Insulated light switch with timer. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Lighting

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There are various ways to isolate the switch from the AC mains and the appliance it controls. This can be done by transmitting on and off commands via radio [1] or using IR radiation [2]. You can turn on the power using an electromagnetic or optoelectronic relay.

Fig. 1

One of the simplest solutions is to use a low-power isolation transformer included in the control circuit of the triac switching mains voltage. A diagram of a switch constructed according to this principle is shown in Fig. 1. Mains voltage 220 V is applied to contacts 1 and 2 of the XT1 block, and one or more incandescent lighting lamps are connected to its contacts 3 and 4 (or vice versa). Fluorescent "energy-saving" lamps should not be used with such a switch, since the current they consume is of a pulsed nature, and the device will be unstable. The triac VS1 is connected in series to the lamp supply circuit. Between its electrodes 2 and the control one, the primary winding of the step-down transformer T1 is connected.

In the initial state, the contacts of the mechanical switches SA2 and SA1 connected to the XT2 block are open. If more switches are required, additional switches can be connected in parallel with these. Only its no-load current (only a few milliamps) flows through the primary winding of the transformer, which is the smaller, the greater the inductance of the winding. Since it is not enough to open the triac VS1, the lighting remains off.

When the contacts of any of the mechanical switches are closed, the transformer T1 operates in short circuit mode. The current in its windings is now greater and sufficient to open the triac VS1. Since the triac opens near the beginning of each half-cycle, almost full mains voltage is supplied to the lamps. And the voltage on the primary winding of the transformer T1 after the opening of the triac does not exceed 2 ... 3 V, so the transformer does not overload.

Fig. 2

The elements of the device are placed on a printed circuit board made of one-sided foil fiberglass, the drawing of which is shown in Fig. 2. XT1 and XT2 - X9777B series screw terminal blocks with a contact pitch of 7,62 mm, but you can use others or do without them at all. Transformer T1 is a low-power network transformer with a secondary winding voltage of 8 ... 12 V and a primary winding current in idle mode of not more than 10 mA.

Fig. 3

In the author's version (Fig. 3), a transformer with an open-circuit current of 5 mA was used from a Sh4300 digital multimeter. Several unified transformers of the TP-112 series, available to the author, turned out to be unsuitable, their no-load current exceeded 15 mA.

Trimmer resistor R1 - SPZ-19. When adjusting the switch, its slider is initially set to the middle position. Then, by connecting an incandescent lamp to the XT1 block and

network, they find such a position of the trimming resistor engine so that when the SA1 (SA2) switch is open, the lamp is turned off, and when it is closed, it is turned on.

The alternating voltage of the secondary winding of the transformer, available between the contacts of mechanical switches, when they are all open, can be used for illumination. It is useful for finding switches in the dark. The main thing is that the current consumed by the backlight node is less than that at which the main lighting devices are turned on.

Fig. 4

A possible diagram of the backlight unit is shown in fig. 4. Its elements are placed in the housing of a conventional switch, using wired wiring and drilling holes for the LEDs. In each half-cycle, only one of them shines, while protecting the other from increased reverse voltage. If desired, one LED can be replaced with a conventional diode of any type, which will only perform a protective function.

With the resistance of the resistor R1 indicated in the diagram, the load current of the secondary winding of the transformer T1 of the circuit breaker does not exceed 1 mA. Given the large transformation ratio, this very slightly increases the current of the primary winding, without creating a danger of untimely opening of the triac VS1. For high-brightness LEDs, 1 mA is enough for a noticeable backlight. If desired, its brightness can be increased by reducing the resistance of the resistor R1, but making sure that the increased current does not cause the switch to malfunction.

Fig. 5

If you want to turn on the lighting only for a certain time, followed by automatic shutdown, instead of a mechanical switch (or in parallel with it), you can connect an electronic timer to the secondary winding of the isolation transformer T1, assembling it according to the circuit shown in Fig. 5. With a two-wire cable, one pair of contacts of the block XT1 (1,2 or 3, 4) of the timer is connected to one of the same pairs of contacts of the block XT2 of the switch (see Fig. 1). The remaining free pairs of contacts on both pads are reserved. Additional mechanical switches or their groups can be connected to them.

In the initial state, the voltage of the secondary winding of the isolation transformer is supplied to the rectifier diode bridge VD1. Through the VD2 diode, the rectified voltage charges the capacitor C1 to 12 ... 15 V. In this state, the HL1 LED illuminates the SB1 timer start button. Since the capacitor C2 is discharged, the field effect transistor VT1 is closed. Lighting remains off.

When you press the SB1 button, even for a short time, the electrical charge accumulated in the capacitor C1 is redistributed between the capacitors C2 and C1. As a result of discharging capacitor C1 and charging capacitor C2, the voltage across them becomes the same and equal to 9 ... 10 V. This is ensured by an appropriate choice of capacitor capacitance. Resistor R3 limits the recharge current.

As soon as the voltage on the capacitor C2 exceeds the opening threshold of the transistor VT1, its opened channel will close the diagonal of the VD1 bridge, and with it the secondary winding of the isolation transformer. Lighting will be on. In this case, the HL1 LED will go out, and the VD2 diode will close. The discharge of the capacitor C2 through the resistor R2 will begin. The field effect transistor will remain open until the voltage across the capacitor approaches the threshold. Then it will begin to gradually close, reducing the current in the transformer windings. The triac will open with an increasing delay relative to the beginning of each half-cycle of the mains voltage. This will lead to a smooth decrease in the brightness of the lighting lamps up to their complete shutdown. Shortly before this, the operation of the proximity switch may become erratic, resulting in several flashes of lighting lamps.

With the ratings of the elements indicated in the diagram, an exposure time of about 3 minutes was obtained before switching off. With a selection of capacitor C2 and resistor R2, it can be changed.

Fig. 6

Fig. 7

All elements of the timer are mounted on a printed circuit board made of fiberglass laminated on one side, the drawing of which is shown in Fig. 6, and the appearance - in Fig. 7. The button and the LED are installed from the side of the printed conductors.

Fixed resistors - C2-23 or imported, capacitors - imported. Replacing the field effect transistor IRFZ30 - IRL2505L or IRL3205, and the diode bridge KTs405A - four separate diodes of the KD105 or 1N4001 - 1N4007 series. The same diodes are suitable instead of 1 N4002.

The L-5013UWC LED can be replaced with another one with increased brightness and any color of glow. Button SB1 - PKn159 or NS-A6PS-130. But other non-latching buttons with a sufficiently long pusher are also suitable. A large button can be mounted on the case in which the board is placed.

Fig. 8

On fig. 8 shows a diagram of another version of the timer. In contrast to the above, here between the circuit that determines the exposure time and the gate of the field-effect transistor VT1 there is a node on the Schmitt trigger elements of the DD1 microcircuit. The supply voltage of this microcircuit comes from the capacitor C1.

In standby mode, the capacitor C2 is discharged, C3 is charged, a high voltage level is set at the outputs of the elements DD1.1 and DD1.2, so it is low at the output of the element DD1.3 and the field effect transistor VT1 is closed. The lighting lamps are off and the HL1 backlight LED is on.

With a short press of the SB1 button, the capacitor C2 is charged, the high level at the output of element DD1.1 will change to low, and the low level at the output of element DD1.3 will change to high. The field-effect transistor VT1 opens, the lighting lamps turn on, the HL1 LED goes out, and the capacitor C3 quickly discharges through the protective diode of the DD1.2 element.

When the capacitor C2 is discharged through the resistor R2 so much that the voltage level at the output of the element DD1.1 becomes high again, the charging of the capacitor C3 will begin. This will set a high level at one of the inputs (pin 5) of the DD1.2 element. The generator assembled on this element will start working, generating pulses with a frequency of about 1 Hz. Through the element DD1.3, they will go to the gate of the field-effect transistor VT1, periodically closing and opening it. As a result, the lighting lamps will flash at the specified frequency. This means that the lighting time is coming to an end.

After some time, the charging current of the capacitor C3 will drop to a value at which the voltage drop across the resistor R4 will decrease to the corresponding low logic level. The operation of the generator on the element DD1.2 will stop, and the timer, having finally turned off the lighting, will return to its original state.

Since the first time the timer is connected to the isolation transformer T1 (see Fig. 1), the capacitor C3 is discharged, the lighting lamps will flash until it is charged. This may indicate that the timer is working.

Fig. 9

All elements of the timer are mounted on a printed circuit board, the drawing of which is shown in Fig. 9. It is made from one-sided foil fiberglass with a thickness of 1,5 ... 2 mm. Clamps XT1.1 and XT1.2 are contact pads with screws and nuts inserted into the holes. The appearance of the mounted board - in fig. 10. The SB1 button and the HL1 LED are installed on the side of the printed conductors.

With the ratings of the elements indicated in the diagram, an exposure duration of about 10 minutes was obtained. It can be changed by selecting capacitor C2. The duration of a series of flashes at the end of exposure depends on the capacitance of the capacitor C3, and the frequency of their repetition depends on the capacitance of the capacitor C4.

Literature

1. Nechaev I. Remote power switch. - Radio, 2011, No. 7, p. 42, 43.
2. Rusin A. IR light switch. - Radio, 2004, No. 2, p. 46-48.

Author: I. Nechaev

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