Wiring a motion sensor for lighting to a spotlight or light bulb

Recommendation points

• Types of motion sensors
• Typical switching schemes
• Is there a difference in the type of lighting fixtures
• Heavy load management
• Setting operating modes
• Connecting multiple sensors
• Eliminating false positives

We offer you comprehensive instructions for connecting lighting devices to various types of motion sensors or presence detectors. Automatic lighting control is used everywhere, providing not only ease of use, but also energy savings.

Types of motion sensors

The correct operation of the sensor depends on the suitability of its type to the operating conditions and the installation site. The detection efficiency depends on the type of built-in detector, but this is a completely separate topic. For now, it is only necessary to determine what differences may be in the signal conversion circuit, starting with the detector output and ending with a contact relay that closes the power circuit of the lighting device.

The simplest type of sensors has a built-in contact pair and does not require additional intermediate devices. This is also true for most of the combined sensors: the built-in comparator compares the signal levels from different detectors and closes the circuit only if they all reported object detection..

The situation is more complicated with sensors designed to work as part of burglar alarm systems. This category includes most microprocessor-based sensors, which are least prone to false positives. At the output, such devices form a digital signal, therefore, to trigger the relay, it is required to install an intermediate module with an ADC on board and amplify the logic level with a transistor..

Typical switching schemes

In general, two types of installation of presence sensors can be distinguished: in the immediate vicinity of the lighting device (including together with it in the same building), or at a more or less significant distance.

Most sensors with built-in relays have three connection terminals. The contacts marked L and N are used to connect the supply circuit, from which voltage is supplied to both the sensor circuit and the normally open relay contact. The L out terminal, sometimes marked with an arrow, is the second part of the contact pair that closes with the L contact when the sensor is triggered. Thus, the voltage is supplied with a two-core cable, the relay contacts are connected to the break of the phase wire.

Wiring diagrams of motion sensor to lighting

If the luminaire is separate from the sensor, it is easier to connect them through a junction box. A three-core cable comes into it from the sensor, in which one core is used as neutral to power the device, and the other two are used for switching. The wiring inside the box follows the same pattern as for a conventional switch.

Connecting a motion sensor through a junction box

It is almost impossible to mix up the connection, in the worst case, the sensor simply will not turn on, which is usually observed when the supply phase is applied to the reverse side of the relay contact. Most motion sensors are not sensitive to polarity, but it is recommended to put the phase wire in the gap – this is the requirement of the PUE.

Is there a difference in the type of lighting fixtures

As many know, the failure of lamps occurs not so much because of their long-term operation, but because of the consumption of the on / off resource. Unlike working in tandem with a conventional switch, the lamp is ignited by the sensor up to 20-30 times more often, which is why the light source quickly breaks down. If the sensor switches a conventional incandescent lamp, halogen floodlight or any other light device with a spiral, it is recommended to install an NTC thermistor of appropriate power in the open circuit of the power supply circuit to anticipate current surges. The resistance is selected experimentally, depending on the required smoothness of the start, usually within 200-1500 Ohm.

Connecting a motion sensor to a light bulb through a thermistor

Energy saving and fluorescent lamps are less sensitive to frequent switching on, but only if their power supply is provided with a hot start. A delay in turning on such lamps can negatively affect the usability. Increase the sensitivity of the sensor and choose the correct type.

The switching frequency has practically no effect on LED devices. In this case, it is recommended to switch the mains voltage with the sensor to the power supply unit. This will not only exclude parasitic consumption at idle, but also eliminate contact bounce due to the snubber built into the power supply.

Heavy load management

Almost all sensors, judging by the marking, are designed for switching a load of 10 or 16 A, and practice shows that even these indicators are greatly overestimated. If the power of the lighting device is more than 1 kW (current 4.5-5 A), it is recommended to add an intermediate switching device to the circuit.

If the lamp is connected to the sensor through a junction box, a compact magnetic contactor will solve the problem. The DIN rail mounts are small enough to attach to the side of the box. It is important to ensure that the supply voltage of the contactor coil is equal to the mains supply. In this case, the coil replaces the lamp in the standard circuit, and the lighting device itself is connected through the main terminals of the starter.

Wiring a motion sensor to lighting through a contactor

In some cases, even a compact modular contactor may not be appropriate. Here you can resort to the method of contactless switching with high-voltage field-effect transistors. The scheme captivates with its simplicity, besides, it can be assembled even by surface mounting. For example, take IRF740APBF – a powerful N-channel transistor with a maximum operating voltage of 400 V and a load current of up to 10 A. To build a circuit, you need a couple of such elements: we are dealing with alternating current, respectively, a half-wave must be passed in both directions.

The power part of the circuit consists of two identical transistors, their drain and source are connected oppositely and are installed in the break of the load supply circuit. The control part of the circuit has no galvanic isolation, it consists of a protective rectifier diode with a breakdown voltage of at least 300 V, as well as a voltage divider of two resistors connected in series. The recommended opening voltage of the IRF740APBF is 25 V, that is, about a tenth of the mains – the divider arm ratings should differ by the same amount. You can safely take the 47 and 4.7 kΩ resistors by connecting the gate through the midpoint. The circuit can be supplemented with a shunt resistor of 100 kΩ or more connected between the gate and the neutral wire through a diode. The shunt is needed to pull up the low level of the control signal in the off state, for such powerful field workers with such a capacity this can be very critical.

Setting operating modes

Most standard sensors have three controls on their housing: SENS, TIME and LUX. Before proceeding to their setting, you need to make sure that the sensor itself is installed in the correct position and its viewing angles completely cover the access control zone. To do this, set the SENS control to the limit position, and then check whether the detector is triggered when a person enters the monitored area. The SENS regulator can be used to adjust the sensor’s viewing range, for example, so that it would be triggered by people approaching the entrance, but not by cars passing by.

Microwave motion sensor DD-MV 201

Using the LUX control, it is determined at what level of illumination the additional light should be switched on. It is recommended to adjust this setting during twilight, setting the vernier to the position where the sensor is just starting to respond. Finally, the TIME knob determines the duration of the light on, this parameter is individually configurable.

Wall-mounted infrared motion detector ERA MD 03

Connecting multiple sensors

Rooms and monitoring areas can be of different shapes and sizes, with the sensor controlling only one lighting fixture or network. To ensure confident presence detection, the sensors can be positioned at a certain angle to each other, opposite and multidirectional, providing maximum detector coverage.

Usually it is required to turn on the light when at least one sensor from the group is activated. In this case, the contacts of the relay groups of all detectors are connected in parallel. It is only important to ensure that all sensors in the group commute either the phase or neutral wire, otherwise a short circuit will most likely occur when turned on..

Parallel connection of motion sensors

If the light is to be turned on only when both sensors are triggered simultaneously, the task becomes more difficult. If one relay contact was not connected to the power terminal, the sensors could be connected in series with contact groups. Because of this feature, it is required to install an intermediate relay, the coil of which turns on when one sensor is triggered. In this case, the load is interrupted first by the contacts of the second sensor, and then by the normally open contact of the additional relay.

Eliminating false positives

To summarize, we will give an important recommendation on how to eliminate false positives. Sometimes the sensitivity adjustment does not give the desired result: the sensor either continues to work idle, or stops responding altogether due to the narrowing of the control zone.

Typically, this behavior is typical for microwave sensors, which are not obstructed by windows with doors or even solid walls. So, a sensor installed in an apartment can be triggered by movement in the entrance or at neighbors. The first thing to do is to make sure the sensor is oriented correctly and, if necessary, change its direction and installation location..

If the above is impossible to do, several sensors should be installed with the expectation of their joint triggering. In this case, the sensitivity of the sensors is chosen close to the minimum, usually such a solution is well suited for small rooms and vestibules, as well as when implementing complex lighting control schemes.