28 LED Clock Timer

The circuit can be arranged in a circular format to represent the 12 hours of a clock face, with an additional 12 LEDs positioned in an outer circle to indicate 5-minute intervals within each hour. Four extra LEDs are utilized to represent time intervals from 1 to 4 minutes within each 5-minute segment. The circuit is powered by a small 12.6-volt center-tapped line transformer, using the 60-cycle line frequency as the time base. This transformer is configured in a full-wave, center-tapped manner, producing approximately 8.5 volts of unregulated DC. A 47-ohm resistor and a 5.1-volt, 1-watt zener diode regulate the supply for the 74HCT circuits. A 14-stage 74HCT4020 binary counter and two NAND gates divide the line frequency by 3600 to generate a one-minute pulse, which resets the counter and advances the 4017 decade counter. The decade counter counts minutes from 0 to 4 and resets on the fifth count, advancing one section of a dual 4-bit binary counter (74HCT393). The four bits of this counter are decoded into one of 12 outputs by two 74HCT138 (3-line to 8-line) decoder circuits. The most significant bit (MSB) is used with an inverter to select the appropriate decoder. During the first eight counts, the low state of the MSB is inverted to enable the decoder driving the first eight LEDs. For counts 9 to 12, the MSB is high, selecting the decoder for the remaining four LEDs while disabling the other decoder. The decoded outputs are low when selected, and the 12 LEDs are connected in a common anode configuration with a 330-ohm current-limiting resistor to the +5-volt supply. The fifth output of the second decoder (pin 11) resets the binary counter so that it counts to 11 and resets to zero on the 12th count. A high reset level is necessary for the 393 counters; thus, the low output from the last decoder stage (pin 11) is inverted using one section of a 74HCT14 hex Schmitt trigger inverter circuit. A 10K resistor and a 0.1uF capacitor extend the reset time, ensuring the counter receives a reset signal significantly longer than the minimum required time. The reset signal is also connected to the clock input (pin 13) of the second 4-bit counter (1/2 74HCT393), which advances the hour LEDs and resets on the 12th hour similarly. Setting the correct time is facilitated by two manual push buttons that feed the Q4 stage (pin 7) of the 4020 counter to the minute and hour reset circuits, allowing the counters to advance at 3.75 counts per second. A slower rate can be achieved using the Q5 or Q6 stages, while Q1 (pin 9) can be used for test purposes to advance the minutes at 30 counts per second. The time interval circuit, depicted below the clock, consists of a SET/RESET flip-flop constructed from the two remaining NAND gates (74HCT00). The desired time interval is programmed by connecting the anodes of six diodes labeled start, stop, and AM/PM to the appropriate decoder outputs. For instance, to activate the relay at 7:05 AM and deactivate it at 8:05 AM, one diode from the start section would connect to the cathode of the LED representing 7 hours, another to the LED cathode for 5 minutes, and the third to the AM line of the CD4013. The stop time is programmed similarly. Two additional push buttons are incorporated to manually open and close the relay. The low start and stop signals at the common cathode connections are capacitively coupled to the NAND gates, allowing the manual push buttons to override the 5-minute time duration. This enables immediate relay reset without waiting for the 5-minute start signal to dissipate. The power supply rectifier diodes utilized are of the 1N400X variety, while the switching diodes can be 1N914 or 4148, although any general-purpose diodes are suitable.

The clock circuit operates effectively by leveraging both digital and analog components, ensuring accurate time representation and control of LED indicators. The arrangement of LEDs in a circular format provides a clear visual indication of time, while the binary counter and decoder setup allows for precise counting and timing functions. The use of a center-tapped transformer ensures stable voltage regulation, and the inclusion of zener diodes helps maintain a consistent supply voltage for the logic circuits. The circuit's design allows for flexibility in programming time intervals, which is essential for applications requiring automated control based on specific timing events. The manual override feature enhances usability, providing immediate control over the relay without waiting for automated sequences. Overall, this circuit design is a robust solution for timekeeping and control applications, integrating various electronic components to achieve reliable performance.can be arranged in a circle to represent the 12 hours of a clock face and an additional 12 LEDs can be arranged in an outer circle to indicate 5 minute intervals within the hour. 4 additional LEDs are used to indicate 1 to 4 minutes of time within each 5 minute interval. The circuit is powered from a small 12. 6 volt center tapped line transformer and the 60 cycle line frequency is used for the time base. The transformer is connected in a full wave, center tapped configuration which produces about 8. 5 volts unregulated DC. A 47 ohm resistor and 5. 1 volt, 1 watt zener regulate the supply for the 74HCT circuits. A 14 stage 74HCT4020 binary counter and two NAND gates are used to divide the line frequency by 3600 producing a one minute pulse which is used to reset the counter and advance the 4017 decade counter. The decade counter counts the minutes from 0 to 4 and resets on the fifth count or every 5 minutes which advances one section of a dual 4 bit binary counter (74HCT393).

The 4 bits of this counter are then decoded into one of 12 outputs by two 74HCT138 (3 line to 8 line) decoder circuits. The most significant bit is used in conjunction with an inverter to select the appropriate decoder. During the first eight counts, the low state of the MSB is inverted to supply a high level to enable the decoder that drives the first 8 LEDs.

During counts 9 to 12, the MSB will be high and will select the decoder that drives the remaining 4 LEDs while disabling the other decoder. The decoded outputs are low when selected and the 12 LEDs are connected common anode with a 330 ohm current limiting resistor to the +5 volt supply.

The 5th output of the second decoder (pin 11) is used to reset the binary counter so that it counts to 11 and then resets to zero on the 12th count. A high reset level is required for the 393 counters, so the low output from the last decoder stage (pin 11) is inverted with one section of a 74HCT14 hex Schmitt trigger inverter circuit.

A 10K resistor and 0. 1uF cap are used to extend the reset time, ensuring the counter receives a reset signal which is much longer than the minimum time required. The reset signal is also connected to the clock input (pin 13) of the second 4 bit counter (1/2 74HCT393) which advances the hour LEDs and resets on the 12th hour in a similar manner.

Setting the correct time is accomplished with two manual push buttons which feed the Q4 stage (pin 7) of the 4020 counter to the minute and hour reset circuits which advance the counters at 3. 75 counts per second. A slower rate can be obtained by using the Q5 or Q6 stages. For test purposes, you can use Q1 (pin 9) which will advance the minutes at 30 per second. The time interval circuit (shown below the clock) consists of a SET/RESET flipflop made from the two remaining NAND gates (74HCT00).

The desired time interval is programmed by connecting the anodes of the six diodes labeled start, stop and AM/PM to the appropriate decoder outputs. For example, to turn the relay on at 7:05AM and turn it off at 8:05AM, you would connect one of the diodes from the start section to the cathode of the LED that represents 7 hours, the second diode to the LED cathode that represents 5 minutes and the third diode to the AM line of the CD4013.

The stop time is programmed in the same manner. Two additional push buttons are used to manually open and close the relay. The low start and stop signals at the common cathode connections are capacitively coupled to the NAND gates so that the manual push buttons can override the 5 minute time duration. That way, you can immediately reset the relay without waiting 5 minutes for the start signal to go away.

The two power supply rectifier diodes are 1N400X variety and the switching diodes are 1N914 or 4148s but any general purpose diodes can be used. 0. 1 u 🔗 External reference