Mushroom picker's alarm clock, also known as a watchdog. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Mushroom picker's alarm clock, also known as a watchdog. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Safety and security

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There are many people who need to get up with the sunrise (for example, mushroom pickers). In order not to oversleep, they can use the proposed alarm clock. At other times of the day, this device is capable of acting as an electronic watchman.

Schematic diagram of the alarm clock is shown in fig. 1. It works like this: when the illumination of the BL1 photo sensor exceeds a certain value, the BF1 telephone capsule will start emitting sound signals that should wake up the "owl".

Mushroom picker alarm clock, aka watchdog

There are five independent nodes in the alarm clock. On the elements DD1.1 and DD1.2, a Schmitt trigger is made, capable of abruptly changing its state. Elements DD1.3, DD1.4 form a waiting multivibrator, DD2.1, DD2.2 - a long pulse generator, DD2.3 and DD2.4 - an audio frequency generator, and a cascade on a transistor VT1 - a power amplifier.

In the initial state, with a darkened photosensor, its resistance is high. At pins 1, 2 of the DD1.1 element, the level is high, and at pin 3, it is low. Accordingly, at pin 4 of the DD1.2 element, a high level. There is no sound in the telephone capsule.

When the illumination level of the photosensor increases, the voltage at the inputs DD1.1 will be less than the switching threshold of this element. At pin 4 DD1.2, the high level will change to low. Through capacitor C3, a negative trigger pulse will be sent to one of the inputs of the waiting multivibrator. At the output of the DD1.4 element, the high level will change to a low one. The duration of this state is determined by the parameters of the parts R6, C4.

A high level from the output of the element DD1.3 goes to the first generator and starts it. From the output of the generator, pulses with a frequency of 4 Hz are fed to the second generator, the pulse repetition rate of which can be 800 ... 1600 Hz. Short series of pulses that appear at the output of the second generator are fed through resistor R9 to the base of the power amplifier transistor. The telephone capsule emits sounds, the total duration of which is approximately 13 seconds. If you install a resistor R6 and a capacitor C4 of much higher ratings, you can increase the duration of the "call" of the alarm clock up to several minutes.

The sensitivity of the device to the level of illumination of the photosensor depends on the set resistance of the resistor R1. Capacitor C1 prevents the alarm clock from being triggered by random light interference, such as lightning flashes. Capacitors C2, C6 - blocking in the power circuit. Diode VD1 protects the transistor from capsular self-induction voltage surges.

The photo sensor is a photodiode FD263 (BL1), which can be replaced by FD263-01, FD256, FD320, a photoresistor, for example, FR1; we can replace the KD522B diode with any of the KD103, KD521, KD522 series; transistor KT3102V - on any of the series KG315, KT503, KT645, KT3102. Chips can be, in addition to those indicated in the diagram, K176LA7, KR1561LA7. Telephone capsule - any, with a resistance of 50 to 3000 ohms, for example, TK-47, TA-4, DEM-4M. Capacitors C1, C4 - K73-17, C6 - K50-35, the rest - K10-17. Variable resistor - SP-I, SPZ-4A, SPZ-29A with a resistance of 2 - 4,7 MΩ, constant - MLT-0,125 or MLT-0,25.

As a power source, you can use a battery of galvanic cells or a stabilized power supply with a permissible load current of at least 30 mA.

All parts, except for a photodiode, a variable resistor, a power switch, a telephone capsule and a power source, are mounted on a printed circuit board (Fig. 2) from one-sided foil fiberglass.

Mushroom picker alarm clock, aka watchdog

The board is installed in a case of suitable dimensions, on the walls of which the remaining parts are mounted, and inside the case a power source is placed from the corresponding number of series-connected galvanic cells.

When checking the operation of the device, you may need to change the tone of the sound. Then you need to pick up the resistor R8, temporarily turning on instead of it a variable resistor with a resistance of 150 kOhm. Having determined the desired resistance of the resistor, a constant resistor of such resistance is soldered in place of R8.

If, instead of a photodiode, you connect the pendulum sensor SA2 (Fig. 3), which works on a circuit, the device will turn from an alarm clock into a simple security alarm system. The sensor parts are mounted in an aluminum case from a K50-6 capacitor with a capacity of 2200 uF for a voltage of 25 V.

Mushroom picker alarm clock, aka watchdog

Such a sensor can be fixed with a bracket on the door. When it is opened, the weight from the M5 screw deviates, hitting the housing wall. Capacitor C1 is discharged, and then the device operates as described above.

Sharing of the photosensor and mechanical is possible. The signaling device will respond to the closing of contacts SA2 only at night.

If the photodiode and the variable resistor are interchanged, the alarm will go off when the illumination drops to a certain level, set by the variable resistor. In this embodiment, it is convenient to use it as a signaling device for interrupting the light flux, for example, a laser pointer beam directed at the photodiode. Such a signaling device can be used in school sports competitions to fix the crossing of the finish line by an athlete. The maximum distance from the pointer to the photodiode depends on the level of ambient light and can reach 10 m.

The current consumed by the device in standby mode does not exceed 100 μA. Nevertheless, if the alarm clock is powered by a battery of galvanic cells, the current consumption can be reduced by setting the variable resistor to the position of the maximum possible resistance, as well as lowering the device's sensitivity by covering the photodiode with a translucent film.

Author: Zh.Mikheeva, village Ivanishchevo, Yaroslavl region

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