Timer for the forgetful. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Timer for the forgetful. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Clocks, timers, relays, load switches

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Probably, many, having sat in the morning on a bus or a fixed-route taxi, begin to remember: "Is the iron turned off?" After a while, doubt turns into panic. We stop the transport and run back home. After some fuss with the entrance door, the elevator and the door to the apartment, after catching our breath, we find that the iron was still turned off. However, the mood for the whole day is spoiled, and a reprimand for being late is provided at work. But this is half the trouble. In a different scenario (and the iron is not turned off, and the saving thought did not visit), everything could have ended much worse.

Having found myself in a similar situation several times and not wanting to bring matters to a disaster, I decided to make a device that limits the duration of the connection of an iron or other electrical appliance to the network. The idea is that by turning on the iron in the network through a similar timer and pressing the "Start" button, you will be able to iron clothes for, for example, 15 minutes, after which the iron will automatically turn off. It doesn’t matter if you don’t manage it, press the button again and continue ironing.

The timer circuit is shown in the figure. Since this is a safety device, particular attention has been paid to reliability during development. The number of parts is reduced to a minimum, electromagnetic relays are used as switching elements.

Timer for the forgetful

The device operates as follows. After pressing the button SB1 "Start" relay K1 is activated and self-blocking closed contacts K1.1. Contacts K1.2 and K1.3 connect the load connected to the XS1 socket and the timer itself to the network. The voltage rectified by the VD1 diode bridge is stabilized at a level of approximately 20 V by the HL1 LED connected in series and the VD3 zener diode. The excess extinguishes the capacitor C1, and the resistor R1 limits the amplitude of the pulse of the charging current of the capacitors C1 and C2 at the moment of switching on. The voltage applied to the zener diode VD2, during normal operation, is less than its stabilization voltage, but in the event of an open in the HL1 LED circuit, this zener diode prevents a dangerous increase in the rectified voltage. A break is quite possible if the LED is installed outside the timer board and connected to it by wires. With guaranteed strength of the connections, the VD2 zener diode can be omitted.

From the moment the relay K1 is activated and a voltage of 20 V appears, the charging of the capacitor C3 through the resistor R2 begins. The voltage on the capacitor slowly rises, and as soon as its value exceeds the threshold for the unijunction transistor VT1, it will open, relay K2 will work, contacts K2.1 will break the winding circuit of relay K1, which will open the contacts of the latter and disconnect the load from the network After a few seconds, the capacitor C3 will be completely discharged through the emitter-base section of the transistor and the winding of relay K2. The timer has returned to its original state and is ready for a new start by pressing the SB1 button.

With the values of resistor R2 and capacitor C3 indicated in the diagram, the exposure time is approximately 15 minutes. If there is a need to change it, instead of a constant resistor R2, you can install a series-connected variable (470 kOhm) and constant (39 kOhm). The adjustment interval will be approximately 0,5...15 min.

The maximum load power that can be connected to the timer depends on the properties of the K1 relay contacts. The author used the RP21 relay with a 220 V AC winding and three groups of contacts powerful enough to switch an electric iron with a power of more than 1 kW.

As K2, a JZK-20F relay with one group of contacts for switching was used. It can be replaced by any other with similar contacts and a response voltage of not more than 12V with a winding resistance of at least 400 ohms.

Author: V.Volodin, Odessa, Ukraine

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