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Relay Circuits
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Last Updated: December 07, 2017 02:04 PM
Circuits Designed by Dave Johnson, P.E. :

Small Pushbutton Controls 10 Amps AC - The circuit below uses a pair of long lasting lithium coil cell batteries to power a latching relay circuit.  With each activation of the small pushbutton, the relay is either latched closed or unlatched open.  Since coin cells canít provide the necessary peak current, a 500uF capacitor is used to store the needed energy.  The circuit standby current is kept very low by using a stack of five 100uF ceramic capacitors instead of the usual electrolytic cap. . . Circuit by Dave Johnson P.E.-January, 2012

Solid State Relay Requires Only 50ua Drive Current - This circuit demands a control current that is 100 times smaller than that needed by a typical optically isolated solid state relays.  It is ideal for battery-powered systems.  Using a combination of a high current TRIAC and a very sensitive low current SCR. . . Circuit by Dave Johnson P.E.-June, 2000

Solid State Relay that is Bidirectional - The circuit below can switch up to 4 amps of current with a peak voltage up to 20v.  It is fully isolated using two inexpensive red LEDs.  One LED forms a red light source, which illuminates a second red LED.  The circuit takes advantage of the 1.5v . . . Hobby Circuit designed by Dave Johnson P.E.-May, 2009

Solid State Touch Activated Switch - It does not get any easier if you want a solid state switch that is activated by the touch of a finger.  Two small metal pins route voltage through the finger skin to a MOSFET switch.  The circuit is great for situations where a membrane type mechanical switch . . . Hobby Circuit designed by David A. Johnson P.E.-June, 2000

SSR Requires Ultra Low Control Current - Most solid state relays require at least 5ma and often more input control current, to fully turn on the device.  But such current levels often force battery powered circuits to use excessively large batteries.  The relay hobby circuit shown below demands only 50uA of input current . . . Hobby Circuit designed by David A. Johnson P.E.-June, 2000

Super Capacitor makes Low Voltage Latching Relay Driver - Using some small super capacitors, this circuit can latch and unlatch a mechanical relay with 10 amp contacts, from a small 3 volt power source.  By using a latching relay, power can be controlled to a load with a tiny battery . . . Hobby Circuit designed by David A. Johnson P.E.-August, 2006

Switches AC or DC power using a Latching Relay - The circuit below switches AC or DC power to a load using an ultra low power 3v latching relay with two coils.  The relay can handle about 10 amps of current, up to 250vac.  Each time the small pushbutton is pressed, the circuit either latches the relay . . . Hobby Circuit designed by David A. Johnson P.E.-December, 2009

Tap Switch uses a Piezo Wafer - 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 . . . Hobby Circuit designed by Dave Johnson P.E.-February, 2002

Thermocouple Temperature Activates Relay - Voltage from a high temperature thermocouple activates a relay when the sensor detects a temperature above 1000 degrees F. . . Circuit by David A. Johnson P.E.-July, 2006

Through Glass Push Button Switch - 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 A. Johnson P.E.-November, 2010

Touch Activated Battery Powered Latching Relay -  A client needed a touch activated switch.  The unit needed to be fairly small and wired into the underside of some kitchen cabinets.  When touched, a string of fluorescent or LED lights, mounted under the cabinets, would turn on.  The total light power is less than 100 watts.  The system into two parts.  (1) A relay is mounted inside a plastic junction box and (2) a touch switch module.    Four wires leading from plastic junction box would connect to a second box containing the touch switch module.  The circuit is a simple relay driver powered by a9v battery. . . . Dave Johnson, Lubbock TX-DC Magazine - Issue 20 Nov 2011

Ultra Low Power Latching Relay Circuit - The circuit below takes advantage of some inexpensive small super capacitors.  The circuit pumps 6 volt pulses into the separate 5 volt latching and unlatching relay coils.  A short 25ms pulse is all that is needed to flip the relay states.  A third super capacitor is used to supply a higher peak current than a small lithium cell might otherwise be able to supplyCircuit Solution by David Johnson P.E.-April, 2007

Universal Flasher using a Latching Relay - Latching relays are nifty devices.  Most contain two separate coils.  When a voltage is applied to one coil the relay latches in one state and stays in that state until voltage is applied to the unlatch coil.  Since the latching and unlatching pulses only . . . Hobby Circuit designed by David A. Johnson P.E.-October, 2005

Water Filling Station Controller - This circuit was requested by a company needing a simple way to fill a large tank with water with little fuss.    They wanted a low tech method to automatically fill the tank with water without constant monitoring.    They wanted to press a single pushbutton switch and then walk away.    In their application, the water tank is first filled to a certain level.    The fixed volume of water is then slowly drained off into a second tank, where the water is mixed with another material.    The empty tank is then ready to be filled again, for another cycle. . . . Hobby Circuit designed by David A. Johnson P.E.-April, 2011

Links to electronic circuits, electronic schematics, designs for engineers, hobbyists, students & inventors:

Solid-state analog data recorder runs for 7.4 days -  11/22/07  EDN Design Ideas:  Using a nonvolatile SRAM, this analog data recorder can storethan seven days' worth of data __ Circuit Design by S Vinay Kumar, Mysore, India

Solid-state breaker for microcontrollers -  10/29/13  EDN Design Ideas:  Electronically protect your load, driver, and microcontroller __ Circuit Design by Didier Juges

Solid-State Flasher Has Short-Circuit Protection -  01/16/97 EDN Design Ideas:  The circuit in Figure 1 switches an inductive or resistive grounded load using a bootstrapped, n-channel, power MOSFET.  The circuit features short-circuit, reverse-polarity, and transient-input-voltage-spike protection.  The input voltage, VDD, can vary from about 18 to 30V, and the circuit can handle as much as approximately 10A with proper heat sinking of the FET.  The circuit flashes at a rate of about 100 flashes/minute, with a duty cycle of about 47%.  You can use this circuit as a solid-state flasher for vehicles with 24V nominal-power systems. __ Circuit Design by Navdhish Gupta, TDF Corp, Naperville, IL

Solidstate Relay -  The basic function of a relay is to switch on power to a load using an electrically isolated, low-power control signal. Hitherto, electromechanical relays have been the components of choice to...__ Electronics Projects for You

Solid-State Relay Saves Battery's Life -  09/01/94 EDN Design Ideas:  The circuit in Fig 1 uses a low-resistance p-channel FET, Q4, to disconnect a load from the battery when the battery's voltage drops below a certain value.  After you disconnect the circuit, it remains off until you remove and replace the battery __ Circuit Design by Tom Gay, Siemens, Regensburg, Germany

Solid-state relays simplify Monitoring electric car battery-voltage -  02/19/04  EDN Design Ideas:  Figure 1 shows the propulsion system of an electric vehicle.  it includes an electric motor, drive electronics, a mechanical transmission, vehicle control/power management, a charging system, and a battery.  The long-term performance __ Circuit Design by Robert Krause, Fairchild Semiconductor, San Jose, CA

Solidstate Signal Lamp -  This solidstate signal lamp is a good alternative to revolving mechanical signaling lamps used in ambulances. it uses ultra-bright blue LEDs for signaling. However, you can replace blue LEDs with bright...__ 

Solidstate Switch For Dc-Operated Gadgets -  This solid state DC switch can be assembled using just three transistors and some passive components. it can be used to switch on one gadget while switching off the second gadget...__ 

Solid-State Tesla Coil, #1 -  Nikola Tesla is considered by some to be the greatest inventor of our modern electrical age, and many experts consider him to be the true father of radio.  However, today he is best remembered for his facsinating wireless power-transmission experiments, using his famous Tesla Coil.  The high-frequency air-core, oscillating Tesla Coil is just as exciting today as it was back in 1899 __ Designed by Tony van Roon  VA3AVR

Solid-State Tesla Coil, #2 -  Nikola Tesla is considered by some to be the greatest inventor of our modern electrical age, and many experts consider him to be the true father of radio.  However, today he is best remembered for his facsinating wireless power-transmission experiments, using his famous Tesla Coil.  The high-frequency air-core, oscillating Tesla Coil is just as exciting today as it was back in 1899 __ Designed by Tony van Roon  VA3AVR

Solid-State Two-Way Switch -  Two-way switches, more profoundly known as staircase switches, are widely used when a load has to be controlled from two ends of the location.  For example, a staircase lamp that needs...__ Electronics Projects for You

Sonoff relay WiFi ESP8266-Lua -  Lua application for management Sonoff WiFi Smart Switch, there is no need cloud, broker or third party server.  The program was tested with the device Sonoff, which you see in the picture, but it is likely that it will also work with older versions Sonoff modules or smart socket. __ Designed by © Pavel Janko

Sound Activated Relay #1 -  This relay remains dormant until the op-amp activates upon sound via the electret-microphone.  I only tested the 2-wire type.  The input stage is a regular off-the-shelf 741 operational amplifier and connected as a non-inverting follower audio amplifier.   __ Designed by Tony van Roon  VA3AVR

Sound Activated Relay #2 -  This relay remains dormant until the op-amp activates upon sound via the electret-microphone.  Only tested with the 2-wire type.  The input stage is a regular off-the-shelf 741 operational amplifier and connected as a non-inverting follower audio amplifier __ Designed by Tony van Roon  VA3AVR

Square Tesla Coil -  Figure 1 shows the schematic diagram of the Tesla Coil circuit.  The circuit consists of little more than a few coils, a step-up power transformer, and a capacitor.  Power from an AC wall receptacle is fed to transformer T1 (a small neon-sign transformer) which steps the voltage up to about 3000-volts AC __ Designed by Tony van Roon  VA3AVR

SSR Requires Ultra Low Control Current -  Most solid state relays require at least 5ma and often more input control current, to fully turn on the device.  But such current levels often force battery powered circuits to use excessively large batteries.  The relay hobby circuit shown below demands only 50uA of input current . . . Hobby Circuit designed by David A. Johnson P.E.-June, 2000

Super Capacitor makes Low Voltage Latching Relay Driver -  Using some small super capacitors, this circuit can latch and unlatch a mechanical relay with 10 amp contacts, from a small 3 volt power source.  By using a latching relay, power can be controlled to a load with a tiny battery . . . Hobby Circuit designed by David A. Johnson P.E.-August, 2006

Switch intelligently controls current -  03/30/00 EDN Design Ideas:  This circuit can intelligently control ac or dc current when connected in series with a load.   PDF contains many circuits, please scroll to find this one __ Circuit Design by Jim Hartmann, Silent Knight LLC, Maple Grove, MN

Switches AC or DC power using a Latching Relay -  The circuit below switches AC or DC power to a load using an ultra low power 3v latching relay with two coils.  The relay can handle about 10 amps of current, up to 250vac.  Each time the small pushbutton is pressed, the circuit either latches the relay . . . Hobby Circuit designed by David A. Johnson P.E.-December, 2009

Switching (PWM) Solenoid Driver -  This is a very simple circuit to interface a current loop sensor to an input which is designed for a voltage, such as that from a standard potentiometer.  This page is technical so that interested persons can build the circuit themselves, but 4QD can also supply the interface on a 'postage stamp' sized (22mm x 27mm) board as shown in the first diagram.  it is a relatively trivial circuit!   __ Designed by Richard Torrens

Tap Switch uses a Piezo Wafer -  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 . . . Hobby Circuit designed by Dave Johnson P.E.-February, 2002

Telephone in Use Relay -  Circuit to close a relay when any telephone extension is off the hook.   __ Designed by Bill Bowden

Temperature Relay -  This simple temperature relay circuit can be used to signal a fire or setpoint for temperature monitoring function.  You need to adjust P1 so that T1′s base voltage is 0.5V smaller than the emitter voltage at a temperature a little bit lower than the desired triggering (switching) temperature.  if the temperature increases __ Designed by Popescu Marian

Thermocouple Temperature Activates Relay -  Voltage from a high temperature thermocouple activates a relay when the sensor detects a temperature above 1000 degrees F. . . Circuit by David A. Johnson P.E.-July, 2006

Through Glass Push Button Switch -  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 A. Johnson P.E.-November, 2010

Time Delay Relay -  A time delay relay is a relay that stays on for a certain amount of time once activated.  This time delay relay is made up of a simple adjustable timer circuit which controls the actual relay.  The time is adjustable from 0 to about 20 seconds with the parts specified.  The current capacity of the circuit is only limited by what kind of relay you decide to use __ Designed by Aaron Cake

Time Delay Relay II -  When activated by pressing a button, this time delay relay will activate a load after a specified amount of time.  This time is adjustable to whatever you want simply by changing the value of a resistor and/or capacitor.  The current capacity of the circuit is only limited by what kind of relay you decide to use __ Designed by Aaron Cake

Time Delay Relay-Power OFF -  The two circuits below illustrate opening a relay contact a short time after the ignition or ligh switch is turned off.  The capacitor is charged and the relay is closed when the voltage at the diode anode rises to +12 volts.  The circuit on the left is a common collector or emitter follower and has the advantage of one less part since a resistor is not needed in series with the transistor base.  However the voltage across the relay coil will be two diode drops less than the supply voltage, or about 11 volts for a 12.5 volt input. __ Designed by Bill Bowden

Time Delay Relay-Power ON -  Here's a power-on time delay relay circuit that takes advantage of the emitter/base breakdown voltage of an ordinary bi-polar transistor.  The reverse connected emitter/base junction of a 2N3904 transistor is used as an 8 volt zener diode which creates a higher turn-on voltage for the Darlington connected transistor pair.  Most any bi-polar transistor may be used, but the zener voltage will vary from about 6 to 9 volts depending on the particular transistor used.  Time delay is roughly 7 seconds using a 47K resistor and 100uF capacitor and can be reduced by reducing the R or C values. __ Designed by Bill Bowden

Time Relay - 12 Sec to 90 min -  Schematic and notes only, no circuit description provided.   __ Designed by Tony van Roon  VA3AVR

Time-delay relay reduces inrush current -  03/07/02 EDN Design Ideas:  A transformer switching onto a line can sometimes cause a circuit breaker to trip or a fuse to blow.  This phenomenon occurs even if the transformer presents no load, such as when the secondary is open.  The problem arises because of the heavy magnetizing inrush current in the transformer.  The amplitude of the current depends on the instant on the ac waveform at which the transformer become __ Circuit Design by P Seshanna, Assumption University, Bangkok, Thailand

Time-delay relay uses proximity control -  03/15/01 EDN Design Ideas:  You can build a circuit that allows a passerby to briefly operate model trains in a display window (Figure 1).  The design uses a proximity detector rather than a pushbutton switch to eliminate the need to mount and wire any equipment outdoors.  The circuit worked well __ Circuit Design by Dennis Eichenberg, Parma Heights, OH

Touch Activated Battery Powered Latching Relay -  A client needed a touch activated switch.  The unit needed to be fairly small and wired into the underside of some kitchen cabinets.  When touched, a string of fluorescent or LED lights, mounted under the cabinets, would turn on.  The total light power is less than 100 watts.  The system into two parts.  (1) A relay is mounted inside a plastic junction box and (2) a touch switch module.    Four wires leading from plastic junction box would connect to a second box containing the touch switch module.  The circuit is a simple relay driver powered by a9v battery. . . . Dave Johnson, Lubbock TX-DC Magazine - Issue 20 Nov 2011

Transformer driver controls bidirectional switch -  07/17/97 EDN Design Ideas:  You can use a transformer-driver IC to simply control a MOSFET on/off switch __ Circuit Design by Gerald Grady, Maxim Integrated Products, Sunnyvale, CA

Transistor Boosted Relay -  This relay driver boosts the input impedance with a regular 2N3904 transistor (or equivalent).  Very common driver.  it can drive a variety of relays, including a reed-relay, and is non-latching. 
Transistor Q1 is a simple common-emitter amplifier that increases the effective sensitivity of the 12 volt relay coil about a 100 times.   __ Designed by Tony van Roon  VA3AVR

Two Lead Solid-state Breaker Resets itself -  03/30/95 EDN Design Ideas:  Simple dc circuit breaker can protect a power supply from the ever-present screwdriver or even isolate a satellites dual power system from a short circuit in one subsystem __ Circuit Design by Phil Harvey, Technology Integration Inc, Bedford, MA

Two Simple Circuit Relay Based Motorcycle Alarms -  Motorcycle Alarm Circuits:  These are two - easy to build - relay-based alarms.  You can use them to protect your motorcycle - but they have many more applications.  if you use relays with 6-volt coils - they'll protect your "Classic Bike".  Both alarms are very small.  The completed boards occupy about half a cubic-inch - 8 cc.  The standby current is zero - so they won't drain your battery.  __ Designed by Ron J.

Ultra Low Power Latching Relay Circuit -  The circuit below takes advantage of some inexpensive small super capacitors.  The circuit pumps 6 volt pulses into the separate 5 volt latching and unlatching relay coils.  A short 25ms pulse is all that is needed to flip the relay states.  A third super capacitor is used to supply a higher peak current than a small lithium cell might otherwise be able to supplyCircuit Solution by David Johnson P.E.-April, 2007

Ultra-Sensitive Solid State Clap Switch -  Here is the circuit of a highly sensitive clap switch that can be operated from a distance of up to 10 metres from the microphone.  Signals picked up by the microphone are amplified by transistors T1, T2, and T3.  Diode D1 detects clap signals and the resulting positive voltage is applied to the base of__ 

Universal Flasher using a Latching Relay -  Latching relays are nifty devices.  Most contain two separate coils.  When a voltage is applied to one coil the relay latches in one state and stays in that state until voltage is applied to the unlatch coil.  Since the latching and unlatching pulses only . . . Hobby Circuit designed by David A. Johnson P.E.-October, 2005

Water Activated Relay -  in his circuit Marin has used two transistors wired as a high gain compound pair.  Transistor T1 may be a 2N2222A and T2 a BC108.  The current gain will be the product of each transistors beta, which will be a minimum of 140 x 110 or 15400.  The power supply used can be any voltage from 4.5 to 15 volts,  __ Designed by Marin Lukas

Water Filling Station Controller -  This circuit was requested by a company needing a simple way to fill a large tank with water with little fuss.    They wanted a low tech method to automatically fill the tank with water without constant monitoring.    They wanted to press a single pushbutton switch and then walk away.    in their application, the water tank is first filled to a certain level.    The fixed volume of water is then slowly drained off into a second tank, where the water is mixed with another material.    The empty tank is then ready to be filled again, for another cycle. . . . Hobby Circuit designed by David A. Johnson P.E.-April, 2011

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