LED string switch. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur
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This switch (fig. 1) controls a garland of sixteen LEDs so that they light up and go out randomly.
(click to enlarge)
A master oscillator is assembled on the logical elements DD1.1 and DD1.2, the frequency of which can be smoothly changed by a variable resistor R2. The signal from the generator is fed to the counting input of the microcircuits DD2 and DD3 - they are connected in such a way that they form a 16-bit shift register. The register input (pins 1, 2 of the DD2 chip) receives a signal from the high-order bits of the DD1.4 chip through the XOR element DD1.3 and the inverter DD3. As a result, the shift register is closed in a ring, which allows you to get a pseudo-random sequence of low and high voltages at its outputs. The state of the register changes along the edges of the clock pulses generated by the master oscillator.
Generator cycle, i.e. the time after which the pseudo-random sequence repeats is equal to 32767 clock cycles. If the master oscillator is running at 3 Hz, the repetition period of the sequence will be about three hours.
The pseudo-random sequence generator has one forbidden state. When the supply voltage is applied to all outputs of the microcircuits DD2 and DD3, the voltage will be high, the state of the shift register will not change, since a high level signal will be applied to its input. To ensure a guaranteed start of the device, an initial installation node was introduced on the elements R4, VD1, C2. After the power is turned on, the capacitor C2 is discharged and a low-level signal is sent to the input R of the microcircuits DD2, DD3, setting them to zero. As the capacitor C2 is charged through the resistor R3, the low level will change to a high one, which will enable the shift register to work. After the power is turned off, the capacitor C2 will be discharged through the diode VD1.
Instead of the K555LP5 microcircuit, you can use KR1533LP5 or K155LP5, but in the latter version, pins 4 and 13 of the DD1 microcircuit connected together should be connected to the positive power wire through a resistor with a resistance of 1 ... 10 kOhm. We can replace the KD522B diode with any low-power silicon diode, for example, the KD521 series. Fixed resistors - MLT-0.125, variable R2, combined with the SA1 power switch, - SPZ-ZgM or other small-sized. Oxide capacitors - K50-35, K50-40.
Any LEDs operating at a current of 10 ... 20 mA. It is permissible to install multi-colored LEDs of the AL307 series. Resistors R5 - R20 should be selected depending on the operating current of the LEDs. For example, for green LEDs AL307V, AL307G, the specified resistors should be 100 ohms. for red AL307A. AL307B - 200 Ohm.
An interesting effect can be achieved using two-color LEDs. for example, the KIPD18 series. The garland will flash not only red and green, but also yellow lights. The current limiting resistors for these LEDs should be 100 ohms.
The details of the device, except for the LEDs of the garland, are placed on a printed circuit board (Fig. 2) made of one-sided foil fiberglass. The dashed lines in the drawing show additional wire jumpers soldered from the side of the parts.
The brightness of the LEDs will increase if you connect them with cathodes to limiting resistors, and anodes - to the plus of the power source. This will reduce the load on the microcircuits.
Author: A.Shitov
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