Photoelectronic alarm clock. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Photoelectronic alarm clock. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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In the article by Zh. Mikheeva "Mushroom picker alarm clock, aka watchdog"Radio", 2002, No. 3, pp. 47, 48 talked about a design that emits a sound signal when the illumination of the photo sensor increases. In other words, the device is a photo alarm clock, convenient for fishermen, hunters, etc.

A similar design can be made on just one microcircuit and one transistor (Fig. 1). It consists of a photodiode VD1, which is a light sensor, a single vibrator on Schmitt triggers DD1.1 and DD1.2, two generators on triggers DD1.3, DD1.4 and an inverter on a transistor VT1.

Photoelectronic alarm clock

In standby mode, when the photodiode is darkened and its resistance is relatively high, at the input of the trigger DD1.1 - a level close to low (almost logical 0). The DD1.2 trigger is in the zero state, so the DD1.3 trigger does not work and its output (pin 10) is high, which closes the transistor. As a result, there is a low level on the collector of the transistor, which prevents the generator from operating on the DD1.4 trigger. The device can be in this state for an arbitrarily long time, consuming a current from a power source of no more than 2 μA.

With increasing illumination, the resistance of the photodiode decreases, and the voltage at the top output of the resistor R1 according to the circuit increases. When it reaches the threshold value, the trigger DD1.1 switches to a state in which its output (pin 3) is set to a low level. Capacitor C2 begins to charge through resistors R2, R3. Appeared high level at the output of the trigger DD1.2 starts the generator on the trigger DD1.3. It starts producing rectangular pulses with a repetition rate of about 1 Hz.

These pulses, inverted by the cascade on the VTT transistor, begin to periodically start the oscillator on the DD1.4 trigger, which generates 3H oscillations with a frequency of about 1500 Hz. The BF1 piezo emitter emits an intermittent sound signal, announcing that the illumination threshold has been reached. The signal will stop when the voltage on the right output of the capacitor C2 according to the circuit reaches the switching threshold of the trigger DD1.2.

The duration of the alarm depends mainly on the capacitance of the capacitor C2 and the resistance of the resistor R3. With the ratings of the indicated parts indicated on the diagram, it is about 30 s. At the end of this time, the trigger DD1.2 switches, a low level appears at its output, the generators on the triggers DD1.3, DD1.4 stop working. The device can also be in this mode for a long time, but the current consumption increases several times.

The alarm will be reactivated only after the light level drops below the threshold. Then the resistance of the photoresistor will increase significantly, a high level will appear at the output of the trigger DD1, the capacitor C2 will quickly discharge through the resistor R2 and the diode VD1. The device will go into standby mode.

It is permissible to use microcircuits of the indicated type of the K564, KR1561 series in the design. It is undesirable to use 2I-NOT logic elements, since they do not have a switching input voltage hysteresis. Transistor - any of the series indicated on the diagram. The photodiode can be, in addition to that indicated in the diagram, FD-236, FD-256 or similar. Diode - any low-power silicon. Resistors and capacitors - any small-sized, it is desirable to use capacitor C2 with a low leakage current. Piezo emitter BF1 - ZP-1, ZP-2, ZP-5, ZP-22 or similar.

The details of the alarm clock, except for the photodiode and the piezoelectric emitter, are mounted on a printed circuit board (Fig. 2) made of one-sided foil fiberglass. The photodiode is placed on the body of the structure so that ambient light falls on it.

Photoelectronic alarm clock

To adjust the sensitivity of the photodiode, it is covered with a film or thin paper of different density, selecting the illumination threshold at which the alarm should go off. The desired result is also achieved by selecting the resistor R1. By selecting resistors R4, R7, the desired frequency of turning on the sound signal and its tone, respectively, are set.

Like the design mentioned above, this one is capable of performing many other functions, for example, turning on various mechanisms and devices at certain times of the day. But for this you need to supplement it with a transistor or trinistor load control cascade of the appropriate power.

Author: I. Potvchin, Fokino, Bryansk region

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