Light switch in utility room. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Lighting
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The automatic light switch, the circuit of which is shown in the figure, turns on the EL1 incandescent lamp when the utility room door is opened, leaves it constantly on when the door is locked from the inside and turns it off after the person has left the room.
Reed switch SF1 is installed on the door frame. Its contacts are closed when the door is closed under the action of the field of the magnet installed on it. As long as the internal door latch is not locked, the mechanically connected SA1 switch is open. Transistors VT1, VT2 and triac VS1 are closed, lamp EL1 is disconnected from the mains.
When the door is opened, the reed switch contacts open and the capacitors C3 and C4 begin to charge. After 1 ... 1,5 s, the voltage across the capacitor C4 will become sufficient to open the transistors and the triac and the EL1 lamp will be turned on. The charging of the capacitors continues for some time, and only after it is completed, the capacitor C4 is discharged through the resistor R5. As a result, after 30 ... 40 s, the EL1 lamp goes out.
If the door is closed (without locking it with a latch), the discharge of the capacitor C4 will begin immediately and the lamp will go out after about 10 s. To re-ignite it, it is enough to open the door for a few seconds (the time sufficient to discharge the capacitor C3 through the closed contacts of the reed switch SF1, the resistor R4 and the diode VD5).
When the door latch is locked, the EL1 lamp will light up and stay on, regardless of the position of the door. In this case, the opening voltage is supplied to the base of the transistor VT1 through the resistor R3 and the closed contacts of the switch SA1.
Lamp EL1 can also be turned on by switch SA2. It can be a "regular" wall or specially installed. They are used if it is necessary to turn on the light in the room for a long time with the door open. There is another option. By setting this switch in such a way that its contacts are closed when the door is open (as in a refrigerator), you can not worry about turning the light on and off if you enter the room for a short time, leaving the door open.
In the design, you can use KT361 transistors with letter indices B - E or other low-power p-n-p structures, for example, MP25B. Diodes VD1, VD2, VD5, VD6 - KD105B or D226B. Zener diodes D814G (VD3, VD4) can be replaced with D814V. Capacitor C1 - K73-17 or MBGCH, oxide - K50-35.
The reed switch is placed in a plastic case (for example, the SMK-1 security alarm transmitter) and fixed on the door frame. The magnet is placed on the door. Switch SA1 is mounted in such a way that its contacts are closed when the door is locked with a latch.
Fusible insert FU1 is necessary to protect the triac VS1 when the EL1 lamp burns out. At this point, an electric arc usually occurs inside the lamp and the current flowing through the triac VS1 increases to a dangerous value.
Author: A.Ivkin, Istra, Moscow region
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