by Dave Johnson P.E., 5/1/05]


Miniature Pushbutton On/Off Switch


In many battery powered devices, you would like to turn the circuit on and off by pressing a small pushbutton switch. One press turns the device on and second press turns it back off. The flip/flop circuit below uses a minimum number of parts to perform… [Hobby Circuit designed by Dave Johnson P.E., 10/16/05]


LED Driver-Wireless


The circuit solves this problem by sending power to the LED through the plastic, using a magnetic coupling technique. The circuit below can route power through plastic enclosures as thick as ¼ inch. The circuit will not work through metal boxes… [Hobby Circuit designed by Dave Johnson P.E., 4/2005]


Power Reminder Beeper Circuit


This hobby electronic circuit produces a short beep once each minute. It is powered directly off the 120vac power line to remind you that a device connected to the circuit is turned on. It is simple enough to be packaged inside a small plastic box… [Hobby Circuit designed by Dave Johnson P.E., 10/26/08]


Test Life of White LEDs


There are a lot of white LED driver circuits floating around. A popular circuit called the “Joule Thief” can drive one white LED from a single 1.2v or 1.5v battery cell. Most of these circuits use one or two transistors to form a voltage boost circuit… [Hobby Circuit designed by David A. Johnson P.E., 10/19/08]


Three-Volt Low Battery Voltage Flasher


Many battery powered devices use two AA alkaline cells. Often you will not know when it is time to replace the batteries until the device powered by them actually stops operating. The hobby circuit below can be connected to a 3v battery, to give you som… [Hobby Circuit designed by Dave Johnson P.E., 10/12/08]


Very Low Voltage Oscillator Circuit


OK, you may not ever need an oscillator to function using a power supply voltage below one volt, but if you ever do need one, here is one way to do it. Getting any electronic circuit to operate below one volt is a real challenge. Typical silicon bipolar… [Hobby Circuit designed by Dave Johnson P.E., 10/5/08]


Watt-Hour Meter Monitor


Have you ever wondered how much power your house or apartment is using? Most watt-hour meters are like the one shown below. Some newer ones have a digital display but there millions of these older models still in use… [Hobby Circuit designed by David A. Johnson P.E., 9/28/08]


Solar Powered Lithium Ion Battery Charger


The circuit below feeds a controlled current and voltage to a 3.6v lithium ion battery. The current is limited to 300ma and the voltage is limited to 4.2 volts. The circuit uses a LTC1734 IC from Linear Technology. No diode is needed between the circuit a… [Hobby Circuit designed by David A. Johnson P.E., 9/21/08]


Power Supply for Digital Panel Meters that is Isolated


Many inexpensive digital panel meters, such as the one shown below, are designed to operate from a 9v battery. However, the nature of the panel meter internal design often requires that the DC supply used to power the device be isolated from the ground … [Hobby Circuit designed by Dave Johnson P.E., 4/2005]


Small Battery Capacity Tester


I have a client who uses a lot of batteries. His sells a device which is powered by a 3v lithium coin cell and he sells thousands of them. It seems that many of the batteries he has been importing from China have a milliamp-hour capacity which is much… [Hobby Circuit designed by David A. Johnson P.E., 9/7/08]


Battery Powered Plus-Minus 15v Supply


Many classic operational amplifier circuits call for a split positive and negative 15 volt supply. The circuit below makes the job of generating such a supply much easier by using an off-the-shelf DC to DC converter, powered from a cheap 6v battery made … [Hobby Circuit designed by Dave Johnson P.E., 8/31/08]


Poor Man's Timer


Often during testing of certain equipment and components, you would like to keep track of the elapsed time in hours, minutes and seconds. There are some nice commercial digital timers and counters available for just this sort of application. However,… [Hobby Circuit designed by David A. Johnson P.E., 8/24/08]


Digital Counter Using Pedometer


There are many occasions when you may want to count something electronically. Perhaps it is car traffic on a street or items moving down an assembly line. It might be the number of times a machine is activated or maybe you want to count the number of pe… [Hobby Circuit designed by Dave Johnson P.E., 8/17/08]


High Power LED Flasher


Power LEDs are becoming more popular these days. The DC input power to these devices ranges from 1 watt to 5 watts. Normal LEDs only receive about 0.05 watts. At this higher power, these devices can emit a lot of light… [Circuit designed by David A. Johnson P.E., 8/3/08]


Reduced Power Relay Driver


Relays can handle a lot of power. However, for certain power sensitive designs you would like to reduce the power needed to hold a relay closed. The circuit below performs such a task. It uses a single CD4093 quad NAND gate. When the “on” logic input … [Circuit designed by Dave Johnson P.E., 8/3/08]


Light/Dark Switch


Often you want to turn on and off power to a device with a single pushbutton switch. The circuit below performs this logic function with just a few parts and can operate from DC supplies between +3v to about 15v… [Circuit designed by Dave Johnson P.E., 4/2005]


12v Brush Motor Speed Controller


In many applications, you would like to hold the speed of a motor constant, even as variations in the power supply voltage or mechanical load try to change its speed. In other applications, the average current to the motor needs to be limited, so the ini… [Hobby Circuit designed by Dave Johnson P.E., 5/1/05]


Computer Controlled 100ma Current Source


Often in industrial control systems a constant current source is needed, which is controlled by a computer and referenced to circuit ground. The circuit below converts a zero to 5v signal from a computer’s analog output into a current, with a full scale of 100ma. The circuit shown requires a 9v DC supply… [Circuit designed by David A. Johnson P.E., 7/8/08]


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., 7/6/08]

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