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Circuits designed by David Johnson, P.E.
Last Updated on: Thursday, April 07, 2016 02:04 PM

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Alarm Circuits
Dave's Circuits:  Alarms Page 1Page 2, Page 3, Page 4

120 VAC Low Voltage Alarm  -  This circuit will activate a beeper alarm whenever the AC line voltage is less than 100vac. . . . [Circuit by Dave Johnson P.E.,  07/09/06]


3v Sweeping Siren Alarm  -  A while back I was challenged by a visitor to the website.  He needed a very loud sweeping siren type audio sound generator powered by 3v.  He tried some of the commercial sirens but they were not very loud when powered by 3v.  He also said that those . . . [Circuit by David A. Johnson P.E.,  06/24/08]


AC Line Under/Over Voltage Alarm  -  Power lines, which deviate much beyond normal voltages can damage expensive electronic equipment.  The circuit below sounds an alarm whenever the line voltage is higher or lower than normal.  I set the alarm limits at about +-15% from standard levels . . . [Hobby Circuit designed by David A. Johnson P.E.,  03/29/09]

AC Power Loss Alarm  -  I’ve been having a problem with my instant hot water system lately.  This system circulates electrically heated water throughout my house using a small pump.  The pump is wired into a ground fault interrupter (GFI) module.  Every now and then, the GFI trips, leaving me without hot water . . . [Circuit by Dave Johnson P.E.,  03/29/09]


Alarm Beeps when the “Check Engine” Light Turns On  -  When wired into a car’s indicator light panel, this circuit will turn on a loud beeper when the “check engine” light turns on . . . [Circuit by Dave Johnson P.E.,  07/09/06]


Alarm Discriminator + Battery Monitor of the Electric Field Disturbance Monitor  -  This schematic is the alarm discriminator plus the batterry monitor of my "Electric Field Disturbance Monitor".  The system can detect human and animal motion by the electric fields they disturb . . . [Hobby Circuit designed by Dave Johnson P.E.,  03/26/02]


Alarm for a Chain Link Fence  -  Long stretches of steel fence can be monitored using the simple circuit below. An insulated wire is woven through the links in the fence.  Where the fence begins, the circuit is connected to an earth ground and the security wire.  At the end of the fence, . . . [Hobby Circuit designed by Dave Johnson P.E.,  10/16/05]


Alarm Sounds At Freezing Temperatures  -  This electronic hobby circuit below is designed to activate a beeper alarm, whenever the outside air temperature is below 0 degrees C (32F) .  A 9v battery powers the hobby circuit.  The average 9ua current is so low that the battery should last for many years . . . [Hobby Circuit designed by David Johnson P.E.,  07/11/06]


Alarm Sounds when Phone Line is Cut  -  Burglars, rapists and other criminals will often cut a phone line before they break into a home.  The cut phone line will prevent many alarm systems from alerting the police.  The battery powered circuit below will sound an alarm when the unit . . . [Hobby Circuit designed by David Johnson P.E.,  10/16/05]


Alarm sounds when Power Cord is Unplugged  -  Many medical devices, such a portable X-ray and ultrasound equipment, carry their own batteries. However, the frantic pace of some hospital emergency rooms might cause the device to be shoved off into a corner without being plugged back into an AC power source to recharge their batteries. So, I designed this circuit to emit an audible alarm whenever the device’s power cord was unplugged from the wall for a period of time . . . [Hobby Circuit designed by David A. Johnson P.E.,  07/08/00]


Alarm sounds when Water is Detected  -  This simple circuit sounds a beeper when its electrodes detect water. It is powered by a single 1.5v N cell. A small 1.5v button battery will also work . . . [Circuit by David Johnson P.E.,  06/11/00]


Alarm-Piezo Device detects Motion  -  Using a piezo device, this circuit will activate a beeper whenever the circuit is moved.  It could be used as an earthquake alarm . . . [Circuit by Dave Johnson P.E.,  07/09/06]


Another Very Loud piezo Alarm Beeper  -  This is yet another beeper circuit that really draws attention. It sweeps the drive frequency slightly to produce a very annoying sound. It uses a transformer to increase the drive voltage across the piezo device to more than 200 volts peak to peak . . . [Circuit by David A. Johnson P.E.,  06/27/00]


Appliance Unplugged Alarm  -  I received a call from an administrator at a large prison.  He told me that he was having problems with inmates who keep unplugging refrigerators, vending machines and microwave ovens in a commons area.  He wanted a battery-powered device, which could be mounted to the outside of the power cord of an appliance. . . . [Hobby Circuit designed by Dave Johnson P.E.,  04/25/10]

 

 

 

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