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Circuits designed by David Johnson, P.E.
Last Updated on: Monday, July 11, 2016 01:59 PM

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Relay Circuits

50ua Current Drives Solid State Relay Requires Only  -  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 Johnson P.E.,  06/03/00]


Bidirectional Solid State Relay  -  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 . . . [Circuit by Dave Johnson P.E.,  05/01/09]

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 Johnson P.E.,  07/31/11]


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 David Johnson P.E.,  12/07/10]


High Temperature Thermocouple activates Relay  -  Voltage from a high temperature thermocouple activates a relay when the sensor detects a temperature above 1000 degrees F . . . [Hobby Circuit designed by David A. Johnson P.E.,  07/09/06]


High-speed 75MHz light receiver  -  This circuit uses one tiny C-MOS inverter IC to form a modulated light receiver with a very fast response. It is designed around a PIN photo diode that is packaged for use with plastic optical fibers. It can be used as an optical fiber receiver. By using the open end of the optical fiber it can "sniff" out any modulated light signals . . . [Circuit by Dave Johnson P.E.,  06/11/00]


Light Receiver Works from 1Khz to Over 70Mhz  -  This circuit uses one tiny C-MOS inverter IC to form a modulated light receiver with a very fast response. It is designed around a PIN photo diode that is packaged for use with plastic optical fibers. It can be used as an optical fiber receiver. By using the open end of the optical fiber it can "sniff" out any modulated light signals . . . [Circuit by David Johnson P.E.,  06/11/00]


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 . . . [Circuit by David Johnson P.E.,  08/13/06]


On/Off Switch With 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 . . . [Circuit by David Johnson P.E.,  12/24/09]


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 Johnson P.E.,  12/07/10]


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.,  02/11/02]


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.,  02/10/02]


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 A. Johnson P.E.,  08/03/08]


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 Johnson P.E.,  08/03/08]


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 Dave Johnson P.E.,  04/06/14]


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 can’t 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.,  01/20/12]


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.,  06/03/00]


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.,  05/01/09]


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.,  06/03/00]


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.,  06/03/00]


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.,  08/13/06]


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.,  12/24/09]


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.,  02/11/02]


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.,  07/09/06]


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.,  11/07/10]


Through Glass Pushbutton 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. . . . [Hobby Circuit designed by Dave Johnson P.E.,  02/10/02]


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 supply . . . [Circuit Solution by David Johnson P.E.,  04/29/07]


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.,  10/16/05]


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.,  04/24/11]


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