Circuits Designed by Dave Johnson, P.E.

• DOORKNOB ALARM
  Many companies offer simple alarm devices for personal use in bedrooms or hotel rooms. A metal chain attached to a box holding the electronics is placed around the inside doorknob of a wood door. Anyone grabbing the knob from the outside is detected by the electrical capacitance change that occurs from the human hand contact between the knob and the box. Almost all of the commercial devices sold use a more expensive and power consuming radio frequency circuit approach to detect the capacitance change. But, a very inexpensive and micro power technique can also work. This circuit schematic should dramatically reduce the cost of the device and allows it to operate for many years from one set of batteries.

Also see Completed Project  by Matthew C. Ruschmann

• Earthquake Alarm Circuit
  Geophones are dandy motion sensors.  They are metal cans containing a powerful magnet, suspended with springs inside a coil of wire.  The magnet functions as a floating mass.  The slightest vertical motion of the device causes the coil to move relative to the magnet.  This induces a small voltage in the wire coil...

• ELECTRIC FIELD DISTURBANCE MONITOR
  This schematic is the power supply and front-end sections of the field monitor that is discussed in more detail at Electric Field Disturbance Monitor. (this link is off-site) The system can detect human and animal motion by the electric fields they disturb.

• ELECTRIC FIELD DISTURBANCE MONITOR
  This schematic is the motion discriminator alarm and battery monitor sections of the field monitor that is discussed in more detail a Electric Field Disturbance Monitor. (this link is off-site)

• Fake Car Alarm Light
  Whenever the car’s ignition is turned off, this circuit activates a flashing LED, which can be positioned to appear as an active alarm system. (added 7/06)

• Freezer Over-Temperature Alarm
  This circuit turns on a beeper whenever the inside temperature of a freezer is greater then zero degrees Centigrade.  The circuit draws only a few microamps from a 9 volt battery.  It uses a glass bead thermistor accurate to 1 degree C.   (added 12/04)

• Freezing Temperature Alarm
  This circuit turns on a beeper whenever the outside air temperature is less than zero degrees Centigrade.  The circuit draws only a few microamps from a 9 volt battery.  It uses a glass bead thermistor accurate to 1 degree C.   (added 12/04)

• Low Voltage 120vac Power Line Alarm NEW
  This circuit will turn on a beeper when the line voltage drops below 100 volt AC.

Daylight Alarm:    The circuit presented here wakes you up with a loud alarm at the break of the daylight. Once again the 555 timer is used here. It is working as an astable multivibrator at a frequency of about 1kHz.  The circuit's operation can be explained as follows:  When no light falls on the LDR, the transistor is pulled high by the variable resistor. Hence the transistor is .....   (added 10/05)

Digital combination lock:    (circuit added 2/07)

Digital Combination Lock:    (schematic added 9/02)

Digital Electronic Lock circuitdb:    The digital lock shown below uses 4 common logic ICs to allow controlling a relay by entering a 4 digit number on a keypad. The first 4 outputs from the CD4017 decade counter (pins 3,2,4,7) are gated...(added 05/07)

Door Alarm:    Hangs up on the door-handle.  Beeps when someone touches the door-handle from outside (added 3/05)

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,....

Doorknob Proximity Alarm :    Reverse engineered circuit diagram of a popular retail doorknob alarm. It contains a small transmitter and the doorknob acts as an antenna so it will not work on a metal door. When a person comes close to the doorknob this loads down the oscillator and the alarm sounds. The proximity sensor circuitry is perfectly capable of driving 74HCT series CMOS logic instead of the noise maker IC's shown in the circuit diagram.  (added 2/05)

Doorknob touch alarm:    The project was originally intended as a present for my brother’s dorm room, but a bad capacitor and the lack of a proper oscilloscope caused delays. It has not made it off the breadboard, and it probably will not until his next semester. The circuit contains a few basic elements, an flip-flop based oscillator, a set of delays, a flip-flop as a sensor, and the audible alarm.  (project added 6/07)

Electronic Buzzer:    This very simple circuit just uses a couple of resistors, a capacitor and the easily available 555 timer IC. The 555 is setup as an astable multivibrator operating at a frequency of about 1kHz that produces a shrill noise when switched on. The frequency can be changed by varying the 10K resistor.   (added 10/05)

Electronic Canary Doorbell:    This circuit is a modified "Hartley Oscillator" with a couple extra parts.  T1, the LT700 (or equivalent, is a small center tapped (ct) audio transformer with an impedance of 1000 ohms at 1000 hertz.  The secondary of this transformer has an impedance of 8 ohms.  Usually noted as 1K:8 ct.  I tried a couple other models like the red and green types (taken out of a $5 made-in-china am pocket radio) and they both work good.  You may have to adjust the caps to get the sound... (added 2/06)

Electronic Card Lock System:    (electronic Circuit / Schematic added 10/04)

Electronic door codelock PIC16F84:    (circuit diagram added 2/07)

Electronic Keypad:    (electronic design added 9/02)

Emergency Light and Alarm:    Powered by2-AA NI-CD batteries Four switchable options (added 6/06)

Emergency Siren Simulator:    This siren circuit simulates police, fire or other emergency sirens that produce an up and down wail.... (electronic design added 6/07)

Enhanced 4 Digit Alarm Keypad:    (circuit diagram added 1/07)

Enhanced 4 Digit Alarm Keypad:    (electronic design added 6/07)

Enhanced 4-Digit Alarm Keypad:    Pressing a single key on the keypad - will energize the relay. Entering a four-digit code of your choice - will de-energize it. The circuit was designed to control the Modular Burglar Alarm System -...(added 05/07)

Enhanced 5 Digit Alarm Keypad:    (circuit added 1/07)

Enhanced 5 Digit Alarm Keypad:    (electronic design added 6/07)

Enhanced 5-Digit Alarm Keypad:    The relay is energized by entering the first four digits of your chosen five-digit security code. Entering the full five-digit security code - will de-energize the relay. When "A, B, C & D" are...(added 05/07)

Enhanced Alarm Keypad:    (diagram added 6/03)

Everything -that-moves alarm:    A crucial failing of proximity detectors is their unreliable and tricky nature. This is where they are used to detect humans, not to speak of smaller living beings. One common approach is to detect.... (electronic design added 10/06)

Factory Siren:    This circuit produces a sound similar to a factory siren.  It makes use of a 555 timer IC used as an astable multivibrator of a center frequency of about 300Hz. The frequency is controlled by the pin 5 of the IC. When the supply is switched ON, the capacitor charges slowly and this alters the voltage at pin 5 of the IC hence the frequency gradually increases. After ....   (added 10/05)

Fire Alarm #2:    This circuit warns the user against fire accidents. It relies on the smoke that is produced in the event of a fire. When this smoke passes between a bulb and an LDR, the amount of light falling on the LDR decreases. This causes the resistance of LDR to increase and the voltage at the base of the transistor is pulled high due to which the supply to the COB (chip-on-board)....  (added 10/05)

Fire Alarm:    (electronic circuit added 02/05)

Four Digit Keypad:    (diagram added 6/03)

Fridge Door Alarm:    (circuit added 9/04)

Frost Alarm #2:    The thermistor used has a resistance of 15k at 25 degrees and 45k at 0 degrees celsius. A suitable bead type thermistor is found in the Maplin catalogue. The 100k pot allows this circuit to trigger over a wide range of temperatures. A slight amount of hysteresis is provided by inclusion of the 270k resistor. This prevents relay chatter when temperature is near the switching threshold of this circuit. (added 2/06)

Frost Alarm zen22142:    (schematic added 9/02)

Fuse Blown Indicator:    It came to me in a flash. A simple way to see if a fuse has blown without removing it from its holder. Its not often you can design a circuit using just two components, but with just one resistor and an LED this circuit provides visual indication of when a fuse has blown....   (added 10/05)

Fuse Monitor/Alarm:    a simple way to see if a fuse has blown without removing it from its holder (added 4/02)

Gate Alarm Circuit zen22142:    (diagram added 6/03)

Gate alarm circuit:    (diagram added 2/07)

Gate Alarm:    A cheap and simple Gate Alarm, that is intended to run off a small universal AC-DC power supply. IC1a is a fast oscillator, and IC1b a slow oscillator, which are combined through IC1c to emit a...(added 05/07)