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Circuits Designed by Dave Johnson, P.E.

Chain Link Fence Alarm  
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, the wire is connected to one of the steel posts, held in the ground.  The typical earth ground resistance will be in the 1000s of ohms range, which is low enough to keep the n-channel FET turned off in the circuit below.  If the wire is cut or broken, ...

12C508 Smart Lock:   base sends a data sequence ( password) which is recognized by  key.  key checks  sequence and -if it recognizes it -sends to  base anor data sequence, as an answer to  password. At  Time when  base gets a correct answer (which means that  key has been introduced),  relay is put into action, and it can command a number of consumers. (added 4/02)

4 Digit Keypad:  (circuit design added 8/03)

5 Zone Alarm Circuit:  (design circuit added 8/02)

6A door lock motor driver for automotive:  AN456AN456AN456AN456AN456  nal (OVV), put at low level  enable of  L9937, confirming  hardware switch-off of  motor ; in this condition an output of  half bridge is in... (app note added 6/06)

Alarm Control keypad:  (circuit design added 8/03)

Analog Video Router:  Intersil Application Block Diagram  (app note added 6/06)

Anti Theft Motorcycle Alarm:  (electronic design added 9/02)

Anti-Bag-Snatching Alarm:  (electronic Circuit / Schematic added 10/04)

Anti-Theft Alarm for Bikes:  (electronic Circuit / Schematic added 10/04)

Anti-Theft Security for Car Audios:  (electronic Circuit / Schematic added 10/04)

Automatic door opener with PIC12C508:  This circuit can be used to operate an electric strike or an electromagnetic lock on a door. It is not  door being opened/closed, but a small electromagnetic strike which unlocks  door…. (added 4/05)

Basic RF Transmitter for PIR Sensors:  (circuit design added 8/03)

Combination Lock:  This circuit is very basic to build. To open a the lock which is connected to the K1 Load. you must press each momentary switch in the correct sequence. The sequence used in this circuit is S1,S2,S3,S4. If any of the other switches are pressed the circuit will reset and you will need to start over. Depending on how you wire the switches, you can use any4 switch combination (added 2/03)

Combination Lock (PIC16F84):  (circuit design added 8/03)

Cut Phone Line Detector #1:  Can be used to alarm security system (added 4/02)

Digital Combination Lock:  (electronic design added 9/02)

Digital Electronic Lock:  (electronic (schematic / circuit added 11/05)

Door/Window Sensor Resists Tampering:  01/21/99 EDN-Design Ideas  / (added 11/05) The simple, inexpensive circuit in Figure 1 detects a failure (or deliberate tampering) on lines connected to normally closed switch sensors. For example, common door interlocks and door/window sensors consist of normally open or normally closed magnetic reed switches. Depending on the monitoring configuration, an open or short on a line may go undetected, thus preventing alarm activation. Embedding a resistor in a normally closed sensor and using bipolar dc power supplies produces the balanced configuration in Figure 1. A short or open on a line (or sensor activation) produces a net positive or negative voltage at the input of the Q1-Q2 pair....