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Boost Infrared TV/VCR Remote Signal - This circuit will boost the signal from any infrared TV or VCR remote, extending the range by a factor of 3X. . . . Hobby Circuit designed
by Dave Johnson P.E.-October 05, 2005
Broad Band 50Mhz Optical Fiber Receiver Version A - if the above circuit it still too slow, you can try this circuit. What it lacks in sensitivity it makes up for in speed. The circuit attaches a plastic fiber optic PIN photodiode assembly to a small box containing a small 3v battery and a standard . . . Hobby Circuit designed
by Dave Johnson P.E.-June 04, 2000
Broken Solar Panels - As illustrated below, one way to salvage a large number of broken solar cells is to wire them in parallel, not in the more conventional series connection. Broken cells of various sizes and shapes are grouped together into panels of about one quarter to on . . . Hobby Circuit designed
by David Johnson P.E.-January 01, 2012
Brownout Voltage Tester - I used this circuit years ago to test AC line powered devices under 95vac line voltage conditions. it has a rating of 250 watts. . . . Hobby Circuit designed
by Dave Johnson P.E.-July 11, 2006
Call-for-Service Beeper - This device functions like a lobby desk service request bell. A short beep is sounded whenever the large pushbutton on top of the device is pressed. Once pressed, the device will not make another beep until at least 2 minutes of time has elapsed . . . Hobby Circuit designed
by David Johnson P.E.-September 25, 2005
Camera Shutter Time Measurement - A while back I received an email from someone who needed a simple way to measure the speed of a photographic camera. I thought about this and suggested the method below. It relies on an oscilloscope and a cheap photodiode to measure the speed . . . Hobby Circuit designed
by David Johnson P.E.-August 14, 2011
Candle Power - A single lit candle in conjunction with an inexpensive thermopile voltage generator provides enough power to charge a small battery when used with a small low voltage DC to DC converter . . . Hobby Circuit designed
by David A. Johnson P.E.-February 09, 2014
Capacitance Proximity Switch-3 volts - This circuit was designed
to provide a touch activated switch function without an external power supply. It draws so little power that a single 3v battery will operate the circuit for many years . . . Hobby Circuit designed
by David Johnson P.E.-February 12, 2002
Capacitance Touch Activated Momentary Switch-5v - This circuit is discussed in more detail in the section on Capacitance Proximity Switch Technology. The circuit is powered from a standard +5v supply. It has both a source and sink output that change state whenever a metal button connected to the circuit . . . Hobby Circuit designed
by Dave Johnson P.E.-July 08, 2000
Capacitance Touch Switch - A single 1.5v silver oxide button cell powers this complete touch activated switch circuit for 5 years. It features both a normally open and a normally closed set of solid state switch thermals. It also has an adjustable sensitivity, which can be set . . . Hobby Circuit designed
by David A. Johnson P.E.-January 18, 2002
Capacitance Type Liquid Level Monitor - This circuit was originally designed
to monitor the level of liquid natural gas in a tank but it can be used to also measure almost any liquid. Two custom insulated metal tubes form a capacitor plate. The capacitance between the two tubes increases as the level of the liquid rises . . . Hobby Circuit designed
by David Johnson P.E.-June 13, 2000
Capacitance-Based Touch Switch - This is another capacitance based touch on/touch off switch, which can control power to a 200 watt 240vac load . . . Hobby Circuit designed
by David Johnson P.E.-July 11, 2006
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.-February 26, 2012
Cell Phone Charger - It seems that as cell phones become more sophisticated, their operating times become shorter and shorter. If you are on the go all the time and have difficulty finding AC or DC outlets to charge your phone, you might consider this simple scheme . . . Hobby Circuit designed
by David A. Johnson P.E.-June 04, 2000
Cell Phone Solar Powered 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 . . . Hobby Circuit designed
by David A. Johnson P.E.-January 02, 2011
Characteristics of CMOS Logic Inverter Amplifiers - This chart shows some of the measured characteristics of three different unbuffered CMOS logic gates, used as voltage amplifiers . . . Hobby Circuit designed
by David Johnson P.E.-February 11, 2006
Charge Pump Battery Charger-+5 Volts - The circuit below will trickle charge a four cell pack of AA or AAA NiMH batteries. The circuit draws current from the +5v available a USB connection and pumps about 70ma of current into the battery. This should be enough current to fully charge a pack of . . . Hobby Circuit designed
by David Johnson P.E.-February 07, 2010
Cheap Close-Up Camera Lens - I often use my digital camera to take photos of some electronic projects. At times, I need to get in close to capture some details. But, without some additional optics, my camera just could not maintain focus, as I received closer to an object. Being a cheap . . . Hobby Circuit designed
by Dave Johnson P.E.-January 01, 2012
Cheap Light Power Meter - Some time ago I was experimenting with the use of some low leakage photovoltaic solar cells, to harvest some energy from the diffuse lighting inside a room. The plan was to use a small array of solar cells (the type often used on solar powered calculators) . . . Hobby Circuit designed
by Dave Johnson P.E.-January 00, 1900
Cheap Low Power 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 . . . Hobby Circuit designed
by David A. Johnson P.E.-June 26, 2000
Check Engine Beeper - When driving down a busy road, you may not notice a “check engine” indicator light that suddenly turns on within the car’s instrument panel. You could drive for many miles before noticing the light. To provide an audible indication of trouble, the hobby . . . Hobby Circuit designed
by David A. Johnson P.E.-July 09, 2006
Check Engine Beeper - When driving down a busy road, you may not notice a “check engine” indicator light that suddenly turns on within the car’s instrument panel. You could drive for many miles before noticing the light. To provide an audible indication of trouble, the hobby . . . Hobby Circuit designed
by David A. Johnson P.E.-July 09, 2006
Check Engine Beeper - When driving down a busy road, you may not notice a “check engine” indicator light that suddenly turns on within the car’s instrument panel. You could drive for many miles before noticing the light. To provide an audible indication of trouble, the hobby . . . Hobby Circuit designed
by Dave Johnson P.E.-July 09, 2006
Check Engine Beeper - When driving down a busy road, you may not notice a “check engine” indicator light that suddenly turns on within the car’s instrument panel. You could drive for many miles before noticing the light. To provide an audible indication of trouble, the hobby . . . Hobby Circuit designed
by David Johnson P.E.-July 09, 2006
Circuit Eliminates 9v Battery - if you use 9v battery powered devices for long periods of time, you may get tired of constantly changing batteries. The circuit below can be wired into any 9v battery powered device, drawing power from an external AC to DC power adapter. I designed
the c . . . Hobby Circuit designed
by David A. Johnson P.E.-January 08, 2010
Circuit 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 Dave Johnson P.E.-January 23, 2011
Circuit indicates Battery Charging Current - Anyone who has ever had a shocking experience on an especially dry day is acutely aware of the kinds of high voltage that can be generated from a simple walk across a carpeted floor. Those same finger to metal sparks can destroy or disrupt sensitive elect . . . Hobby Circuit designed
by David A. Johnson P.E.-January 23, 2011
Circuit Indicates Time to Dust - I have used the circuit below for many different applications, which require ultra low power drain. It uses a single CA4013 flip/flop. One section forms a pulse generator. The generator produces 100uS pulses at a rate of about 1 per second . . . Hobby Circuit designed
by Dave Johnson P.E.-May 07, 2006
Circuit Monitors 120 Power Line - 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 David A. Johnson P.E.-June 03, 2000
Circuit Monitors 12v Lamp Current - This circuit monitors DC current and turns on a LED indictor light whenever DC current flows into a 12v lamp . . . Hobby Circuit designed
by Dave Johnson P.E.-July 09, 2006
Circuit Monitors Power Loss - For some medical equipment it is important for an operator to know when power is lost to the machine. The beeper is powered from a 9v battery and requires the machine to have a power switch with a third set of contacts . . . Hobby Circuit designed
by David A. Johnson P.E.-March 26, 1999
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 . . . Hobby Circuit designed
by Dave Johnson P.E.-June 09, 2000
Classic Linear 5v Supply using 6.3VAC Transformer - There are times when you wish to control a pulse width modulation (PWM) signal with a DC voltage, ranging from 0v to 5v. This voltage is often created by a computer interface. The circuit below performs this voltage to PWM conversion. PWM signals are often . . . Hobby Circuit designed
by David Johnson P.E.-October 31, 2010
Classic Linear 5v Supply using 6.3VAC Transformer - There are times when you wish to control a pulse width modulation (PWM) signal with a DC voltage, ranging from 0v to 5v. This voltage is often created by a computer interface. The circuit below performs this voltage to PWM conversion. PWM signals are often . . . Hobby Circuit designed
by David Johnson P.E.-October 31, 2010
Clip-on AC Current Indicator Circuit - Several times over the last few years I have received calls from people looking for a simple circuit that would tell them when a sump pump or water well pump motor was running. These people told me that the electrical circuits that turn on and off . . . Hobby Circuit designed
by David A. Johnson P.E.-November 18, 2004
CMOS IC Flashes LED - Judging from the numerous references I have received, this electronic circuit, hobby circuit seems to be rather popular. Many published circuits which flash LEDs need 3 or more volts. This electronic circuit uses only a single inexpensive C-MOS IC and . . . Hobby Circuit designed
by Dave Johnson P.E.-June 04, 2000
CMOS Inverter forms a Resonant LD Oscillator - This circuit uses a single CMOS inverter to form a series resonant LC oscillator. The values shown set the oscillation at about 125KHz but the other frequencies are possible by changing the main LC values . . . Hobby Circuit designed
by Dave Johnson P.E.-March 12, 2002
CMOS Inverters Form 125Khz Oscillator - This circuit inverts the LC components so the inductor is grounded. Two inverters are needed to produce the needed oscillation. Again, the values shown set the frequency at 125KHz but can be changed to produce other frequencies . . . Hobby Circuit designed
by David A. Johnson P.E.-March 12, 2002
CMOS Oscillator, Draws Only 0.5ua - if truly low power oscillators interest you, this circuit draws a mere 2 microwatts (500nA) from a 6v battery. It uses a very inexpensive C-MOS IC to produce a frequency of 2Hz. However, by changing the component values you can push it to 300Hz . . . Hobby Circuit designed
by Dave Johnson P.E.-June 03, 2000
Collection Active High Pass 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 achieve any frequency cut-off you may need . . . Hobby Circuit designed
by David Johnson P.E.-February 11, 2002
Colman_Lantern3_Modified - While shopping at a sporting goods store one day, I spotted a compact LED lantern. It looked like the kind of lantern I could modify. The lantern LEDs were grouped into three sections of 7 LEDs each, oriented into 120 degree angles. The LEDs were standard T 1-3/4 epoxy types. The lantern had a three mode switch which selected between off, full power and half power modes. At the half power mode, only half of the 21 LEDs were illuminated. At full power all LEDs were illuminated. . . . Hobby Circuit by David Johnson P.E.-September 21, 2017
Computer Controlled Power Relay - Often there is a need to turn on and off a 120vac device using a low power +5v logic signal from a computer. The circuit below shows how this could be done. The circuit uses a power relay with a 120vac coil and 10 amp contacts. A small solid state relay turns on and off the relay. The solid state relay provides 5KV isolation from the computer . . . Hobby Circuit designed
by David A. Johnson P.E.-July 31, 2011
Computer, Long Period, Watch Dog Timer - This circuit uses a simple 4060 IC oscillator/timer that is reset periodically by a computer. Should the computer fail to send a pulse, the output changes state. The time can easily be set from seconds to hours. . . Hobby Circuit designed
Dave Johnson P.E.-December 26, 1998
Constant Current Drives LEDs - LEDs are current driven devices. The light they produce is proportional to the current flowing though them. One way to maintain a fairly steady light output is to keep the current through the LED fixed, even as the supply voltage may change. A classic . . . Hobby Circuit designed
by David A. Johnson P.E.-December 13, 2010
Constant Current LED Driver - Bob Pease from National Semiconductor came up with this circuit and I have used it many times. It maintains a constant current through one or more LEDs. A single resistor controls the desired current. . . . Hobby Circuit designed
by David A. Johnson P.E.-November 25, 2004
Control both Power & Direction of a DC Motor - TTL type Q and inverted Q inputs control a classic H-bridge circuit, rated at 50 volts and about 10 amps. The circuit can control power and direction of a DC motor . . . Hobby Circuit designed
by David Johnson P.E.-July 09, 2006
Control Variable LED Intensity - The circuit below uses power from four rechargeable AA NiMH cells to drive 3 white LEDs. A potentiometer varies the duty cycle of a pulse width modulator circuit to vary the intensity of the LEDs from 0% to 100%. The beauty of the circuit is that when the . . . Hobby Circuit designed
by Dave Johnson P.E.-September 26, 2009
Controlled Up/Down Ramping Light - There are times when you want a lamp to gradually change from dark to light and from light to dark, with each push of a pushbutton switch. The circuit below performs this control function using a flip/flop and a classic pulse width modulation or PWM . . . Hobby Circuit designed
by David A. Johnson P.E.-March 09, 2009
Controller for a 9v Powered Xenon Photoflash - This 9v battery powered circuit is designed
for remote control flash needs. A charge control circuit turns off the high voltage generator when the photoflash capacitor is fully charged. A neon lamp is included to indicate when the system is ready to flash . . . Hobby Circuit designed
by David A. Johnson P.E.-June 03, 2000
Convert AC to DC signal with is Full Wave Rectifier - This is a classic circuit that can accurately convert an AC signal to DC. Using a LF412 high speed dual op amp, this circuit will rectify any signal up to about 100KHz with a minimum amplitude of about 50mv peak to peak. At 40KHz the input signal can be . . . Hobby Circuit designed
by David A. Johnson P.E.-July 11, 2006
Converts a Square Wave input signal into a Voltage - This is a test circuit converts a square wave input signal into a voltage. But, the voltage produced is proportional to the time between edges (period) of the signal, not the frequency. The range is from 100uS to to 10mS, which produces a voltage from 100mV to 10 volts. Other scale factors are also possible . . . Hobby Circuit designed
by David A. Johnson P.E.-June 04, 2000
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.-February 06, 2011
Count Cars with Laser - This is an illustration how a laser could be used to count traffic and measure the speed of each car passing through the sensor area . . . Hobby Circuit designed
by David A. Johnson P.E.-July 09, 2006
CT Hour Meter - Many systems require routine maintenance based on machine operation time. The circuit below is a simple way to turn on a hour meter whenever AC power is supplied to the machine. An inexpensive snap-on current transformer from Magnetics inc, is used to . . . Hobby Circuit designed
by David Johnson P.E.-February 13, 2010
Current Source is Computer Controlled - 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 . . . Hobby Circuit designed
by Dave Johnson P.E.-July 08, 2008
CW & CCW Alternating Motor Driver - A while back a company needed a motor controller for a custom door latching system. The system used a small brush motor with a speed reduction gear to both latch and unlatch the door. When pulsed for about 250ms in the clockwise direction, the motor would unlatch the door . . . Hobby Circuit designed
by Dave Johnson P.E.-September 05, 2010
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