Figure 1 represents 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 high pip-pip-pip warning sound when a gate (or window, etc.) is opened. The circuit is intended not so much to sound like a siren or warning device, but rather to give the impression: “You have been noticed.” R1 and D1 may be omitted, and the value of R2 perhaps reduced, to make the Gate Alarm sound more like a warning device. VR1 adjusts the frequency of the sound emitted.
IC1d is a timer which causes the Gate Alarm to emit some 20 to 30 further pips after the gate has been closed again, before it falls silent, as if to say: “I’m more clever than a simple on-off device.” Piezo disk S1 may be replaced with a LED if desired, the LED being wired in series with a 1K resistor.
Figure 2 shows how an ordinary reed switch may be converted to close (a “normally closed” switch) when the gate is opened. A continuity tester makes the work easy. Note that many reed switches are delicate, and therefore wires which are soldered to the reed switch should not be flexed at all near the switch. Other types of switches, such as microswitches, may also be used.
By Rev. Thomas Scarborough
Cape Town
E-mail scarboro@iafrica.com
Source :: http://www.zen22142.zen.co.uk/schematics.htm
Related Links
More circuit about IC 4011 cmos
More Security Circuits
More circuit digital counter
Frost Alarm or Cold Activated Switch by IC 741
Opto-Thermo Alarm with IC 741, 4001
Single Zone Digital CMOS Alarm
Event Failure Alarm By IC Timer 7555
Miniature Loop Alarm
Gate Alarm by IC 4011
Enhanced 4 Digit Alarm Keypad by And Gate
This entry was posted on Thursday, June 21st, 2007 at 12:12 pm and is filed under Circuit diagram, Digital, Electronic Control.
