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Circuits of the Week - June to January 2012
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

06/24/12

Charge Pump Efficiently Divides Supply Voltage

 

This circuit uses a few FETs and logic devices to down convert a DC voltage to about one half its value … [Circuit designed by Dave Johnson P.E., 7/8/00]

06/17/12

Alkaline Low Battery Voltage Indicator

 

In many battery powered systems a user would like to know when it was time to replace the batteries before they were completely discharged, causing the device being powered to fail completely. Alkaline batteries have an open cell voltage of about 1.5 volts. As they are discharged, the voltage slowly drops. When the cell voltage reaches about 1.25v, they have delivered about 90% of their stored energy. The circuit below monitors the voltage of two alkaline cells in series and begins flashing an efficient red LED when the total battery voltage drops below 2.5v, alerting the user that the batteries need to be replaced. The circuit takes advantage of some new voltage comparators from Touchstone Semiconductor. The TSM9118 has an internal 1.25v reference and an open drain output while drawing an astoundingly low 0.7ua… [Circuit designed by Dave Johnson P.E., 6/17/12]

06/10/12

H Bridge Motor Controller uses Low Power

 

This circuit uses a few logic gates & a 74AC240 to control power to a small gear motor… [Hobby Circuit designed by Dave Johnson P.E., 7/3/05]

06/03/12

Doggie Water Dispenser Alarm

 

I have two Beagle dogs. They drink a lot of water; they go through about 3 gallons a week. They have two water dispensers. One is on the inside and second unit on the outside of my house. Often, I forget to fill one of their two jugs and discover them completely empty. I do worry that my dogs might get dehydrated if I ever forget to fill both of their containers. To help prevent this, I decided to design and build a battery powered alarm system which would monitor the level of water in the jugs and sound an alarm when the water level dropped below a certain volume… [Circuit designed by David A. Johnson P.E., 6/3/12]

05/27/12

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… [Hobby Circuit designed by Dave Johnson P.E., 1/22/08]

05/20/12

Automatic DC Motor Brake

 

In many DC motor powered systems you would like the motor to quickly come to a full stop.This circuit provides an automatic electronic braking action to any DC motor ranging from 6v to 24v up to 1 amp of current. Some of the component values could be… [Hobby Circuit designed by David A. Johnson P.E., 4/30/06]

05/13/12

Cooling Fan Thermostat (12v)

 

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… [Hobby Circuit designed by David A. Johnson P.E., 1/10/10]

05/06/12

Isolated Miniature AC/DC Power Supply

 

This circuit uses a novel approach to produce a fully isolated and regulated 5 volts @30ma from the 120vac power line. It uses two tiny SCRs that alternately discharge two capacitors through a miniature high frequency transformer… [Hobby Circuit designed by David A. Johnson P.E., 6/9/00]

04/29/12

Crystal Controlled 1KHz Signal Generator

 

A while back I got an email from a Discover Circuits visitor. He needed a way to calibrate a RPM display for a big diesel engine. The engine had a tachometer output which produced 12 pulses per rotation with an amplitude of about 5v RMS. The full scale reading on the engine’s RPM speed display dial was 5,000. This translates to a pulse frequency of 1KHz at full scale. To calibrate the display, he needed an accurate 1KHz signal. The 32KHz crystal controlled oscillator below performs this function. It uses a CD4060 binary counter, which has internal inverters configured for a classic crystal oscillator. The Q5 output divides the 32KHz crystal frequency by 32 yielding an output of 1KHz. The output is AC coupled to the input of the tachometer… [Circuit designed by Dave Johnson P.E., 4/28/12]

04/22/12

Finger Touch Activated Switch

 

It does not get any easier if you want a solid state switch that is activated by the touch of a finger. Two small metal pins route voltage through the finger skin to a MOSFET switch. The circuit is great for situations where a membrane type mechanical switch… [Circuit designed by David A. Johnson P.E., 1998]

04/15/12

Isolated AC Current Monitor

 

This circuit uses a small AC current transformer from Magnetek to produce an isolated voltage proportional to the AC current in the primary winding. The transformer contains a single turn primary with a low 0.001-ohm resistance. It can easily handle 30 … [Circuit designed by David A. Johnson P.E., 1/20/01]

04/08/12

AC Current Monitor

 

This circuit uses a small AC current transformer from Magnetek to produce an isolated voltage proportional to the AC current in the primary winding. The transformer contains a single turn primary with a low 0.001-ohm resistance. It can easily handle 30 … [Hobby Circuit designed by David A. Johnson P.E., 10/16/05]

04/01/12

Detector for Human Footsteps

 

This circuit uses a surplus geophone to detect human footsteps from 100 feet away. It was designed for monitoring the Mexico/USA border. This preamp part of the system draws about 50 microamps from a 12 volt supply… [Hobby Circuit designed by Dave Johnson P.E., 4/2005]

03/25/12

12v – 24v AC and DC Day/Night Switch

 

The circuit uses a bridge rectifier circuit to convert AC into DC. A 12v zener diode limits the operating voltage of the circuit to 12v. A tiny Schmitt trigger inverter and a phototransistor detects the ambient light intensity. If there is sufficient light, the inverter disables a push-pull oscillator circuit. The oscillator, with a differential output, drives a charge pump circuit which when activated, provides sufficient gate-source voltage to a pair of n-channel FETs to turn them on fully. The transistor switch can handle both AC and DC with a current in excess of 5 amps… [Hobby Circuit designed by Dave Johnson P.E., 3/25/12]

03/18/12

3Khz Low Pass Filter Plus Audio Amp

 

This circuit uses a switched capacitor filter IC from National Semiconductor to filter signals with frequencies higher than the 3KHz needed for voice audio. The schematic includes an audio amplifier that is designed to drive a standard audio head phone. T… [Circuit designed by David A. Johnson P.E., 12/16/07]

03/11/12

Lead-Acid Battery Low Voltage Indicator

 

Lead-acid batteries should not be discharged below a certain voltage. Discharging a cell below about 1.8v and can often damage it, which will result in a lower amp-hour capacity. Large battery banks are critical in off-grid energy storage systems. One … [Circuit designed by Dave Johnson P.E., 7/30/06]

03/11/12

Lead-Acid Battery Low Voltage Indicator

 

Lead-acid batteries should not be discharged below a certain voltage. Discharging a cell below about 1.8v and can often damage it, which will result in a lower amp-hour capacity. Large battery banks are critical in off-grid energy storage systems. One … [Circuit designed by Dave Johnson P.E., 7/30/06]

03/11/12

Low Voltage Indicator for Lead-Acid Battery

 

Lead-acid batteries should not be discharged below a certain voltage. Discharging a cell below about 1.8v and can often damage it, which will result in a lower amp-hour capacity. Large battery banks are critical in off-grid energy storage systems. One … [Hobby Circuit designed by David A. Johnson P.E., 7/30/06]

02/26/12

Car Ignition 12v Switch

 

A Discover Circuits visitor needed a way to redirect +12v DC in his car to some auxiliary loads whenever the car’s engine was running. Many cars have cigarette lighter outlet jacks, which are either live all the time or become live when the car’s engine is running… [Hobby Circuit designed by Dave Johnson P.E., 2/26/12]

02/19/12

Modulated Light Detector -40Khz

 

This circuit uses a unique cascode amplifier circuit to convert the current from a PIN photo diode to a current without any feedback network. It is very stable and very sensitive. The circuit shown has the potential for a conversion factor of 10 volts per… [Hobby Circuit designed by Dave Johnson P.E., 6/14/00]

02/12/12

30Khz Light Receiver Amp

 

This circuit uses NPN darlington transistor to amplify the signal produced from short light flashes, as detected by a PIN photo diode. The circuit draws only about 330uA from a 6v battery… [Circuit designed by Dave Johnson P.E., 3/7/00]

02/05/12

Voltage Monitor for a 9v Battery

 

Nine voltage batteries are used in a wide variety of electronic devices. But, you often don’t know when it is time to change the battery until the device stops operating. The hobby circuit below provides a visual indication when the battery needs to be … [Hobby Circuit designed by David A. Johnson P.E., 4/2005]

01/29/12

Use Color Film Negative to Make An Optical Filter

 

This isn’t an electronic circuit but the file shows the curve for an optical filter made from exposed color film. The material blocks most visible light but is very transparent to 880nM infrared light. Kodak 100ASA is exposed to "cool white" fluorescent light for about 5 seconds… [Hobby Circuit designed by Dave Johnson P.E., 2000]

01/20/12

Small Pushbutton Controls 10 amps AC

 

The circuit below uses a pair of long lasting lithium coil cell batteries to power a latching relay circuit. With each activation of the small pushbutton, the relay is either latched closed or unlatched open. Since coin cells can’t provide the necessary peak current, a 500uF capacitor is used to store the needed energy. The circuit standby current is kept very low by using a stack of five 100uF ceramic capacitors instead of the usual electrolytic cap. An aluminum capacitor, even a tantalum capacitor leaks too much current, shortening the battery life. By using the ceramic caps, the overall standby current is only a few microamps… [Circuit designed by David A. Johnson P.E., 1/20/12]

01/15/12

1Khz to Over 70Mhz Light Receiver

 

This circuit uses one tiny C-MOS inverter IC to form a modulated light receiver with a very fast response. It is designed around a PIN photo diode that is packaged for use with plastic optical fibers. It can be used as an optical fiber receiver. By using the open end of the optical fiber it can "sniff" out any modulated light signals… [Hobby Circuit designed by David A. Johnson P.E., 6/11/00]

01/08/12

On/off Pushbutton Switch with 30 Minute Timeout

 

I received an email from a Discover Circuits visitor. He wanted to control power to a LED camping light he was building. He wanted to turn the light on and off with a small pushbutton switch. He also wanted a timer so the light would not stay on longer than 30 minutes at a time. It seems that his kids often turned lights on when going to bed and then forgot to turn them off… [Circuit designed by Dave Johnson P.E., 1/8/12]

01/01/12

Darkroom Camera Shutter Timer

Happy
New Year

This circuit was designed to control a film exposure shutter for a darkroom. It has 8 time steps ranging from 0.35 seconds to 4 seconds. It is activated by a foot switch and draws power from an external 12-volt DC supply… [Circuit designed by Dave Johnson P.E., 6/11/00]


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


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