Night light with LEDs. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Night light with LEDs. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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LEDs with increased brightness that appeared not so long ago allow a new approach to such a device as an ordinary home night light. By excluding incandescent lamps from it, it will be possible to create an almost "eternal" small-sized structure that can be placed, for example, in an installation box under a light switch.

The device (see figure) operates in two modes. The first of them we will conditionally call "day", the second - "night". When the night lamp is in "day" mode, when the VD2 photodiode is illuminated, the EL1-EL3 LEDs glow with reduced brightness. If the level of ambient light becomes less than the threshold, the LEDs will burn with maximum brightness.

LED night light

The alternating mains voltage is supplied through the quenching capacitor C1 and the current-limiting resistor R2 to the diode bridge VD1. The rectified voltage from the output of the bridge is supplied to the main part of the structure.

When the night lamp is operating in the "day" mode, a noticeable current flows through the illuminated photodiode VD2. Therefore, the field-effect transistor VT1 is open and shunts the emitter junction of the high-voltage transistor VT2 with a low resistance of the open channel. Transistors VT2 and VT3 are included as a composite, and transistors VT1 and VT2 are included according to the Schmitt trigger circuit. Therefore, when the transistor VT2 is closed, VT3 is also closed. The LEDs glow due to the small current flowing through the resistor R8.

When the photodiode is dimly lit, the device switches to "night" mode. The resistance of the photodiode is high, the transistor VT1 is closed, VT2 and VT3 are open. The brightness of the LEDs is maximum. Transistors VT1 and VT2 are covered by positive feedback in order to provide two stable states of the transistor VT3, since it works without a heat sink and during the transient process (when VD2 is moderately lit) it would not be fully open and it would not dissipate a lot of power.

LEDs EL1-EL3, in addition to their direct purpose, also play the role of a low-power zener diode, which protects the photodiode and gate of the field-effect transistor from overvoltage.

The resistance of the resistor R2 may seem somewhat overestimated, however, even with its resistance of 240 ohms and sparking in the power circuit, the transistor VT3 may fail.

Resistors can be used any small-sized specified or higher power. Resistor R3 can be made up of several resistors of lower resistance connected in series. Capacitor C1 - K73-16, K73-17 for a voltage of at least 400 V, C2 - any ceramic or film, for example, KM-5, K73-9. Instead of the diode bridge indicated in the diagram, you can use KTs407A or replace it with four diodes of the KD209 series. Instead of the field effect transistor KP501V, it is permissible to use any other from the KP501 series or "telephone" assemblies KP1014KT1, KP1064KT1 with letter indices A, B). We replace the KT940AM transistor with KT940A, KT969A, 2N6517, and VT3 - with 2N6520, 2SA1625 with letter indices M, L, K.

Instead of the FD265A photodiode, FD265B, FD256 or another similar small size or high-resistance photoresistor, for example, SF2-8, SFK-1M, is suitable. The author used imported red LEDs with a diameter of 5 mm with a brightness of 3500 mcd, but any others with a brightness of at least 3000 mcd and even different colors will do. Domestic LEDs of the KIPD36 series work well in this device, which have a high luminosity at the same current.

A properly assembled sensor does not require adjustment. It may only be necessary to select the resistor R3 to obtain the necessary sensitivity, which is chosen high enough - after all, the operation of a night lamp at full power at twilight does not make sense.

When the device is operating in the "night" mode, the brightness of the LEDs is high, so it is advisable to direct the main light flux from them to the ceiling. If the ceiling is made in light colors, using LEDs of different colors, you can get a simple color composition on it.

If desired, the number of LEDs can be easily increased to five without correcting the details of the device.

When checking and adjusting the device, you must remember that it has a transformerless power supply, and take the necessary precautions.

In the "day" mode, the night light consumes a current of about 1 mA, and in the "night" mode - about 20 mA. The luminous flux from the LEDs should not fall on the photodiode.

Author: A.Butov, village of Kurba, Yaroslavl region

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