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Circuits of the Week - December to July 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

12/30/12

Solar Lithium Ion Battery Charger Using LT1129

 

I have been designing a number of solar powered devices lately. Many of them use just a single 3.6v lithium ion battery. These batteries like to be charged from a fixed 4.2v source with a charging current limited to about C/8, where C is the milliamp-hour rating of the part. When being charged from a small solar panel, the panel should be selected so the charging current does not exceed C/8. In such cases, there is no need for additional charging current control. However, some mechanism is still needed to prevent the battery from discharging back into the solar panel, when the panel is in darkness. Usually, a diode is inserted between the battery and the charge control circuit… [Circuit designed by Dave Johnson P.E., 12/30/12]

12/23/12

Lithium Battery Tester

 

I bought some lithium ion rechargeable batteries a while back. I was planning on using them to modify some solar powered LED spot lights. What I like about these new batteries is that they are the same size as a standard 1.5v alkaline AA cell. With such a package, I can use a standard AA battery holder. The battery manufacturer claims an amp-hour capacity of 2.2 amp-hours. Usually, these amp-hour figures are made using rather small loads, conducted over a long period of time… [Hobby Circuit designed by David A. Johnson P.E., 2/5/12]

12/16/12

Day-Night Switch powered by 24VAC

 

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… [Hobby Circuit designed by Dave Johnson P.E., 12/26/10]

12/09/12

On/Off Flip/Flop Circuit With Automatic Timeout

 

This circuit is ideal when a device needs to be turned on and off with a single pushbutton switch, but also needs to turn itself off after some period of time. With the components shown, the output will stay on for only about 20 seconds… [Circuit designed by David A. Johnson P.E., May 2012]

12/02/12

Shop Safety Light

 

My 1200 square foot metal shop only has one small entrance door and no skylights. If the lights are turn off while inside, there is not enough light to safely find the door to exit the building. On a couple occasions, while I was quietly working at my test bench, my wife would walk into the shop, grab a garden tool and rush back out, turning off the lights and closing the door without thinking. I was left in total darkness and only my very loud screams would bring her back to her senses. I thought it would be wise to install some kind of light, which would turn on for a few minutes every time the ceiling lights were turned off… [Hobby Circuit designed by Dave Johnson P.E., 12/2/12]

11/25/12

Clear Fluid Detector-2

 

Using an ultra low power technique, this circuit detects when a clear fluid is inside a piece of clear plastic tubing. It can be used for fluid level control and monitoring… [Circuit designed by David A. Johnson P.E., 10/22/05]

11/18/12

AC to DC Power Supply requires Low Power

 

Last month I showed how to use a small common mode choke as a fully isolated transformer and a 130v sidac to generate about 100mw of DC power from the AC line.  This second circuit still uses the same inexpensive tiny off-the-shelf 5mH common mode filter coil with a split bobbin as a high frequency transformer but uses two SCRs instead of a sidac to discharge a capacitor… [Hobby Circuit designed by David A. Johnson P.E., 12/11/11]

11/11/12

Xenon Flash Transmitter is Line Powered

 

This line powered xenon flash circuit drives a small camera type flash tube. It has an optical isolator to allow the flash to be safely triggered from some remote device. A flash rate of 2Hz is possible with the circuit… [Hobby Circuit designed by Dave Johnson P.E., 6/9/00]

11/04/12

Locate a Plastic Pipe

 

I used this circuit once to track down the location of a plastic pipe, buried underground. It transmits a weak RF signal, which can be picked up by a portable AM radio from about 6 feet away… [Hobby Circuit designed by David A. Johnson P.E., 5/1/05]

10/28/12

Audio Amp + 3Khz Filter

 

This circuit is the audio amp section for a complete optical transmitter. The circuit amplifies and filters the voice audio signals from an electret microphone. The circuit is described in more detail in… [Circuit designed by Dave Johnson P.E., 6/4/00]

10/21/12

Plastic Pipe Finder

 

I used this circuit once to track down the location of a plastic pipe, buried underground. It transmits a weak RF signal, which can be picked up by a portable AM radio from about 6 feet away… [Circuit designed by David A. Johnson P.E., 5/1/05]

10/14/12

3Khz Filter + Audio Amplifier

 

This circuit is the audio amp section for a complete optical transmitter. The circuit amplifies and filters the voice audio signals from an electret microphone. The circuit is described in more detail in… [Hobby Circuit designed by David A. Johnson P.E., 6/4/00]

10/07/12

Servo Pulse to Voltage Converter

 

A while back a Discover Circuits visitor needed a way to convert servo pulses from a radio controlled device into a voltage. The drawing below, illustrates the pulse train of the received signal. The time between pulses is a fixed 20ms while the pulse widths vary from a low of 1ms to a high of 2ms. The visitor wanted to convert the pulses into a voltage such that a 1ms pulse width would produce 1 volt, a 1.5ms pulse would produce 5v and 2ms pulse would produce an output of 9v. The circuit below performs this conversion… [Hobby Circuit designed by Dave Johnson P.E., 10/7/12]

09/30/12

FET Input High Speed Light Detector

 

This circuit is yet another design that converts current from a PIN photo diode to a voltage. It has a bandwidth that extends beyond 50MHz… [Circuit designed by David A. Johnson P.E., 6/2000]

09/23/12

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 Dave Johnson P.E., 2/6/11]

09/16/12

12v Lamp Current Monitor

 

This circuit monitors DC current and turns on a LED indictor light whenever DC current flows into a 12v lamp… [Hobby Circuit designed by David A. Johnson P.E., May 2012]

09/09/12

Footstep Alarm for Human Traffic

 

This circuit processes the amplified signals from the footstep detector circuit and activates a microprocessor when the circuit detects footsteps. At has an adjustable sensitivity… [Hobby Circuit designed by David A Johnson P.E., 7/11/06]

09/02/12

CMOS Nand Gate Forms Gated 125Khz Oscillator

 

This circuit is uses a NAND gate as an inverter. The gate allows the oscillator to be gated on and off. Again, the values shown set the frequency at 125KHz but can be changed to produce other frequencies… [Circuit designed by Dave Johnson P.E., 10/16/05]

08/26/12 Coin Operated Telescope
  Their first attempt at an electronic system was not very successful.  That system used a power  hungry circuit using a solenoid, powered by a large sealed battery.  The battery had to be… [Circuit designed by Dave Johnson, 2/2/10]

08/19/12

Power Reminder Beeper

 

This 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. It might be ideal for… [Circuit designed by Dave Johnson P.E., 10/16/05]

08/12/12

ELC signal between 200Mhz-400Mhz

 

If you need a clean emitter coupled logic (ECL) type signal between 200MHz and 400MHz this circuit works fine. It uses four voltage-controlled capacitors to change the frequency… [Hobby Circuit designed by Dave Johnson P.E., 1998]

08/05/12

Produces Beep once each Minute

 

This 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. It might be ideal for c… [Hobby Circuit designed by Dave Johnson P.E., 10/16/05]

07/29/12

AC Power Loss Alarm

 

I’ve been having a problem with my instant hot water system lately. This system circulates electrically heated water throughout my house using a small pump. The pump is wired into a ground fault interrupter (GFI) module. Every now and then, the GFI trips, leaving me without hot water. What I need is an alarm to tell me when AC power on that outlet has been turned off by the GFI. I figured a 9v battery powered loud chirping beeper would be the ticket for this kind of alarm. The chirping sound would draw my attention and let me know that I need to reset the GFI switch, without waking the whole neighborhood. My circuit is shown below… [Circuit designed by Dave Johnson P.E., 7/29/12]

07/22/12

12kv Power Supply

 

If you need about 12,000 volts DC for an ion generator this circuit might be the ticket. It draws power from the 120vac power line but it uses a small 6KV camera flash trigger coil. The output signal is isolated from the power line… [Circuit designed by Dave Johnson P.E., 3/27/99]

07/15/12

Circuit tests the self-resonance frequency

 

This circuit tests the self-resonance frequency is of an inductor. During the test, only a few picofarads of parallel capacitance are added to the inductor. When connected to an oscilloscope and a signal generator, you can measure resonant frequencies fr… [Hobby Circuit designed by Dave Johnson P.E., 4/2005]

07/08/12

Low Power 12,000 Volt Power Supply

 

If you need about 12,000 volts DC for an ion generator this circuit might be the ticket. It draws power from the 120vac power line but it uses a small 6KV camera flash trigger coil. The output signal is isolated from the power line… [Hobby Circuit designed by David A. Johnson P.E., 1998]

07/01/12

Freezer Over-Temperature Alarm

 

This circuit turns on a beeper whenever the outside air temperature is less than zero degrees Centigrade. The circuit draws only a few microamps from a 9 volt battery. It uses a glass bead thermistor accurate to 1 degree C… [Circuit designed by Dave Johnson P.E., 7/20/06


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