Security alarm sensor: overview of the working mechanism.

• Optical proximity sensorsare small cubes with a convex frosted glass facing the security zone. The frosted glass is called a Fresnel lens, it is divided into sectors with different angles of inclination and density relative to the surface. Light transmitted through such lenses penetrates fog for many nautical miles. The security area varies depending on the type of lens built into the optical proximity sensors, such as vertical, wide in depth, focused or blurred area. Security alarm sensorsare characterized by the following mechanism of operation: a heat emitter appears in the protection zone, the security alarm sensor detects a change in thermal radiation in the infrared spectrum, then this change is amplified and controlled by the final power stage. The basis of the device is a pyroelectric detector — it reacts to a change in the IR background, so a moving object does not cause a change in the state of the sensor. In order to be able to effectively use the device as an automatic light switch in a room, a switch-off delay unit is introduced into the security alarm design. The sensitivity of the device is adjusted by changing the angle of inclination and approaching the lens of the sensor itself, as well as electronically — by adjusting the gain of the first stage of the circuit.
• Infrared sensor— is a pyroelectric detector consisting of sensitive ceramic surfaces covered with a quartz window that transmits only IR rays. For the protection of indoor spaces, the most widely used are passive IR sensors of motion and combined passive + microwave alarm sensors. Combined sensors are distinguished by much higher reliability and resistance to false alarms. Passive IR sensors are capable of operating without interruption around the clock.

The sensor operates as follows:

A rapid change in the thermal field in the sensor's active zone causes small voltage surges at the detector's output; this signal is amplified by the first stage of the amplifier. The signal is fed to the non-inverting input. The IR sensor circuit includes four independent identical operational amplifiers, united by power supply, implemented.

Some tips for safe construction of the structure: solder passive IR sensors carefully, preferably with an antistatic grounded bracelet, without overheating the sensor leads — solder each lead for no more than 1 second. Overheating can disable the device or worsen its sensitivity. It is better to take the Fresnel lens from the security system sensor, since it gives the greatest effect.

Security alarm device

Security alarm device assumes that the sensitive side of the sensor is directed to the controlled area and is placed at a distance of 1.7-2.5 cm from the surface of the Fresnel lens. The Fresnel lens is glued to the body, while you need to ensure that the glue does not get on the protective quartz window of the sensor. If there is no industrial lens, you can use a regular flat matte plexiglass, 40 x 60 mm in size. This is a simple IR sensor circuit for a security alarm.