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Circuits designed by David Johnson, P.E.
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David Johnson's Circuits
Circuits + Descriptions

from Dave's Circuits & Circuit Solutions


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Photo Transistor interface Circuit  -  This page shows different ways to improve the response time of a photo transistor circuit . . . Circuit by David Johnson P.E.-July 11, 2006

Piezoelectric Beeper that is Very Loud  -  This circuit generates a continuous tone instead of a pulsing one.  The 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 maxim . . . Circuit by David A. Johnson P.E.-June 03, 2000

Piezoelectric Tap Switch  -  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 battery . . . Circuit by David A. Johnson P.E.-November 07, 2010

Piezoelectric Triggered Switch  -  Two different switch circuits are shown.  One sources current and the second sinks current.  Both switches are connected to a piezo wafer.  When the wafer is tapped, the switches are activated . . . Circuit by David Johnson P.E.-July 09, 2006

Piezoelectric Vibration Sensor Activates Switch  -  An inexpensive piezo device is used as a motion sensing device for this motion alarm . . . Circuit by Dave Johnson P.E.-July 09, 2006

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 by Dave Johnson P.E.-December 18, 2004

Pop Free Microphone On/Off Switch Circuit  -  This circuit uses a simple technique to eliminate the usual “pop” sound when an electret type microphone is switched on or off.  The circuit is completely passive and will work on dynamic and electret type microphones . . . Circuit by David A. Johnson P.E.-February 15, 2002

Pot Controlled Variable LED Intensity Circuit  -  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 . . . Circuit by Dave Johnson P.E.-September 26, 2009

Power Line Alarm  -  This circuit will activate a beeper alarm whenever the AC line voltage is less than 100vac.   . . . Circuit by David A. Johnson P.E.-July 09, 2006

Power Loss Beeper  -  For some medical equipment it is important for an operator 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 . . . Circuit by David Johnson P.E.-March 26, 1999

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 . . . Circuit by David A. Johnson P.E.-June 03, 2000

Power Supply-24 Volts AC to 24 Volts DC  -  The idea for the circuit below came from a company needing to power several 24v DC LED assemblies from an existing 24vac supply.  They did not need a rock solid and regulated 24 volts but a voltage which hovered around that voltage.  Running the AC through . . . Circuit by David A. Johnson P.E.-February 19, 2011

Precision 40Khz Triangle Generator  -  This circuit generates a precision 40KHz triangle waveform . . . Circuit by Dave Johnson P.E.-July 09, 2006

Precision AC Peak Detector  -  This unique circuit uses a very inexpensive voltage comparator to form a peak detector.  The DC voltage produced tracks the positive peak of the input signal.  It works from about ten millivolts to about 10 volts peak to peak.  The maximum frequency is about . . . Circuit by Dave Johnson P.E.-February 11, 2002

Precision Fullwave Rectifier  -  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 . . . Circuit by David Johnson P.E.-February 12, 2002

Precision Ultra Lower Power Oscillator  -  This circuit works much like the classic 555 timer, but draws only about 1.5 microamps from a 3 volt battery.  It is highly stable under varying temperature and supply voltages . . . Circuit by Dave Johnson P.E.-May 29, 2000

Pulse Period to Voltage Converter  -  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 . . . Circuit by David Johnson P.E.-June 04, 2000

Pulsed LED Test Circuit  -  This circuit is designed to test visible and infrared LEDs in pulsed mode operations.  It can drive the LED with peak currents in excess of 10 amps.  A light detector nearby can monitor the response time and intensity of the LED under test . . . Circuit by David Johnson P.E.-June 17, 2000

Pump Motor Monitor  -  This circuit can turn on a light or sound a beeper, whenever an AC line powered pump motor turns on.  It is designed to detect the motor current using a small inductor placed on the outside of the motor’s power cable.  No direct wire contact is needed . . . Circuit by David Johnson P.E.-November 18, 2004

Pushbutton Activates Service Request Beeper  -  A single press of a pushbutton switch turns on a beeper for one second but can’t be activated again for 60 seconds.   . . . Circuit by David A. Johnson P.E.-July 09, 2006

Pushbutton 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 . . . Circuit by David Johnson P.E.-March 09, 2009

Pushbutton Oneshot & Latch  -  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 . . . Circuit by David A. Johnson P.E.-March 26, 2002

Pushbutton Switch works 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.   . . . Circuit by David Johnson P.E.-February 10, 2002

Ramping LED Driver  -  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 . . . Circuit by Dave Johnson P.E.-December 21, 2007

Rapidly Stops DC Motor Drive   -  This circuit will bring a fast moving DC motor to a stop in seconds.  The braking action is automatically applied whenever DC power is removed . . . Circuit by Dave Johnson P.E.-July 13, 2006

Reduce voltage to Photovoltaic Solar Panel with Ideal Diode  -  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 . . . Circuit by David Johnson P.E.-March 06, 2011

Reduced Power Relay Driver  -  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 . . . Circuit by David Johnson P.E.-August 03, 2008

Remote Doorbell Beeper  -  This circuit will activate a beeper alarm whenever the AC line voltage is less than 100vac . . . Circuit by David A. Johnson P.E.-July 11, 2006

Remote Extender Circuit  -  This circuit is designed to be placed directly in front of a standard TV or VCR remote.  The exiting light pulses produced by the circuit match the pulses from the remote but are about 10 times more powerful.  Using the device, the remote can operate a TV or VCR over three times the normal distance . . . Circuit by Dave Johnson P.E.-June 04, 2000

Remote LED Indicator Light  -  There are times when you would like to transmit a signal from one LED indicator light to second LED at another location.  The circuit below works well for this application.  It takes advantage of the fact that the internal infrared LED inside an opto-isolator . . . Circuit by David A. Johnson P.E.-December 26, 2008

Remote Relay Driver  -  I received a call from a Discover Circuits visitor who wanted a simple way to turn on an outside lighting system using a low voltage control line.    I suggested the simple circuit below.    The circuit is powered by a 9v battery and uses a very efficient latching relay.    Two low voltage wires lead from the a toggle switch to the remote relay.    A toggle switch selects either the on or off condition of the relay.    The relay contacts are rated at 10 amps, so the small relay can handle a sizeable amount of power.   . . . Circuit by David Johnson P.E.-March 31, 2014

Resistance Tester got UL Ground Requirement  -  UL requires that an electrical system using a metal chassis earth ground connection have a resistance of less than 0.1ohms.   This resistance must be tested with actual line current in excess or 10 amps.    The circuit shown uses a couple of LEDs to indicate . . . Circuit by David Johnson P.E.-April 28, 2001

Restaurant Table Service Request Flasher  -  A single 3v lithium coin cell battery powers the device.  Four super bright green LEDs are flashed when the assembly was rotated into the “Requested Service” position. A tilt switch senses when the device is in that position.  A simple timer turns off the flasher after 3 minutes, if left in the flashing position.   . . . Dave Johnson, Lubbock TX-DC Magazine - Issue 13 Nov 2010

RS232 Data Bus Status Monitor  -  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 volt . . . Circuit by David A. Johnson P.E.-December 15, 2004


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