Pulse illumination of an alarm clock. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Clocks, timers, relays, load switches
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Waking up at night or early in the morning, the first thing you want to know is the time so as not to oversleep for work or school. The room is dark and you don't want to get up to turn on the light. To facilitate this task, the proposed device is intended (Fig. 1).
The dial and clock hands are illuminated by the HL1 LED, which has a very high brightness (3,5 ... 4,5 cd) at a current of about 20 mA. The device turns on automatically when it gets dark, when the resistance of the photodiode VD1 increases. Transistor VT1 closes, the generator starts to work, made on transistors VT2, VT3. Its advantage is that during the pause between pulses, both transistors are closed, so the current consumed from the power source is minimal.
Pulse duration (illumination of the alarm clock with LED HL1) - 4 s, pause duration 26 s. As practice has shown, this is quite enough to see the time. But if you wish, you can set a more convenient mode for you by selecting resistor R3 or capacitor C2. In addition, the GB1 battery draws a small amount of current from the GBXNUMX battery during this operation, which increases the service life of the device.
When the room is bright, the current consumption of the device does not exceed a few microamperes, so the supply voltage switch is not provided.
Resistors and capacitors can be any small-sized (C1 any ceramic or film). Transistors are any of the series indicated in the diagram. Instead of the FD256 photodiode, FD256A, FD256B or a small-sized high-resistance photoresistor, for example, SF2-8, SFK-1M, are suitable. The LED indicated in the diagram can be replaced by any with a high luminous intensity. Battery GB1 may be composed of galvanic cells or small batteries connected in series.
The parts are mounted on a printed circuit board (Fig. 2) made of one-sided foil fiberglass. For the LED, it is desirable to provide a small reflector, which will increase the illumination of the alarm clock and protect the photodiode from the light flux of the LED.
(click to enlarge)
When setting up the device, you may have to select the resistor R1 to set the threshold for a given minimum illumination. If you need to slightly change the brightness of the LED flashes, you should more accurately select the resistance of the resistor R4.
The device is operational when the supply voltage drops to four volts. At the same time, the current through the LED decreases, which means its brightness, but the pulse duration increases to five seconds.
Author: V. Grichko, Krasnodar
See other articles Section Clocks, timers, relays, load switches.
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