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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Flashing LED indicators on CMOS chips. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Lighting Annotation. Operating mode indicators are most widely used in electronic devices, for example, as part of an intruder alarm system, or, in an individual design, also to simulate its presence. Such an indicator can be used in electronic toys for creating aesthetic effects or as a controller for controlling flashing beacons in special-purpose vehicles. As light-emitting elements, it is advisable to use super-bright LEDs, which, due to the high load capacity of CMOS microcircuits of the KR1554 and KR1564 series, can be connected directly to their outputs, without key transistors.
Principle of operation. As the basic circuit solutions for LED indicators, the simplest designs on two and three CMOS microcircuits of the standard logic of the KR1554 series, respectively, considered in [1] and [2], are used. The first version (Fig. 1) of the device generates two flashes of each LED with a duty cycle of four. This means that the LED flash time is 25% of the flash period, which subjectively corresponds to the clearest flashing of the LEDs. In addition, such a duty cycle doubles the life of low-power cells when the device is battery powered. We will consider the operation of the device, assuming that at the initial time the counters DD2.1 and DD2.2 are in the "zero" state. On the elements DD1.1, DD1.2, a rectangular pulse generator is made, with a repetition rate of about 10 Hz. When switching the element DD1.2 to the opposite state, the voltage on the left, according to the scheme, plate of the capacitor C1, is added to the previous value and reaches almost twice the value of the power supply voltage. For the input protective diodes of the DD1.1 element, this mode of operation is unacceptable, therefore, a resistor R1 is introduced into the device, which limits the current pulses at the level of 1 mA, which is already quite an acceptable value. This resistor prevents the failure of the protective diodes and thus significantly increases the reliability of the device during long-term operation. The counter DD2.1 is triggered by the negative differences of the counting pulses, and, when the "third" state is reached, it generates the levels of logical units at the outputs "1" and "2" (pins 11 and 10, respectively), which, when fed to the inputs of the element DD1.3. 1.4 cause a "zero" level to appear at its output. This logic level is input to the element DD2 and, inverting the last one, causes the HLXNUMX LED to light up.
This happens due to the fact that the counter DD2.2, as noted above, is in the initial "zero" state, and the level of the logical "one" is formed at the output of the element DD1.4 (see the timing diagram in Fig. 2). The transition of the counter DD2.1 to the "fourth" state leads to the extinction of the HL2 LED, and the transition to the "seventh" - to its re-ignition. Further, the negative drop of the next counting pulse, the counter DD2.1 is transferred to the "eighth" state, and the negative drop from the output of its "third" bit (pin 4) leads to an increase in the state of the counter DD2.2 by one. Now, at the moment when the level of logical "zero" appears at the output of the element DD1.3, the red LED HL1 lights up. Thus, there are two successive flashes of each LED. The flash frequency can be changed by trimming resistor R2, and the upper limit of the oscillator frequency range can be changed by selecting resistor R3. If you need to get not two, but four flashes of each LED, you need to apply counting pulses to the input DD2.2 from the output of the fourth (pin 8), and not the third bit (pin 9) of the counter DD2.1.
The electrical circuit diagram of a three-LED indicator is shown in Fig. 4. The device generates three successive flashes of each LED, also with a duty cycle of four. Unlike the first version of the device, the DD2.1 counter is reset by a short positive pulse from the output of the DD1.4 element when the "twelfth" state is reached. If zeroing is not performed, but the reset input "R" (pin 12) is connected to the "common" wire, then not three, but four flashes of each LED will occur. Counting pulses from the output of the high-order digit DD2.1 are fed to the input DD2.2, which generates code combinations to select one of the three flashing LEDs HL1 ... HL3. A duty cycle equal to four is achieved through a combination of control signals coming from the outputs of the least significant digits of the counter DD2.1 (pins 11 and 10) to the inverse "permission" inputs "V (&)" of the DD3 decoder (pins 4 and 5). Its direct "enable" input ("V", pin 6) is connected to the power rail, according to the logic of operation. In this case, the ignition of one of the three LEDs HL1 ... HL3 occurs only when the inputs "V (&)" of the decoder DD3 (pins 4 and 5) match two levels of logical zero, according to the timing diagram in Fig. 5.
Each counting pulse received at the input of the counter DD2.2 from the output DD2.1 leads to an increase in its state by one. Upon reaching the "third" state, thanks to the chain VD1, VD2, R4, the counter DD2.2 is reset, and, then, the cycle of the device is completely repeated. It should be noted that the specified chain (VD1, VD2, R4) is a fully functional equivalent of two elements connected in series DD1.3, DD1.4, i.e. performs the function of logical "multiplication" of signals.
An improved version of the three-LED indicator is shown in fig. 7. Here, the counter DD2.2 is not reset, so it operates in a cyclic mode with a full set of states, which allows you to generate negative pulses at the four outputs of the DD3 decoder. The number of LEDs is still three, but they are connected not directly to the outputs of the decoder, but through the elements DD4.1 ... DD4.3. The level of logical zero appears at their outputs, and, consequently, the corresponding LED lights up when the specified elements of the same logical level arrive at any of the inputs, according to the timing diagram in Fig. 8. When the counter DD2.2 reaches the "third" state (at the outputs "1" and "2" - the levels of logical units), the same level appears at the output "3" (pin 12) of the decoder DD3, but only if the condition of coincidence of two logic "zero" levels at the inputs of its resolution "V(&)" (pins 4 and 5). Thus, after three consecutive flashes of each of the three LEDs HL1 ... HL3, all LEDs are ignited three times simultaneously. The inputs of the element DD4.4 (not shown in the diagram) are connected to the power bus.
It became possible to significantly change the algorithm of the device operation due to the use of a microcircuit containing four identical RS-flip-flops with inverse control inputs in one package (Fig. 10). This means that the transition of the RS-flip-flop to the corresponding state occurs according to the level of logical "zero" coming to the corresponding input "R" or "S". At the same time, the levels of logical units must be preliminarily fixed at the specified inputs before applying the active level of logical zero. This mode of operation is provided using the decoder DD3, the active output logic levels of which are just "zero". At the initial moment of time, the counters DD2.1 and DD2.2 are in the "zero" state, therefore, at the output of the element DD1.3, a logic unit level is formed, which prohibits the decoding of the states of the counter DD2.2, the output logic levels of which are fed to the address inputs " 1" and "2" of the DD3 decoder. Thus, the levels of logical units are formed at all its outputs, which corresponds to the initial state of the device. Since at the end of the previous cycle, a short negative pulse was generated at the output of the DD1.4 element, all RS-flip-flops were set to the "single" state, so all the LEDs were off. When the counter DD2.1 passes from the "zero" to the "first" state, the level of logical zero from the output of the element DD1.3 allows the decoding of the states of DD3 and at its output "0" (pin 15) the level of logical "zero" appears. This level flips the first (upper in the diagram) RS-flip-flop, which is part of the DD4 chip, to the zero state, and, at the same time, goes to the anode of the HL1 LED. But the ignition of the LED at this point in time does not yet occur, since the potential difference at its terminals is zero. When the counter DD2.1 reaches the fourth state, the decoding of the DD3 states will be again prohibited, and a logical unit level will be formed at its output "0" (pin 15). Since the "1Q" output (pin 4) of the first, according to the scheme, RS-flip-flop DD4, the "zero" level was formed, this will lead to the ignition of the HL1 LED. This will be followed by three flashes, with a duty cycle equal to four, as in previous cases, according to the timing diagram in Fig. 11. In this case, negative pulses at output "0" (pin 15) of the DD3 decoder lead precisely to the extinction of the HL1 LED, therefore, during the transition counter DD2.2 from zero to the first state, at the indicated output "0" (pin 15) of the decoder DD3, a fixed (static) level of logical unit is formed, and the HL1 LED remains on. Each subsequent counting pulse from the output of the generator leads to an increase in the states of the counter DD2.1 and, after it, and DD2.2. In this case, three successive flashes of the LEDs HL2 ... HL4 occur, followed by their fixation in the on state. When the counter DD2.2 reaches the "fourth" state, a short positive pulse is generated at its output "4" (pin 9), which, inverted by the element DD1.4, leads to the installation of all RS-flip-flops DD4 in the "single" state and the LEDs go out . Further, the cycle of operation of the device is completely repeated.
An improved version of the four-LED indicator is shown in fig. 13. The simplest timer was introduced into its composition, consisting of a rectangular pulse generator assembled on elements DD2.1, DD2.2, and counters DD4.1, DD4.2. The timer significantly expands the functionality of the LED indicator and allows you to choose almost any duration of the device operation cycle, starting from a single flash of the HL1 LED, and ending with a certain time delay for all LEDs to glow after the entire working cycle has passed.
The logic of the device operation is fully consistent with the timing diagram shown in fig. 11, with the difference that the signal for setting the RS flip-flops of the DD6 chip is generated by the counter DD4.2 of the additionally introduced timer. Unlike the previous one, in an improved version of the device, two independent rectangular pulse generators operate, the frequency of which is set independently. This allows you to separately change both the frequency of LED flashes (using R3) and the duration of the entire operation cycle (using R6).
Construction and details. All devices are made on printed circuit boards made of double-sided foil fiberglass 1,5 mm thick. PCB dimensions: first option (Fig. 3): 35x50 mm; second option: (fig. 6): 40x70 mm; third option: (fig. 9): 40x70 mm; fourth option: (Fig. 12): 40x75 mm; and the fifth option: (Fig. 14): 50x90 mm.
The devices use fixed resistors of the MLT-0,125 type, trimmers SP3-38b in horizontal design, non-polar capacitors of the K10-17 type, oxide capacitors of the K50-35 or imported ones. CMOS microcircuits of the KR1554 series have a high load capacity (up to 24 mA), which allows you to connect LEDs to their outputs directly, without switching transistors. If super-bright LEDs are not available, standard brightness LEDs can also be used, but, in this case, it is necessary to use only KR1554 series ICs, the output currents of which can reach 24 mA. In the circuits of rectangular pulse generators in place of KR1564LA3 (74HC00N), you can also use KR1564TL3 (74HC132N), which contains four Schmitt triggers. This option is most preferable for battery-powered devices, to increase their efficiency by significantly reducing through currents when switching logic elements. Due to the high load capacity of CMOS microcircuits of the KR1564 and KR1554 series, it is possible to combine CMOS (KR1564, KR1554, KR1594) and TTLSH (KR1533, K555) and even TTL (K155) series chips in one device. Only microcircuits of the K561 and KR1561 series are not applicable in devices, the load capacity of which does not exceed 1 mA, even for devices of the CD40xxBN series. For example, in place of DD1 (KR1564LA3), its fully functional TTLSH analogue of the KR1533LA3 type can work. Since the input currents of TTLS-series microcircuits are much higher than the corresponding values for CMOS microcircuits, it is necessary to install a trimmer resistor (R2) with a resistance of 1 kOhm, and replace the constants (R1 and R3) with jumpers. In this case, the non-polar capacitor C1 is replaced by an oxide capacitance of up to 100 μF in order to maintain the time constant of the generator. When powering devices from low-power elements with a total voltage of 3 V, the integral stabilizer and protective diode must be excluded, and the LEDs must be selected with the lowest possible operating voltage of the glow. When using the KR1564TL3 (74HC132N) chip generator on site, the battery life will be enough for several months of continuous operation. Devices assembled from serviceable parts and without errors do not need to be adjusted and work immediately when turned on. Literature.
Author: Odinets A.L.
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