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Circuits of the Week - June to January 2013
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

06/30/13

On/Off Latch Circuit with 2 Second Delay

 

A Discover Circuits visitor needed a latch circuit which could operate using a power supply voltage ranging from 3v to 24v. He wanted to use a tiny pushbutton switch to turn on and off power to a load. However, he wanted a 2 second delay between the switch activation and the state change of the output. The delay would prevent accidental activation of the circuit from a quick push button switch closure. The circuit below performs this function. A dual Schmitt trigger inverter IC and a single flip/flop IC form the heart of the circuit. The A1A section performs the 2 second delay function… [Circuit designed by David A. Johnson P.E., 6/30/13]

06/23/13

Ultra Pure 125kHz Sine Wave Signal Source

 

For some RFID systems operating at 125KHz, a very low distortion signal source reference is needed. The circuit shown on this page produces a 10-volt peak-to-peak signal into a 50-ohm load, with a distortion of only 0.01%… [Circuit designed by David A. Johnson P.E., 2/11/02]

06/16/13

8-Solar Cell Lithium Ion Battery Charger Version 2

 

Jim Wilber looked at my design for a battery charger and sent me some suggested changes. He suggested using the LTC4412 ideal diode IC to switch off the current path from the solar cell to the battery in darkness. He also noted that the 2.5v regulator from Seiko I had selected was no longer in production and suggested using the shunt reference. I think his suggestions made a lot of sense. In my design, in dark conditions, the first MOSFET Q1 would not turn off until well after the point where the solar panel no longer produces current, allowing some leakage current to flow from the battery into the solar panel… [Circuit designed by David A. Johnson P.E., 6/16/13]

06/09/13

Earthquake Alarm Circuit

 

Geophones are dandy motion sensors. They are metal cans containing a powerful magnet, suspended with springs inside a coil of wire. The magnet functions as a floating mass. The slightest vertical motion of the device causes the coil to move relative to… [Hobby Circuit designed by Dave Johnson P.E., 2012]

06/02/13

Tester 4-20ma Current Loop

 

This circuit injects an adjustable current through a wire loop. Using a digital current meter, the current can be adjusted from near zero to over 24 milliamps… [Hobby Circuit designed by David A. Johnson P.E., May 2012]

05/26/13

Touch Switch 240VAC

 

This is another capacitance based touch on/touch off switch, which can control power to a 200 watt 240vac load… [Hobby Circuit designed by Dave Johnson P.E., 4/2005]

05/19/13

Eight Solar Cell Lithium Ion Battery Charger

 

A single lithium ion battery cell needs 4.2v from a current source to insure it is fully charged. A solar panel with just enough individual cells wired in series to produce the needed voltage would be more efficient than one with more cells for a given surface area. If you divide 4.2 by eight you come up with a figure of 0.525 volts… [Circuit designed by David A. Johnson P.E., 5/19/13]

05/12/13

Light Receiver Amp -40Khz

 

This circuit iprovides more gain and operates up to 40KHz.However it draws more power supply current… [Hobby Circuit designed by Dave Johnson P.E., 1/29/06]

05/05/13

Modulated light receiver circuit

 

The circuit uses a very inexpensive C-MOS IC that is connected to a small photodiode. Using a unique inductive feedback network, the circuit provides high sensitivity under high ambient light conditions. It is a great circuit when you want to extend the range of an optical remote control transmitter… [Circuit designed by David A. Johnson P.E., 1998]

04/28/13

NPN/PNP Transistor Tester

 

I designed this circuit about 30 years ago and still use it from time to time. It is powered from a 3v battery. It makes a tone when the transistor tests good. It can be used to determine device type (NPN or PNP) and the base, collector or emitter transist… [Hobby Circuit designed by David A. Johnson P.E., 06/28/00]

04/21/13

Infrared Remote Extender Circuit

 

This circuit is designed to be placed directly in front of a standard TV or VCR remote. The exiting light pulses produced by the circuit match the pulses from the remote but are about 10 times more powerful. Using the device, the remote can operate a TV o… [Circuit designed by David A. Johnson P.E., 1998]

04/14/13

Electrical Current Indicator

 

I designed this circuit as a simple current indicator for any load ranging from 40 watts to 250 watts. The circuit turns on a small LED, whenever it detects current flowing to a remote load… [Circuit designed by David A. Johnson P.E., 10/16/05]

04/07/13

Detect the narrow 1uS pulses

 

This circuit is designed to detect the narrow 1uS pulses produced by the above amplifier circuit. The clean logic type pulses produced by the discriminator are then sent to a frequency to voltage converter. The circuit is designed to process a pulse frequ… [Circuit designed by David A. Johnson P.E., 1998]

03/31/13

Moving Air Indicator

 

Many months ago a Discover Circuits reader wanted a method to indicate when a fan was blowing air. He didn’t want to route any electrical wires to a sensor near the fan. Instead he wanted all the electronics for both the fan and other circuits mounted s… [Hobby Circuit designed by Dave Johnson P.E., 1/1/12]

03/24/13

Chicken Coop Door Opener with Timer

 

I received an email from a guy in Australia who was trying to build a chicken coop system. He was planning on using a conventional garage door mechanism to open and close the 6 doors to the coop. The commercial door opener opened and closed the doors with a single pushbutton. He wanted to install a standard 220vac powered timer so he could open the doors in the morning and close them at night. He was not sure how to perform this function. I suggested the circuit shown below… [Circuit designed by David A. Johnson P.E., 3/24/13]

03/17/13 Vertical Lifting Security Gate Power Source
  The circuit that I came up with is shown below.  It uses 5 identical circuits.  Each one is wired in parallel with it assigned 650F super capacitor.  A LM385 provides a 1.2v voltage reference.  A resistor divider measures the voltage across the capacitor and feeds the voltage to a low power, low voltage op amp. The output of the op amp drives an n-channel FET.  I designed the circuit for a maximum charge current of 600ma.  This would allow the use of a 10 watt solar panel, if needed, in the future.  Each of the five circuits forms a shunt type voltage regulator, limiting the voltage across each capacitor to about 2.6v.  … [Circuit designed by Dave Johnson P.E. for DC Magazine, 11/1/09]

03/10/13

NMH/NiCd 6v Battery Tester

 

I designed this circuit to test rechargeable six volt battery packs under constant current conditions. As designed, the circuit applies a 10 amp load to the battery pack. A heat sink must be used on the main power transistor… [Hobby Circuit designed by David A. Johnson P.E., 6/25/06]

03/03/13

Low Voltage Latch Circuit

 

A Discover Circuits visitor wanted a simple on/off latch circuit which can turn on and off power to a small motor. The kicker was that he wanted to power the circuit and the motor from a single 1.2v NiMH battery cell. Texas Instruments comes to the rescue with their family of low voltage single gate ICs. Specifically the SN74AUP1Q14 Schmitt trigger inverter will operate down to 0.8v and lower. When used with a good NPN transistor, the circuit below works quite nicely… [Circuit designed by Dave Johnson P.E., 3/3/13]

02/24/13

Tester for Pulsed LEDs

 

This circuit is designed to test visible and infrared LEDs in pulsed mode operations. It can drive the LED with peak currents in excess of 10 amps. A light detector nearby can monitor the response time and intensity of the LED under test… [Hobby Circuit designed by Dave Johnson P.E., 1998]

02/17/13

Air Transparency Monitor, Xenon Flash Receiver

 

I designed this receiver circuit many years ago to monitor the quality of a mile long column of air for future optical communications experiments. The transmitter system (circuit 72 below) uses a powerful xenon flash in conjunction with a large 12 inch… [Circuit designed by Dave Johnson P.E., 1998]

02/17/13

Xenon Flash Receiver-Air Transparancy Monitor

 

I designed this receiver circuit many years ago to monitor the quality of a mile long column of air for future optical communications experiments. The transmitter system (circuit 72 below) uses a powerful xenon flash in conjunction with a large 12 inch… [Hobby Circuit designed by Dave Johnson P.E., 1998]

02/10/13

Flexible Outputs from Square Wave Driver

 

This circuit can produce an output signal ranging from DC to 100KHz. It can source a voltage ranging from 1v to 30v. It can sink a voltage ranging from zero volts to –30v. It can drive up to 200ma of current and can even be switched to a floating tristate… [Hobby Circuit designed by Dave Johnson P.E., 1998]

02/03/13

2-12v Battery Isolator Circuit with a LTC4412

 

Linear Technology has just announced a neat little chip (LTC4412) . It has been designed to be used in conjunction with an external P-channel power FET, to form an ideal diode function with a very low 0.05v voltage drop. The chip monitors the voltage… [Hobby Circuit designed by David A. Johnson P.E., 2012]

02/03/13

Improved Solar LED Spot Light

 

Nearly all the Chinese made solar powered LED lights I see being sold at Lowe's and Home Depot home improvement centers are poorly designed. Their solar panels and battery packs are all undersized. The light from their Chinese made LEDs fade rapidly and the devices never make it through a complete night before the light dies… [Circuit designed by Dave Johnson P.E., 2/3/13]

01/27/13

Inductive Pulse Receiver (175Khz)

 

This circuit is discussed in more detail in the Experimenters Journal. The receiver’s six inch diameter coil detects the ring signal from the above transmitter and use a single NPN transistor to provided enough amplification for the signal to be easily viewed on an oscilloscope… [Hobby Circuit designed by David A. Johnson P.E., 1998]

01/20/13

240VAC to 5VDC Power Supply

 

This is simple way to power some 5v logic from a 240vac source. If a 120vac power adapter is used, the circuit will also work for 120vac power lines… [Circuit designed by Dave Johnson P.E., 2/12/00]

01/13/13

HO Train Lighthouse Flasher Revisited

 

HO train sets often have authentic looking scale model homes and buildings. A while back someone asked me to design a flashing LED light, which he could mount inside a model lighthouse and have it operate so it would appear to rotate and flash. I designed a working circuit but I was never pleased with the results. I’m still not 100% pleased but the revised circuit below seems to operate a bit better than the older circuit… [Hobby Circuit designed by Dave Johnson P.E., May 2012]

01/06/13

175Khz Iinductive Pulse Transmitter

 

This circuit is discussed in more detail in the Experimenters Journal. The transmitter’s six-inch diameter coil launches powerful magnetic 175KHz ring pulses that can be detected by the circuit below… [Hobby Circuit designed by Dave Johnson P.E., 7/30/06]


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