Detector/Switch for Audio Signal - For some audio systems it is desirable to have an automatic switch circuit turn on other devices, such as an amplifier, if an audio signal is detected on one line. The hobby circuit below will activate a transistor switch when it detects at least 50mv peak to peak of an audio signal. The switch could be used to turn on a relay, routing the signal to where it is needed. A single 3v lithium cell will power the electronic circuit for about 5 years . . . Hobby Circuit designed
by David Johnson P.E.-July 09, 2006
Detects Low Voltage 120VAC Power Line - This hobby circuit will turn on a beeper when the line voltage drops below 100 volt AC . . . Hobby Circuit designed
by Dave Johnson P.E.-July 09, 2006
Detects Xenon Lamp Flash - This circuit has a very low standby current requirement yet has very high sensitivity toward the light flashes from a xenon lamp. When connected to a flip/flop it can serve as an on on/off Controller . . . Hobby Circuit designed
by David Johnson P.E.-December 14, 2004
Digital Counter using Pedometer - There are many occasions when you may want to count something electronically. Perhaps it is car traffic on a street or items moving down an assembly line. It might be the number of times a machine is activated or maybe you want to count the number of . . . Hobby Circuit designed
by David Johnson P.E.-August 17, 2008
Digital noise generator - When you need to test an audio circuit with broadband noise, this circuit works great. It uses just three inexpensive C-MOS ICs that generate a series of output pulses whose widths vary randomly. I included a level control pot . . . Hobby Circuit designed
by David A. Johnson P.E.-July 04, 2000
Display RF Signal Strength - 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 David Johnson P.E.-July 09, 2006
Divide a TTL Clock Signal by 1.5 - Two inexpensive ICs divide a TTL clock signal by 1.5. By following the circuit with another flip/flop, you could also generate a divide by three function . . . Hobby Circuit designed
by David Johnson P.E.-July 06, 2000
DIY 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 Johnson P.E.-June 04, 2000
Doorbell heard in Basement - if you can't hear your doorbell when you are in your basement try this circuit. This circuit takes advantage of the 24vac power source located near the furnace. Using a simple current transformer technique, the circuit sounds a beeper whenever the . . . Hobby Circuit designed
by David A. Johnson P.E.-June 04, 2000
Drive a LED with an Intensity Ramping Mode - The circuit below was designed
to drive a LED with an intensity ramping mode. Two 555 timers generate two different triangle waveforms. The upper device generates a 10KHz signal while the lower unit produces a 1Hz signal. The two signals are fed to a . . . Hobby Circuit designed
by David Johnson P.E.-December 21, 2007
DVD Recorder + TV Camera Make Cheap Data Logger - Sometimes recording the results of a test requires as much ingenuity as designing the product. Consider a situation which requires the collection of many different variables over a long period of time . . . Hobby Circuit designed
by David A. Johnson P.E.-April 01, 2005
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 David A. Johnson P.E.-July 08, 2005
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 David A. Johnson P.E.-June 03, 2002
Exciter Circuit-12v Touch Switch - This circuit generates a 20KHz pseudo sine wave signal that can power approximately 50 remote touch activated switch circuits. This hobby circuit can support a cable length of approximately 2500 feet. A typical remote switch circuit is also shown as . . . Hobby Circuit designed
by David Johnson P.E.-February 10, 2002
FETs & Logic Devices Divides Supply Voltage on Charge Pump Efficiently - This circuit uses a few FETs and logic devices to down convert a DC voltage to about one half its value . . . Hobby Circuit designed
by David A. Johnson P.E.-July 08, 2000
Finger Tap Piezo Relay - This circuit uses a flat piezo wafer, glued inside a plastic box, as a finger tap sensor. With each tap of a finger to the box holding the wafer, the circuit turns on and off AC or DC power to an external device. The circuit is powered by a 9v 9v battery . . . Hobby Circuit designed
by Dave Johnson P.E.-November 07, 2010
Fireflies_In_Jar_Solar_Light - When I was a kid, growing up in Illinois, I remember catching fireflies and putting them into a jar. I loved watching them flashing away at night. Later, I would let them out to live another day. . . . Hobby Circuit by David Johnson P.E.-September 21, 2017
Flasher_&_Beeper_for_Well_Pump - Several years ago I installed a box on my water well pump head. At the well head, the solar powered box flashed a very bright LED light and chirped a very load siren, whenever the pump motor was in operation. After many years of service, the box finally became corroded from rain water and died. So, I thought it was time to redesign the circuit and build up a replacement. . . . Hobby Circuit by David Johnson P.E.-June 29, 2017
Flashing LED Advertising Badge #1 - I have seen numerous flashing light badges at trade shows and conventions. They are often handed out as gifts to promote some business. The devices often use inefficient circuits, which cause the battery power source to be quickly depleted . . . Hobby Circuit designed
by David Johnson P.E.-March 26, 2002
Flashing Light to simulate Car Alarm - Whenever the car’s ignition is turned off, this circuit activates a flashing LED, which can be positioned to appear as an active alarm system . . . Hobby Circuit designed
by David Johnson P.E.-July 09, 2006
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.-July 08, 2000
Fluorescent Lamp Intensity Modulator - This hobby circuit was designed
to experiment with using small fluorescent lamps as a broad pattern source of modulated light. This hobby circuit hits the small lamp with narrow 1us pulses at a rate of 10KHz. Each pulse launches about 10 watts of visible light. The lamp starting method is a bit crude but the circuit does work. . . . Hobby Circuit designed
by David A. Johnson P.E.-June 27, 2000
Fluorescent Lamp Night Light Needs Only 1 Watt - if you are tired of replacing small night light lamps, try this circuit. The line powered circuit uses a long life cold cathode fluorescent lamp. A simple charge pump technique runs the lamp without any transformer . . . Hobby Circuit designed
by David Johnson P.E.-March 26, 1999
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 Johnson P.E.-July 11, 2006
Form Push-Pull Driver using N-Ch & P-Ch Transistors - This circuit can produce high speed output signals with fast rise and full times. The unique change pump action allows the voltage of the upper P-ch device to range from millivolts to hundreds of volts. The output current is only limited by the rating of . . . Hobby Circuit designed
by David A. Johnson P.E.-June 27, 2000
Front Desk 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 Dave Johnson P.E.-September 25, 2005
Front End + Power Supply Circuits of the Electric Filed Disturbance Monitor - This schematic is the power supply and front-end sections of my field monitor, "Electric Field Disturbance Monitor". The system can detect human and animal motion by the electric fields they disturb . . . Hobby Circuit designed
by David A. Johnson P.E.-March 26, 2002
Front-End Circuit-Modulated Light Receiver - 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. This hobby circuit shown has the potential for a conversion factor of 10 volts per microwatt at 900nm. I included a simple JFET post-amplifier with a gain of about 20. . . . Hobby Circuit designed
by David Johnson P.E.-March 12, 1999
Fully Isolated 50/60hz Sync Generator - This circuit will produce a single pulse at the zero voltage cross points of the power line voltage. An opto-coupler provides a very safe 5KV isolation . . . Hobby Circuit designed
by David A. Johnson P.E.-December 12, 2004
|