In residential spaces, there's often a need for emergency lighting soft illumination that turns on only when a person enters. This lighting is especially convenient in hallways, bathrooms, storage rooms, and other areas where constant light isn't required, but comfort and safety are important.

Today, the market offers a variety of ready-made solutions, from infrared to microwave (radar) motion sensors, as well as devices based on them. However, most of them are designed for mains power, have redundant functionality, and, as a result, consume an unreasonable amount of power for such a simple task detecting a person's presence and turning on lighting.

This paper discusses a cost-effective solution based on the LD2410S radar sensor with a current consumption of approximately 1.5 mA. The device is designed for direct connection to a battery with a voltage of 9 to 55 V, which covers virtually the entire range of batteries used in UPS and inverter systems. This approach ensures uninterrupted operation of emergency lighting regardless of the state of the industrial power grid.

Despite its low power consumption, the sensor is capable of directly switching loads up to 100 W, which is more than sufficient for modern LED luminaires. This allows for the creation of a stand-alone, reliable, and energy-efficient emergency lighting system without the need for additional control electronics. Unlike infrared sensors, a radar sensor reliably detects the presence of a person even when stationary.

The schematic diagram of the device is shown in Figure 1, and its connection diagram is shown in Figure 2. They clearly demonstrate the composition of the main components and the interconnections between them.

Power from the battery is supplied to connector CN1 and then to the low-power pulse converter U1. This converter ensures the device operates over a wide range of input voltages from 9 to 55 V.

The output of converter U1 generates a stabilized voltage of 5 V, which is used to power a second converter U2. This unit steps down the voltage to 3.3 V, which is necessary for the correct operation of radar presence detection sensor U3.

The sensor's output signal (pin OT2) controls MOSFET transistor Q1, which switches the load. The light is connected to connector CN2 and turns on only when a person's presence is detected.

The sensor connection diagram is shown in the following figure. The battery is connected to the CN1 input connector, and the lighting fixture is connected to the CN2 output connector.

Please note that when using an LED lamp, which is highly desirable for maintaining the overall energy-saving operation of the autonomous system, it is necessary to maintain the polarity of its connection. In this circuit, power is supplied to the lamp through SENSOR1 from the same battery that powers all other components of the device.

For the primary voltage conversion, the circuit uses DC-DC converter U1 — a module based on the LX8015 microchip. The exact markings of the microchip are usually not specified by manufacturers, but it is crucial that the input voltage is in the range of 5–80 V, and the output voltage is a stable 5 V. In practice, the actual operating range may be somewhat narrower. The external appearance of the module is shown in Figure 4.

If the module's dimensions are not critical, any other DC-DC converter with similar parameters can be used. However, please note that the device's current consumption may differ from the stated value.

The second step-down converter U2 was also selected for efficiency. DThe L78L33A is well suited for this task, generating a voltage of 3.3 V, which is necessary for the operation of the next module in the circuit, U3.

The main element determining the operation of the entire device is the human presence sensor U3. Similar sensors are widely available in electronic component stores, but the HLK-LD2410S model exhibits the lowest power consumption.

As an alternative, you can consider the HLK-LD2420 sensor. However, keep in mind that when using it, the current consumption of the entire circuit in standby mode will increase by approximately 4-5 times.

The following MOSFETs are suitable for output switch Q1: IRLZ24N, FQP30N06L, IRLZ44N, IRLZ34N. Their distinctive feature is a low turn-on threshold from 2V. To reduce the height of the printed circuit board, these transistors must be used in a TO-262 package (with a cut flange), for example IRFSL3306PBF or IRLZ24NL.

Any lamp with the appropriate voltage and power rating of up to 100 W can be used. The author recommends LED lamps with a rated voltage range of 12 to 80 V, for example.. In this case, you are completely independent of the battery's discharge level, or even its operating voltage.

Figure 5 shows the circuit assembled on a circuit board. A professionally designed circuit board, taking into account all the sensor's features, as well as a file for 3D printing the housing, can be found below. Photo 6 shows a version of the sensor mounted on a wall next to a lamp. It is mounted with double-sided tape, eliminating the need for drilling.

If the circuit is assembled correctly, adjustment may only be required for the LD2410S sensor. All other components of the circuit are not adjustable and begin operating immediately after power is applied.

By default, the LD2410S sensor is set to its highest sensitivity, which in practice often proves excessive, especially when used in small spaces. In such conditions, the sensor can detect a person's presence even through a wall, resulting in the lighting not actually turning off.

There are two ways to reduce the sensor's sensitivity: software and hardware.

The software method involves connecting to the LD2410S via the UART interface and configuring operating parameters using specialized software. This method will not be discussed in this paper, as it is already described in detail in the official documentation here and here.

Of much greater interest is a hardware-based method for reducing sensitivity, experimentally tested by the author and not described in the available documentation. Simply shielding the sensor with conductive or dielectric materials does not produce the desired result under such conditions, the sensor begins to operate unstable and unpredictably.

During the experiments, a material was selected that effectively absorbs the sensor's radiation while introducing minimal distortion into its operation. This material turned out to be two nanocrystalline rings measuring 4.3×7.2×3 mm (core ring 0703). Each ringIt is glued directly to the sensor's emitter, as shown schematically in the figure on the left. Ferrite has proven ineffective for this purpose.

As can be seen from the illustrations above, the device can be easily assembled on a standard circuit board. However, if a more accurate and professional implementation is required, below is an implementation option designed for a full-fledged 62x31 mm printed circuit board.

Note to the board: Connectors CN1 and CN2 can be any type of connector, as long as the pin spacing is 5 mm. The author used the following: DB302-5.0-2P-GN-S.

It is recommended to solder the U3 radar module at a slight angle so that after installation, the plane of the two antenna elements located on the module's board is oriented toward the monitored area of ​​the room.

The files for 3D printing the sensor housing are provided below. The housing design is tailored to the specifics of the radar module and does not significantly affect its operation.