Periodic load timer

The design of a simple timer that allows you to turn on and off the load, at predetermined time intervals. The operating time and pause time are independent of each other.

Varieties of Timers

The use of timers in everyday life has now become quite common. Therefore, such a device can simply be bought at an electrical goods store. Most often, these are multichannel timers that allow you to program switching on / off at a certain time of the day, and even taking into account the day of the week.

But sometimes a timer is required that works simply according to the “work - pause” algorithm. You can turn it on simply by hand, but the operating time and pauses can be adjusted independently of each other. One example where you might need just this time relay, can serve as a "Chizhevsky chandelier."

A bit of history

Chizhevsky's chandelier is a device for saturating air with negative oxygen ions. The inventor of the chandelier, the famous Soviet scientist Alexander Leonidovich Chizhevsky, began to engage in experiments on aeroionization of air back in 1922 in one of the laboratories of Glavnauka. But, as often happened at that time, in 1942, the scientist was repressed and stayed in exile in Karaganda until 1950. But Chizhevsky continued his work there: aeroionotherapy sessions in the regional Karaganda hospital helped many patients with wound healing. In 1958, the scientist returned to Moscow, where until the last days of his life he was engaged in the implementation of aeroionization.

In addition to wound healing, the Chizhevsky chandelier is an excellent prophylactic that prevents the development of many diseases, and also improves performance, both mental and physical. There has been much debate in the literature about the benefits or dangers of a chandelier, and even articles titled "DIY Chizhevsky Chandelier."

It is recommended to use the Chizhevsky chandelier starting with short sessions, gradually increasing their number and time. But, if the chandelier is turned on constantly, the concentration of aero ions in the air can exceed the optimum, which is not entirely good for health. You can control this concentration simply by turning the device on and off manually, which, you see, is not very convenient. To simplify this process will help the simplest timer, performed on just one logical chip.

Of course, such a timer can find many more applications when periodic switching on - switching off the load is required. Figure 1 shows a circuit diagram of a timer.

Figure 1. Timer for periodic load on.

Actually the timer in this case is a rectangular pulse generator on the elements DD1.1 ... DD1.4. The duty cycle of the pulses can be adjusted, and both the pulse time and the pause time are independently set.

The entire device is powered by a transformerless power source with ballast capacitor C1 and a rectifier bridge VD1. The transistor VT1 is used as a zener diode. The stabilization voltage in this case is about 10 V — the K561 series microcircuits are operable in the range of power supply 3 ... 15 V. Therefore, a voltage of 10 V is enough for normal operation of the circuit as a whole.

Load turn on triac VS1, which, in turn, is turned on by a low-power triac optocoupler pair U1.1. The latter contains a built-in circuit for determining the transition through zero of the mains voltage. Therefore, there will be no switching interference in the network. It is this circumstance that explains the absence of an input line filter in the circuit.

To control the optocoupler pair, a key cascade made on the transistor VT2 is used. The LED of the optocoupler pair U1.1 and the LED HL1, which indicates the inclusion of a load, are included in its collector circuit. Resistor R10 limits the current through the LEDs.

The scheme works as follows. In the initial state, all capacitors are naturally discharged. When you turn on the power through resistors R3 and R4, capacitor C3 starts charging. Until it is charged, the input of the DD1.1 element is logic zero, and, of course, one at the output. This state leads to the fact that at the output of the element DD1.4 is also a logical unit that opens the transistor VT2, through its collector-emitter junction, the optocoupler U1.1 LED is turned on. The latter includes a triac VS1, connecting the load. The HL1 LED also lights up to indicate that the load is on. This timer position is called “Operation”.

In this position of the generator, the output of the element DD1.2 is a logic zero voltage, which does not allow charging of the capacitor C4.

Capacitor C3, do not forget about it, is already charging from the moment the power is turned on. When the voltage across it reaches the level of a logical unit, a low level will appear at the output of the logical element DD1, and a high level at the output of the DD1,3 element. This state of the circuit leads to the closing of the transistor VT2, and, consequently, to the disconnection of the load.

Capacitor C4 will start charging through the element DD1.3 and resistors R6 ... R8. In this case, the capacitor C3 is quickly discharged through the diode VD2, resistor R6, logic element DD1.2, which is at this time in a state of logical zero at the output.

When the capacitor C4 is charged, at the output of the element DD1.2, the level of the logical unit will be established. This will result in a low setting on the output of DD1.3. Therefore, through the element DD1.4, the transistor VT2 opens, the load will be connected. Also, through the element DD1.3 and resistors R6 ... R8, the capacitor C4 is discharged.

In addition, the appearance of a logical unit at the output of the element DD1.2 prevents the discharge of the capacitor C3 through the diode VD2 and the resistor R5. With charging capacitor C3, a new timer cycle begins.

The duration of the operating time and pause is set using the variable resistors R4 and R7, respectively. With the values ​​indicated on the diagram, it can be changed within 3 ... 30 minutes. In this case, the pause time does not depend on the operating time, since the charging circuits of the capacitors are different. The adjustment device assembled from serviceable parts does not require, except setting the desired operating time and pause.

If you still need to set up, you should remember that the device does not have galvanic isolation from the network. Therefore, it is better to use a safety transformer for commissioning. In this case, as a load, you can use a conventional lighting lamp with a power of 25 ... 100 watts.

A few words about the details. The ratings of the parts are mainly indicated on the circuit diagram. All permanent resistors such as MLT or imported, most likely Chinese, variables SPO, SP4-1. Capacitor C1 for a working alternating voltage of at least 250V, such is usually used in line filters, or of type K73-17 for a working voltage of at least 400V. Electrolytic capacitors C3 and C4 with a low leakage current, otherwise the shutter speeds will be unstable. Here, too, imported capacitors, for example, the JAMICON brand, are better suited.

If the load power does not exceed 400W, the VS1 triac can be installed without a radiator.

The KT 816B transistor can be replaced with a Zener diode D 815B. In this case, its cathode should be connected to the + capacitor C2.

Design

The device can be made in a plastic case of a suitable size, there are plenty of them on sale now. It should not be forgotten that the design has transformerless power, that is, it is under voltage. Therefore, the handles of variable resistors are also better made from plastic.

Boris Aladyshkin