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Oneshot & Latch Pushbutton - This circuit uses a single IC to convert a noisy pushbutton switch signal into a clean pulse or a sustained push on-push off signal. It can operate from 3v to 18v . . . Hobby Circuit designed
by David Johnson P.E.-March 26, 2002
One-Watt White LED Night Light - As many of you know, I have a pet peeve with poorly made LED night lights. Often, the light from the LED quickly fades, so within months, the light is useless. I have posted several versions of modified night lights using higher quality components. This circuit is yet another version, which produces much more light than those other designs. . . . Hobby Circuit designed
by David A. Johnson P.E.-November 06, 2008
Operation of Cascode Light Receivers - This page provides a detailed explanation of how the modified cascode light receiver circuit operates. The cascode technique in conjunction with an inductive load provides very high current to voltage conversion as well as very high speed . . . Hobby Circuit designed
by David Johnson P.E.-February 15, 2002
Optical Fiber Receiver-5Mhz Broad Band - This circuit is a simple broad band light detector that uses a very inexpensive IC and a PIN photodiode that is packaged for use with plastic optical fibers. It has a bandwidth from 1KHz to over 5MHz. It is great for experimenting with various modulated . . . Hobby Circuit designed
by David Johnson P.E.-June 04, 2000
Optical Filter using Color Film Negative - 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 David A. Johnson P.E.-January 01, 1998
Oscillator Draws 2 Nanoamps - if you have hundreds of touch switches that need an excitation signal, then this circuit is what you need. Its 20KHz 20v peak to peak output signal can supply up to 3 watts of touch switch excitation power . . . Hobby Circuit designed
by David A. Johnson P.E.-July 11, 2006
Oscillator with Ultra Low Current - Here is a challenge. Design an astable oscillator which draws only a few nanoamps of current from a +3v supply. I gave this some thought and came up with the circuit below. I used some pretty standard parts except for three surface mounted 1000M resist . . . Hobby Circuit designed
by David A. Johnson P.E.-March 08, 2009
Over-Temperature Micro-Power Alarm - The circuit is powered for years by a single 3 volt lithium battery. It sounds an alarm when the temperature exceeds a certain point. With some minor changes the circuit could also be configured for an under temperature (freeze) alarm. The circuit uses a . . . Hobby Circuit designed
by David Johnson P.E.-June 04, 2000
Parallel LC Oscillator with CMOS Inverter - I have used this parallel resonant LC oscillator circuit countless times. The oscillator frequency is determined by the inductor and capacitor values. I have shown an adjustable inductor to make it easy to set the frequency to a specific value . . . Hobby Circuit designed
by Dave Johnson P.E.-March 12, 2002
Peak to Peak 300v Signal Generator - This circuit converts a square wave signal to a +-150 volt output signal with fast 100nS rise and fall times . . . Hobby Circuit designed
by Dave Johnson P.E.-June 03, 2000
Photovoltaic Solar Panels-Ideal Blocking Diode Circuit - Most photovoltaic solar panels are used to charge a battery during the daytime. Nearly all panels come equipped with a blocking diode. The diode prevents DC current from flowing backwards from the battery bank into the panel at night. The usual blocking . . . Hobby Circuit designed
by David Johnson P.E.-January 26, 2009
Piezoelectric Device used for Motion Alarm - An inexpensive piezo wafer is used to detect vibration and when the vibration is sufficient a switch is activated . . . Hobby Circuit designed
by David Johnson P.E.-July 09, 2006
Piezoelectric Water Flowing Indicator - The vibrations associated with water flowing through a pipe are picked up by an inexpensive piezo wafer. The signal from the wafer is first boosted by a micropower transistor amplifier and then fed to an ultra low power voltage comparator . . . Hobby Circuit designed
by David Johnson P.E.-July 26, 2009
Plus & Minus DC Power Supply - This is a classic example of a regulated DC power supply that produces both a positive 15v and a negative 15v from a 20vac wall adapter . . . Hobby Circuit designed
by David A. Johnson P.E.-December 13, 2004
Poor Man's Timer - Often during testing of certain equipment and components, you would like to keep track of the elapsed time in hours, minutes and seconds. There are some nice commercial digital timers and counters available for just this sort of application. However, . . . Hobby Circuit designed
by Dave Johnson P.E.-August 24, 2008
Power Supply for Digital Panel Meters that is Isolated - Many inexpensive digital panel meters, such as the one shown below, are designed
to operate from a 9v battery. However, the nature of the panel meter internal design often requires that the DC supply used to power the device be isolated from the ground . . . Hobby Circuit designed
by Dave Johnson P.E.-September 13, 2008
Precision Low Capacitance Meter - This circuit was originally designed
to measure the volume of fluid inside a medical syringe. As designed
, it produces a zero to 5 volt output, corresponding to a capacitance change of about 10 picofarads . . . Hobby Circuit designed
by David A. Johnson P.E.-December 12, 2004
Press & Hold Latching Circuit - 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 . . . Hobby Circuit designed
by David Johnson P.E.-July 02, 2013
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 . . . Hobby Circuit designed
by David Johnson P.E.-June 03, 2000
Protects from Battery Polarity Reversal - This simple circuit can protect a sensitive electronic circuit from an accidental connection of a battery with a reversed polarity. The N-channel FET connects the electronic device to the battery only when the polarity is correct. The circuit shown was . . . Hobby Circuit designed
by David Johnson P.E.-March 09, 2002
Pulse Generator uses Micropower - it is hard to design a pulse generator that produces clean logic pulses while drawing very low current. This circuit is designed
to produce short 2mS pulses at a rate of one per second while drawing only 1 microamp from a 9 volt battery . . . Hobby Circuit designed
by David A. Johnson P.E.-June 03, 2000
Pushbutton On-Off Latch Circuit - Often you want to turn on and off power to a device with a single pushbutton switch. The circuit below performs this logic function with just a few parts and can operate from DC supplies between +3v to about 15v. It uses two inverters within a 74C14 or a CD4069 hex inverter package. Each press of the button toggles between the two on/off logic states. . . . Hobby Circuit designed
by David Johnson P.E.-January 09, 2010
Pushbutton Switch Through Glass - This simple technique allows a mechanical pushbutton switch assembly to activate a reed type relay through a pane of thick glass. Pushing the button moves a magnet near the reed relay, closing the relay contacts . . . Hobby Circuit designed
by Dave Johnson P.E.-February 10, 2002
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 Johnson P.E.-July 09, 2006
PV Solar Panels-Ideal Blocking Diode Circuit - Solar panels usually include a diode at their outputs. The diode prevents current from back feeding into the panel at night from the battery being charged by the panel. But, this diode does waste some power and does reduce the available panel voltage . . . Hobby Circuit designed
by David Johnson P.E.-January 26, 2009
PWM Circuit for Motor Speed Control - Sometimes you want to slow down a brush type DC motor. The most efficient way to do this is with a pulse width modulation (PWM) technique. The hobby circuit below can operate from about 3 volts to 15 volts. The frequency is fixed at about 2KHz but the pulse . . . Hobby Circuit designed
by David A. Johnson P.E.-December 16, 2007
PWM Motor Controller uses a 555 Timer - Using a CMOS version of the 555 timer, this circuit can be used to control the speed of a motor by adjusting the duty cycle of the pulses sent to the motor . . . Hobby Circuit designed
by David Johnson P.E.-June 30, 2006
Quartz_Clock_Solar_Powered - Connect a solar panel to a quality AA size 1One of my homes in Colorado had a large battery powered quartz clock above the fireplace. It was powered by an AA size 1.5v alkaline cell. The battery would run the clock for about a year or so, then I would have to break out a tall step ladder and replace the battery for another year or so of operation. That gave me a great 10:1 energy margin. . . . Hobby Circuit by David Johnson P.E.-September 21, 2017
Radio Tower Lamp Monitor - The circuit below uses two 3 amp diodes to produce a fairly constant 2 volts whenever current is routed to the tower lamp. The voltage is used to power a bright LED. The LED will only turn on when current is actually flowing to the lamp. With the component values shown, the circuit will work with lamps ranging from 40 to 250 watts. . . . Hobby Circuit designed
by David Johnson P.E.-January 00, 1900
Ramping Lamp Driver - This circuit drives any 12v lamp, incandescent or LED type, in a ramping fashion. Each press of the button toggles the lamp from a fully off mode to a fully on mode. But, instead of an instantaneous lamp intensity change, the power is slowly ramped up a . . . Hobby Circuit designed
by Dave Johnson P.E.-December 21, 2007
Really Loud Output Beeper Circuit - I you need a real loud beeper, this circuit delivers about 110db (12 inches away) from a 9v battery using a single inexpensive C-MOS IC . An off-the-shelf piezoelectric beeping device is driven at resonance to insure maximum efficiency. By changing the control IC to a 74AC14, the same circuit can operate from 3v and 1.5v batteries . . . Hobby Circuit designed
by Dave Johnson P.E.-June 03, 2000
Rectifier-Fullwave - I have used this handy circuit many times. It accurately converts an AC signal into pulsing DC, which can be filtered to provide an average of the input voltage. It works from millivolts to volts. The circuit shown requires a stable +5v reference if a . . . Hobby Circuit designed
by David Johnson P.E.-February 12, 2002
Reduced Power Sequential LED Flasher - The 74HC4017 is a neat decade counter which has 10 decoded outputs. When supplied with a low frequency clock and with a LED attached to each of the outputs, an interesting ten step sequential flasher effect can be produced. The LEDs can be configured in . . . Hobby Circuit designed
by David A. Johnson P.E.-November 30, 2008
Reflected Infrared Light Switch - infrared light reflected off a finger is used to activate this switch circuit. Drawing only 30uA from a 3v supply, this circuit will detect a human finger with a range of about 1 inch. The sensor uses an inexpensive infrared LED and a matching photo diode . . . Hobby Circuit designed
by Dave Johnson P.E.-July 09, 2006
Relay Driver with Reduced Power - Relays can handle a lot of power. However, for certain power sensitive designs you would like to reduce the power needed to hold a relay closed. The circuit below performs such a task. It uses a single CD4093 quad NAND gate. When the “on” logic input . . . Hobby Circuit designed
by David A. Johnson P.E.-August 03, 2008
Remote Range Extender TV/VCR - I often get frustrated by how poor some infrared remotes in many new electronic devices operate. I used to be able to turn on the TV from 30 feet away. Now, it seems that I have to get within 15 feet. To address this DC Magazine - Issue, I designed
the circuit shown below. The circuit taps into the TV remote’s existing infrared LED and drives 8 more efficient devices. The result is a communications range some 4 to 5 times longer. . . . Hobby Circuit designed
by David Johnson P.E.-December 14, 2009
Remote TV Camera Power - A while back I received an email from a company with a problem. They needed a security camera installed at some remote location. The camera was a high quality color camera sending out standard NTSC video through a long 500 foot 50 ohm coax cable. With some difficulty they got the coax strung. They were told that there would be AC power near the camera location to power that device. But, they then learned that in fact, there was no power available. They thought about using a solar panel and a battery but they decided to call me for help instead. Was there some way to power the camera without stringing another cable? . . . Hobby Circuit designed
by David Johnson P.E.-July 03, 2011
Replace a Pushbutton Switch with this Infrared Safety Switch - Using a slotted optical sensor, this circuit can be used as a replacement for a large mushroom pushbutton safety switch . . . Hobby Circuit designed
by Dave Johnson P.E.-July 11, 2006
Revisited-Model Train Lighthouse Flasher - This was originally designed
for a model in a HO train set. It simulates the behavior of the light from a lighthouse. The LED intensity gradually increases, then flashes with a bright light and finally decreases slowly in intensity . . . Hobby Circuit designed
by David Johnson P.E.-July 11, 2006
RF 2.45Ghz Signal Detector - This passive RF indicator is made from a few simple parts. A 100uA moving coil meter is used to display the relative intensity of the RF signal. This circuit can be used with some cell phones and many cordless telephones . . . Hobby Circuit designed
by Dave Johnson P.E.-July 09, 2006
RF Signal Detector at 2.45GHz - Many cordless telephones, computer networks and cell phones use a RF transmitter lefted on the 2. 45GHz band. Using a microwave diode detector and 100uA meter, this passive circuit will display the strength this signal. The drawing shows how a bent piece of wire can be used as the antenna. . . . Hobby Circuit designed
by Dave Johnson P.E.-July 09, 2006
RFID Smart Key Detector - The hobby circuit below was designed
to indicate if a key held over the small coil of wire contains a 125KHz RFID chip. These chips are often used in smart car ignition keys. The left side circuit forms a 125KHz power oscillator, which drives the 680uH a . . . Hobby Circuit designed
by Dave Johnson P.E.-July 09, 2006
RFID Test Circuit - I designed
this test the concept of using light techniques to send identification data instead of RF. . . . Hobby Circuit designed
by David Johnson P.E.-July 08, 2000
RS-232 Serial Interface Status Indicator Circuit - I designed
this circuit many years ago and have used it several times since. It uses a single logic IC to indicate the TXD and RXD line activity of a typical RS-232 serial data communications link. The thresholds are set at plus 3 volts and minus 3 volts. Two dual color LEDs are used as indicator lights . . . Hobby Circuit designed
by David A. Johnson P.E.-December 15, 2004
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