Proximity switch based on PIR module. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Clocks, timers, relays, load switches

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LED light sources - lamps, tapes - are widely used today for interior lighting. And although their power is calculated in units of watts, the total power of all lighting fixtures installed even within the same room can be significant. Of course, leaving the lights on in a room where there are no people is irrational. This habit will lead to additional costs when paying electricity bills.

Recently, proximity switches have appeared based on the so-called PIR sensors that respond to the presence of people. These switches are completed, for example, with street spotlights, which are placed in front of the front door of a country house. The sensors react to the infrared radiation of a person or a group of people that appear in their "field of view", and turn on the lighting. PIR modules can be purchased at radio amateur shops or online stores both separately and as part of various electronic kits. In fact, the PIR module is a functionally complete design. To control the lighting, depending on whether there are people in the room or not, it is enough to supplement it with only a few external components.

The appearance of one of the PIR modules - HC-SR501 - is shown in fig. 1. On one side of the board are electronic components, including a microcontroller, on the other - under a plastic Fresnel lens - the PIR detector itself. The built-in integrated stabilizer allows you to power the module with a constant voltage of 5 to 15 V. The operation mode is set by a switch installed in the upper right corner of the module, two trimmers allow you to adjust its sensitivity (from 1 to 6 ... 8 m) and the shutdown delay time in the absence of PIR sensor signal (from 1 to 5 min). At the bottom (in Fig. 1) there are three pin contacts for connecting power and the controlled device. The designations of these pins can only be seen on the opposite side of the board by removing the Fresnel lens. The control signal at the output of the module takes two values: 0 V if the PIR sensor does not detect movement, and 3,3 V when people appear in front of the PIR sensor.

Rice. 1. Appearance of the PIR module - HC-SR501

A small output current (less than 5 mA) and a low voltage (only 3,3 V) at the output of the module do not allow connecting an executive relay directly to it. However, if you use a switching power supply from a personal computer to power LED lighting devices, you will need one single element - an optocoupler - to create an automatic lighting control system! True, such a system, when motion is detected, will turn on the lighting, regardless of whether it is day or night outside the window.

The reason is that the module does not have a light sensor - a photoresistor, although printed areas for its connection are provided (they are located slightly above and to the left of the already mentioned pins for supplying power and removing the output signal). The problem can be easily solved by soldering two pins of suitable sizes to these pads (they are accompanied in Fig. 1 by the inscription "Pins for connecting to a photoresistor").

A diagram of an automatic non-contact lighting switch based on a modified PIR module is shown in fig. 2. Here U1 is an ATX form factor computer power supply, B1 is a PIR module, EL1-EL3 and EL4 are LED lamps powered by U1, R1 is a photoresistor, R2 and R3 are current-limiting resistors of lamps. A high logic level that occurs at the output of the module when the movement of an infrared radiation source is detected is fed to the emitting diode of the optocoupler U2. As a result, the optocoupler transistor opens, closing the PS-ON input of the power supply U1 to a common wire, and it turns on, lighting the lamps connected to it.

Rice. 2. Scheme of an automatic non-contact light switch based on a modified PIR module

Installation of the device can be hinged, especially since the photoresistor must be moved outside the room where the PIR module is installed, otherwise it will go into trigger mode, turning the lighting on and off in turn. You can make the design more compact and reliable by placing all components (except for the photoresistor) on the adapter board. A drawing of a possible variant of such a board is shown in Fig. 3.

Rice. 3. Board drawing

To connect a PIR module and a communication cable with a photoresistor, the board has sockets cut out from a 40-pin socket with a 2,5 mm pitch. They securely fix the module on the board, and additional fastening is not required. A 20-socket socket from an old computer motherboard was used to connect to the power supply. Instead of the EL817 optocoupler, you can use any transistor optocoupler with similar technical characteristics (for example, the PC817 series). However, it should be borne in mind that the case, the number and purpose of the pins may be different for them than for the EL817, so the board drawing may need to be corrected. A Chinese-made GL5516 photoresistor was used as a photo sensor (dark resistance - 0,5 MΩ, resistance at illumination of 10 lux - 5 ... 10 kΩ).

If the PIR module is to be used as a switch, the jumper on its board must be set to the upper position (according to Fig. 1). Then the module controller will withstand the time interval set by the trimmer. When the jumper is set to the lower position, the module will turn off as soon as the signal from the PIR detector disappears. This mode is used in photo and video surveillance systems.

Author: I. Tsaplin

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