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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) |
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Digital Combination Lock - (schematic added 9/02) |
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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) |
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Door Alarm - Hangs up on the door-handle. Beeps when someone touches the
door-handle from outside (added 3/05) |
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Door Knock Beeper - In some apartment buildings and homes, not equipped with a door
bell, it may be hard to hear someone knocking on the front door. The circuit below
provides a means to activate a loud beeping sound inside, whenever someone knocks on the
door from the outside. The circuit is powered by a 9v battery, which should provide
years of service. An on/off switch allows the device to be turned off if desired....
….(designed by David A. Johnson) |
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Door/Window Sensor Resists Tampering - 01/21/99 EDN-Design Ideas: 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,.....(design idea added 11/05) |
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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. ….(designed by David A. Johnson) |
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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) |
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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) |
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Dozen Small CMOS Alarm Circuits - This is a selection of small self-contained alarm
circuits. They have a very low standby current; and are suitable for battery operation.
Some are triggered by normally-open and normally-closed switches. Some react to changes in
light or temperature. The result is a variety of output times and patterns. (circuit
design added 04/05/06) |
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Dummy Alarm - This project makes a superbright LED flash briefly once every 5 seconds
to imitate the indicator light of a real alarm. The circuit is designed to use very little
current to prolong battery life so that it can be left on permanently. This is a simple
project, ideal for a beginner.....Designed by John Hewes (circuit added 06/07/08) |
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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... ….(designed by David A. Johnson) |
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Electric Field Disturbance Monitor-1 - 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. The system can detect human and animal motion by the electric
fields they disturb. ….(designed by David A. Johnson) |
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Electric Field Disturbance Monitor-2 - 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) ….(designed by David A.
Johnson) |
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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) |
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Electronic Canary Doorbell - This circuit is a modified "Hartley Oscillator" with a
couple extra parts. T1, the LT700 (Tony van Roon's or equivalent, is a small center
tapped (Tony van Roon's 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 (Tony
van Roon's 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... (Tony van Roon's electronic design
added 2/06) |
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Electronic Card Lock System - (electronic Circuit / Schematic added 10/04) |
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Electronic door codelock PIC16F84 - (circuit diagram added 2/07) |
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Electronic Door Release - This circuit is designed to operate an electrical
door-release mechanism, but it will have other applications. When you enter the four-digit
code of your choice, the relay will energize for a preset time period. Use the relay
contacts to power the release mechanism. The standby current is virtually zero, so battery
power is a realistic option. |
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Electronic Keypad - (electronic design added 9/02) |
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Electronic Lock circuitdb - The 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) |
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Emergency Light & Alarm - Powered by2-AA NI-CD batteries Four switchable options
(added 6/06) |
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Emergency Siren Simulator - This siren circuit simulates police, fire or other
emergency sirens that produce an up and down wail. (circuit design added 08/08) |
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Enhanced 4 Digit Alarm Keypad - (circuit diagram added 1/07) |
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Enhanced 4-Digit Alarm Control Keypad - This Keypad is suitable for the Modular
Burglar Alarm. However, it has other applications. Pressing a single key will energize the
relay. Entering the Four-Digit code of your choice will de-energize it….(circuit design
added 08/08/08) |
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Enhanced Alarm Keypad - (diagram added 6/03) |
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Enhanced Shed/ Garage Alarm - This is an enhanced version of the simple Garage/Shed
Alarm. The Entry and Exit delays have been increased to about 30-seconds, and I've added a
timed Siren cut-off and automatic Reset. I've also replaced the LED with an entry Buzzer.
These enhancements mean that the new version will have a much wider application. Photo
(circuit design added 04/05/06) |
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Ethernet Based Security System - ….(electronic schematic added 08/08) |
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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) |
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Expandable Multi-Zone Modular Burglar Alarm - This circuit features automatic Exit and
Entry delays and a timed Bell Cut-off. It has provision for both normally-closed and
normally-open contacts, and a 24-hour Personal Attack/Tamper zone. By using the Expansion
Modules, you can add as many zones as you require; some or all of which may be the inertia
(shock) sensor type. (circuit design added 04/05/06) |
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Expandable SCR Based Burglar Alarm - This is a simple SCR based burglar alarm circuit.
Its features include automatic Exit and Entry delays, together with a timed Bell Cut-off
and Reset. The basic alarm has a single zone. This will be adequate in many situations.
However, the modular design means that you can add as many zones as you like to the
system. Details of a Four-Zone expansion module are provided. (circuit design added
04/05/06) |
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Expandable Transistor Based Burglar Alarm - This transistor based alarm features
automatic Exit and Entry delays, together with a timed Bell Cut-off and system Reset.
Along with the Exit/Entry zone, the basic alarm board has one Instant Zone. This will be
adequate in many situations. However, the modular design means that it's easy to add as
many zones as you like to the system. Details of a Four-Zone expansion module are
provided. (circuit design added 04/05/06) |
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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) |
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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) ….(designed by David A. Johnson) |
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Fire Alarm - 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 8/06) |
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Four Digit Keypad - (diagram added 6/03) |
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Four Small Transistor Alarm Circuits - This is a selection of simple self-contained
transistor based alarm circuits, complete with photographs. They are designed….(circuit
design added 04/05/06) |
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Four-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) |
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Freezer Over-Temperature Alarm - 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.... ….(designed by David A. Johnson) |
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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. ….(designed by David A.
Johnson) |
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Fridge door Alarm - Beeps if you leave open the door over 20 seconds. 3V battery
operation, simple circuitry. (circuit added 9/04) |
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Frost Alarm (Andy Collinson) - 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) |
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Fuse Monitor/Alarm - a simple way to see if a fuse has blown without removing it from
its holder (added 4/02) |
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Gate alarm circuit - Figure 1 represents a cheap and simple Gate Alarm, that is
intended to run off a small universal AC-DC power supply….(diagram added 2/07) |
