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Circuits of the Week - June to January 2011
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


DC Brushless Motor Monitor


The circuit below was requested by a person needing a way to keep track of the number of hours a 24v DC brushless motor was in operation. The circuit taps into a bit of the power going into the motor using a bridge rectifier and filter capacitor. The voltage produced feeds the input of a small solid state relay. The output of the relay routes AC power to an hour meter, every time the brushless motor is turned on. This same circuit could be used with other DC motors with some simple modifications… [Hobby Circuit designed by Dave Johnson P.E., 4/2005]


1-Nimh Cell powers White LED


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]


Touch Switch controls a 120 VAC load


This is a capacitance based touch on/touch off switch, which can control power to a 200 watt 120vac load… [Hobby Circuit designed by David A. Johnson P.E., 5/25/06]


PV Solar Cell Test Fixture


This is an illustration of a test fixture, which can be used to test individual solar cells for short circuit current.  Using a PIN photo diode and a control circuit, the solar cells can be tested under constant light level conditions… [Hobby Circuit designed by David A. Johnson P.E., 10/1/06]


Isolation Transformer Circuits


A while back I needed to test a circuit at 240vac. Since I needed to safely test the circuit using my oscilloscope, I needed the 240vac to be fully isolated from the power line. So, to step up the voltage, I bought a small isolation transformer from Digikey. This device is made by Triad and has four separate windings. As shown below this transformer can be wired up in several different configurations to either provide a 1:1 isolated voltage conversion or it can step up the 120vac input voltage to 240vac or even 360vac.… [Circuit designed by David A. Johnson P.E., 5/29/11]


Garage Door Timer


The circuit below is designed to automatically close a garage door, if it was left open for a long period of time. A reed relay and magnet combination sense when the door is fully open. An ultra low power voltage comparator oscillator and two 74HC4040 ICs are used to generate the time delay. A jumper or a switch can be used to select a time of 30 minutes, one hour or two hours. The circuit draws only about 2ua so a pair of AA alkaline cells would keep the unit running for many years… [Circuit designed by Dave Johnson P.E., 5/15/11]


High Pass Active Filters


This is a collection of inverting and non-inverting active high pass filter circuits. I included one, two three, four and six pole filter circuits. You can change the component value ratios shown to achive any frequency cut-off you may need… [Circuit designed by Dave Johnson P.E., 2/11/02]


Chain Link Fence Alarm


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,… [Circuit designed by Dave Johnson P.E., 10/16/05]


Water Filling Station Controller


This circuit was requested by a company needing a simple way to fill a large tank with water with little fuss. They wanted a low tech method to automatically fill the tank with water without constant monitoring. They wanted to press a single pushbutton switch and then walk away. In their application, the water tank is first filled to a certain level. The fixed volume of water is then slowly drained off into a second tank, where the water is mixed with another material. The empty tank is then ready to be filled again, for another cycle… [Hobby Circuit designed by David A. Johnson P.E., 4/24/11]


Low Power Sequential Flasher using 10 LEDs


A while back I posted a flasher circuit which flashed 10 LEDs sequentially. That circuit was powered by four AA alkaline cells. The circuit below operates much the same way but has been modified to run from a single 3v CR2032 lithium coin cell… [Hobby Circuit designed by Dave Johnson P.E., 4/17/11]


Another Very Loud Piezo Alarm Beeper


This is yet another beeper circuit that really draws attention. It sweeps the drive frequency slightly to produce a very annoying sound. It uses a transformer to increase the drive voltage across the piezoelectric device to more than 200 volts peak to peak… [Circuit designed by Dave Johnson P.E., 6/27/00]


Bidirectional DC Current Indicator


This circuit is handy for battery powered systems. The circuit is self-powered using a single 3v lithium coin cell. The current draw is so low that the battery should last for years. The hobby circuit flashes a red LED when DC current is drawn from the battery and flashes a green LED whenever current flows into the battery. Two “probe” input wires connect across a section of cable, which acts as a shunt resistor. A voltage of only 5mv in either direction will trigger a LED indication… [Hobby Circuit designed by David A. Johnson P.E., 4/3/11]


Universal DC Sink to Source Converter


As illustrated below, many control systems use an open collector transistor to turn something on. Perhaps it is a LED light, a relay or a small motor. However there are other systems which require a common ground connection instead of a common V+ suppl… [Hobby Circuit designed by David A. Johnson P.E., 3/27/11]

03/20/11 Convert Solar Powered Light to Flasher
  The circuit converts an off-the-shelf solar powered LED path light from producing a steady state light to one that produces a flash.  This circuit includes a tiny dual Schmitt trigger inverter IC which form the pulse generator and the daylight sensor circuit.  The circuit produces a positive pulse, which feeds the constant current LED driver circuit.… [Circuit designed by Dave Johnson P.E. for DC Magazine, 2/27/10]


Solar Path Light Improvement


Use 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… [Circuit designed by David A. Johnson P.E., 3/13/11]


HO Model Train Lighthouse Flasher


HO train sets often have authentic looking scale model homes and buildings. The hobby circuit below can be mounted inside a model lighthouse. The electronic circuit drives a single LED lamp in such a way that it produces light which simulates the rotati… [Hobby Circuit designed by David A. Johnson P.E., 3/6/11]


Ultra Low Current 3v LED Flasher #3


This circuit takes advantage of a tiny LMC7215 low current draw voltage comparator from National Semiconductor and a super efficient green LED from King Bright. The 3v powered circuit forms a classic pulse generator circuit, which drives the LED. The pulse width is set at about 20ms, the pulse rate is about 0.5Hz and the peak LED current is only 100ua… [Circuit designed by David A. Johnson P.E., 2/27/11]


Power Supply -24 Volts AC to 24 Volts DC


The idea for the circuit below came from a company needing to power several 24v DC LED assemblies from an existing 24vac supply. They did not need a rock solid and regulated 24 volts but a voltage which hovered around that voltage. Running the AC throug… [Circuit designed by David A. Johnson P.E., 2/20/11]


Air Transparency Monitor, Xenon Flash Transmitter


This is the matching transmitter for the above receiver. The transmitter launches powerful 1000-watt light pulses that last about 20 microseconds… [Circuit designed by Dave Johnson P.E., 6/2000]


12v Cooling Fan Thermostat


This circuit will turn on a 12v DC powered cooling fan when the air temperature reaches a certain high temperature and will keep the fan turned on until the temperature drops below a second lower level. Both the high and low temperatures are both adjustable… [Circuit designed by Dave Johnson P.E., 2/6/11]


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… [Circuit designed by Dave Johnson P.E., 6/2000]


Indicates Battery Charge Current


This circuit was inspired by a company needing a DC current indicator light which was small enough to be molded into a medium size 28v rechargeable battery assembly. They specified that they wanted the circuit to turn on the LED indicator light with 500ma or more of battery charge current… [Hobby Circuit designed by David A. Johnson P.E., 1/23/11]


Water Seepage Monitor


A while back I received a request from a company who needed a water seepage detector. They wanted a device that ran on 24vac and detected water using a fully isolated circuit. They wanted a circuit which switched off 24vac to a load when water was detected… [Circuit designed by Dave Johnson P.E., 1/16/11]


Wireless LED Flasher


There are times when you would like to flash a LED through a thick plastic case without wires. The circuit below performs this feat. It can be powered from a supply ranging from 3v to 6v. It draws about 300ua of average current at 5v… [Circuit designed by Dave Johnson P.E., 1/9/11]


Solar Powered Cell Phone Charger


The simple circuit below regulates the voltage from a 6v solar panel to a fixed +5v. That voltage can be fed to any cell phone or USB connected portable device, to charge its battery. The circuit uses a Reg113-5 voltage regulator from Texas Instruments… [Circuit designed by Dave Johnson P.E., 1/2/11]

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