Digital kitchen timer. Encyclopedia of radio electronics and electrical engineering

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

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The hostess puts the dish in the oven and remembers it only when smoke poured out of the oven ... A similar situation is familiar to many. A timer will help to avoid it, which after a certain time gives a signal, warning that the dish in the oven needs attention.

The main advantage of the proposed kitchen timer - ease of use. As long as the mains plug is inserted into a 220 V socket, the device is ready for use, just press the button. On the indicators at any time you can see how much time is left before the end of exposure. After its expiration, a beep will sound with a smoothly increasing volume, after which the timer will automatically turn off, waiting for a new command. The mains plug can remain plugged into the outlet all the time, since in the passive state (before the start of the exposure and after the end of the sound signal), almost all timer nodes are de-energized. From the network at this time, only the no-load current of the power transformer is consumed.

Schematic diagram of the timer is shown in fig. one.

(click to enlarge)

The clock generator is assembled according to a typical circuit on a K176IE12 (DD2) chip with a ZQ1 quartz resonator with a frequency of 32768 Hz. Its pulses (with a period of 1 s or 1 min, depending on the position of the switch SA1) are fed to the timing unit - counters DD3 and DD4 K561IE14. They are included in such a way that they work "by subtraction", so setting the shutter speed and counting down the time. Elements DD5, DD6, HG1, HG2 are designed to decrypt and indicate the remaining time.

To the transfer outputs of counters DD3 and DD4, a matching element of diodes VD11, VD12 and resistor R24 ​​is connected. A low logic level at its output serves as a signal for the end of exposure. The differentiating circuit R1C1 and the threshold element - the Schmitt-tbDDI.1 trigger - form an impulse during which the sound signal is turned on.

Element DD1.3 is an audio frequency signal generator, transistor VT5 is its amplifier. The sound frequency is modulated by triangular-shaped pulses coming through the resistor R19 from the generator on the DDI.2 element. This gives the signal a more noticeable, vibrating character. The tuning resistor R20 can change the vibrato frequency.

Thanks to the R23R27VD13C11 circuit, at the time the timer is triggered, the signal sounds at a reduced volume, which gradually increases as the capacitor C11 charges. Variable resistor R26 set the maximum volume of the signal.

The basis of the power supply unit of the timer is the transformer T1. The voltage from its winding II, rectified by the diode bridge VD6, is supplied to the stabilizer (transistors VT3, VT4). The voltage drop across the HL1 LED and the VD5 diode, through which the rectifier load current flows in the forward direction, increases the potential difference between the cathodes and anodes of the HG2,3 and HG2,6 indicators by 1 ... 2 V, which leads to a noticeable increase in the brightness of the generated digits .

The alternating voltage of the glow of the indicators comes from the winding 111 of the transformer T1. When the timer is in the passive state, the electronic switch on transistors VT1 and VT2 breaks the heating circuit. The drop of alternating voltage on the key open during exposure does not exceed 0,15 ... 0,2 V. Therefore, at a nominal heating voltage of 6 V for IV-1 indicators, the voltage on winding III of transformer T1 should be equal to 1,2 V.

When the timer is connected to the network, but is in passive mode, of all its nodes, only the rectifier on the VD6 diode bridge works. There is no exemplary voltage on the basis of transistor VT3, therefore transistors VT3 and VT4 are closed and there is no voltage in the +9 V circuit. The VT1VT2 key is open, since to open the transistors it is necessary that the voltage in the +9 V circuit exceed the stabilization voltage of the VD10 zener diode.

When you press any of the SB2 "Set units" or SB3 "Set tens" buttons, the voltage from the capacitor C3 of the rectifier filter through the resistor R8 is supplied to the zener diode VD8, which limits it to approximately 9 V. Through the VD3R3 circuit, voltage is supplied to the base of the transistor VT3, opens it and activates the stabilizer. The +9 V voltage is supplied to all timer nodes, and the VT1VT2 key turns on the glow of the indicators.

In the future (after releasing the buttons SB2, SB3), the voltage in the circuit for forming the exemplary voltage of the stabilizer is present due to the diodes VD1 and VD4. Their anodes are connected to such timer points that during the entire exposure time and signal sounding, the voltage in at least one of them has a high logic level. At the end of the exposure and the signal, the levels at both points become low, the reference voltage at the base of the VT3 transistor drops to almost zero, and the timer goes into a passive state. During exposure, you can force it into this state by pressing the SB1 "Off" button.

At the moment the +9 V voltage is turned on, the C9R15 circuit generates a short pulse that writes the code of the number 3 to the unit counter DD9 (it is set by the corresponding connection of the inputs D1-D8), and the zero code to the tens counter DD4. These figures appear on the indicators, and if the pressed button is immediately released, the timer will work out an exposure of 9 minutes or seconds (depending on the position of the SA1 switch). But if the button SB2 or SB3 is kept pressed, pulses with a frequency of 2 Hz are fed to the clock input of the corresponding counter (DD3 or DD4), decreasing its contents by 1 with each pulse.

In order for the pulse to appear at pin 10 of the DD2 microcircuit exactly 60 s after the end of the time setting (release of the buttons), the second counter of the DD2 microcircuit is blocked by a high logic level supplied to its pin 9 from the VD8 zener diode, while at least one of the buttons is pressed.

After releasing the buttons, the clock input of the counter DD3 through the switch SA1 is connected to the output of minute or second pulses of the microcircuit DD2, and the clock input of the counter DD4 is connected to the transfer output of the counter DD3. The dwell time is counting down. After it expires, the contents of both counters will become zero, a low level will simultaneously appear at their transfer outputs (pins 7), the diodes VD11, VD12 will be closed and the capacitor C1 will start charging along the R1R2R24 circuit. A high level at the output of the element DD1.1 will allow the operation of the audio signal generator. After about 5 s, the capacitor C1 will be charged to the switching level of the element DD1.1, the high level at the output of which will change to a low one, which will stop the sound signal and turn the timer into a passive state. The signal from the output of the element DD1.1 goes to pin 5 of the DD2 chip, prohibiting its operation. Otherwise, in the "second" exposure interval, the counters DD3, DD4 may change state during the sound of the signal, which will lead to a failure.

The timer circuit board shown in fig. 2 is made of one-sided foil fiberglass.

(click to enlarge)

The conclusions and the body of the sliding switch PD-9 (SA1) are soldered directly to the pads. Fixed resistors - MLT, S2-33 or imported power indicated on the diagram. Trimmer resistor R20 - SPZ-16, SPZ-38a or SPZ-38v, you can install a pre-selected constant resistor instead. Variable resistor R26 - SP4-1 or SPZ-6a. It is mounted on a U-shaped bracket made of sheet brass or steel with a thickness of 0,5 ... 0,8 mm. Bracket development - in fig. 3. If there is no need to adjust the volume, the variable resistor R26 can also be replaced with a selected constant.

All oxide capacitors are K50-6, K50-16, K53-35 or K53-1a, the rest are ceramic KM, KD or imported. Diodes KD521A can be replaced by KD522 with any letter index. Two or three similar diodes connected in series will replace the HL1 LED.

Transistors VT1, VT2 - KT817 with any letter index and coefficient h21E of at least 80. Transistors VT3, VT4 must have h21E of at least 60 (transistors of the KT315, KT361 series are also suitable here). The composite transistor KT972A can be replaced with a conventional KT503 with any letter index (h21E - at least 100), and a diode of the KD521 or KD522 series must be connected in the forward direction to its emitter circuit.

Power transformer T1 - TP-321. Using a rather large gap between its secondary (II according to the timer circuit) winding and the magnetic circuit, winding III is additionally wound on the same frame - 38 turns of PELSHO wire 0,25 ... 0,31 mm. If not IV-1, but other seven-element fluorescent indicators (IV-2, IV-Za, IV-6) are used as HG3, HG22, the number of turns of winding III must be changed in such a way as to obtain the required heating voltage. Instead of the above transformer, you can use others that are suitable in terms of overall power (at least 3 W) with secondary windings of 12 ... 18 V (II) and 1,2 V (III) or allow winding such windings. It is desirable to choose a transformer from among those having a separate insulated section of the frame for the secondary windings. This will provide the necessary electrical safety for the timer.

Resistor R5 reduces the no-load current of the transformer permanently connected to the network, reduces its heating and increases reliability [1]. Indicated in the diagram of Fig. 1 rating and power of the resistor are optimal for the transformer TP-321, the no-load current of which does not exceed 5.. .7 mA. With a transformer, the idle current of which is more than 10 mA, you can use the MLT-2 resistor with a nominal value of 1,2 ... 1,5 kOhm, but better - reactive ballast resistance, which will reduce heat generation. The windings of the PCM, RES6, RES22 relays for an operating voltage of 24 V are well suited for this. The relay armature must be fixed in a position attracted to the magnetic circuit.

The timer board is placed in a plastic case with a closed green glass window for indicators. Buttons SB1 - KM1-I SB2, SB3 - KM2-I, sound emitter BF1 - DEMSh-1A. They are attached to the case, and connected to the printed circuit board with flexible wires. When using the ZP-1 piezo emitter as BF1, it is necessary to connect a resistor with a nominal value of 4,3 kOhm in parallel to it, installing it in the holes provided on the board. On fig. 2 this resistor is shown by a dashed line.

It is recommended to mount the T1 transformer, VD6 diode bridge, C3 capacitor and voltage regulator parts first. To check, a load equivalent is temporarily connected to the output of the stabilizer - an MLT-1 resistor with a resistance of 470 ... 560 Ohms, and between the base of the transistor VT3 and the positive terminal of the capacitor C3 - an MLT-0,25 resistor with a nominal value of 15 ... 18 kOhm. The output voltage of the stabilizer should be within 9 ... 9,5 V. If necessary, it can be slightly changed by installing other instances of the VD7 and VD9 zener diodes.

Before turning on the timer for the first time in the network, it is a good idea to fix a plate of insulating material 1x47 mm in size on its board at the T35 transformer from the side of the printed conductors (for example, by pressing it with the transformer fastening screws). The plate should close all the conductors and leads of the elements connected to the 220 V network. It is also desirable to isolate the resistor R5 and the fusible link FU1 well. This will allow you to check and adjust the timer without fear of electric shock.

After checking the operation of the stabilizer, you can remove the temporarily installed resistors and continue installation. It's better to start with jumper wires, because some of them will end up under the chips later on. One of the jumpers, highlighted in Fig. 2 thick line, serves not only as a connecting wire, but also as an element for fastening IV-6 indicators. Its dimensions, shown in Fig. 4 are selected taking into account the height of the transformer TP-321 and the diameter of the indicator cylinders. It is better to mount this jumper after the DD1 chip, but before installing the indicators. They are placed parallel to the board so that the top of the cylinders can be attached to the jumper with thread and glue.

After installing all the parts and visually checking the correct installation, make sure that there are no short circuits in the power circuits by "ringing" them with an ohmmeter. A correctly assembled timer starts working immediately after being plugged into the network and pressing one of the SB2 or SB3 buttons - the numbers on the indicators are lit, the time is counting down. Otherwise, you will have to check the supply voltage of the microcircuits (9 V) and the alternating voltage of the indicators glow (0,95 ... 1,05 V). If, when the timer is on, there are no pulses at the outputs of the DD2 microcircuit, or their repetition rate differs from the nominal one, the most common cause is a malfunction of the quartz resonator. If greater exposure accuracy is not needed, it can be dispensed with by replacing it with an RC circuit, as suggested, for example, in [2].

The timer is suitable not only for giving an audible signal, but also for turning on any load for a specified time, for example, a heater or a fan. Of course, this will require an electronic key or a relay of the appropriate power. Point A will serve as the output of the signal controlling the key (see Fig. 1). Unfortunately, the load capacity of this output is small. A more powerful signal is a voltage of 9 V directly from the output of the stabilizer. But keep in mind that its duration is longer than the specified shutter speed for the duration of the sound signal.

Please note that on the printed circuit board (see Fig. 2), the inputs of the unused element of the DD1 microcircuit are connected to point A. Therefore, at the output of the mentioned element (pin 11 DD1) during the exposure count - log. 0, during the sound signal - log. 1, and in the passive state of the timer, it is in a high impedance state.

Set the exposure as follows. When you press any of the SB2 or SB3 buttons, the timer turns on (unless, of course, the mains plug is inserted into the socket) and the number 09 is displayed on its indicators. When you hold down, for example, the SB2 button, the number in the units digit decreases by 1 twice per second (9 , 8, 7,..., 1, 0, 9,...). When the "Set tens" button is pressed, the digits in the tens digit change with the same frequency (9, 0, 18, 9, 0...). Thus, you can set any shutter speed in the range of 0-99 minutes or seconds. The unit of reference is selected using switch SA1. The shutter speed is automatically counted from the moment the buttons are released.

Memorization of the selected shutter speed is not provided. Therefore, it is asked anew each time. The order in which the setup buttons are pressed is in most cases irrelevant. However, in order to avoid failures, it is not recommended to press the SB3 "Set tens" button if the units digit is zero. You must first set any non-zero number of units, then - the desired number of tens, and only after that - zero units.

Literature

1. Andreev V. Reducing the heating of transformers of low-power power supplies. - Radio, 2003, No. 6, p. 24.
2. Lukyanov D. Unusual "professions" of microchips for watches. - Radio, 1988, No. 12, p. 31.

Author: B. Andreev, Togliatti

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