28 LED Clock Timer

This is a programmable clock timer circuit that utilizes individual LEDs to indicate hours and minutes. Twelve LEDs 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. Four additional LEDs are used to indicate 1 to 4 minutes of time within each 5-minute interval. The circuit is powered by 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, 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 are employed to divide the line frequency by 3600, producing a one-minute pulse that resets the counter and advances 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 as common anode with a 330-ohm current-limiting resistor to the 5-volt supply. The fifth 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 a 0.1 µF capacitor are used to extend the reset time, ensuring that the counter receives a reset signal that is significantly 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 that 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, Q1 (pin 9) can be used to advance the minutes at 30 per second. The time interval circuit consists of a SET/RESET flip-flop 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 7:05 AM and off at 8:05 AM, one diode from the start section is connected to the cathode of the LED representing 7 hours, the second to the LED cathode representing 5 minutes, and the third to the AM line of the CD4013. The stop time is programmed similarly. 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, allowing the manual push buttons to override the 5-minute time duration. This enables immediate relay reset without waiting for the 5 minutes to elapse. The two power supply rectifier diodes are of the 1N400X variety, while the switching diodes can be 1N914 or 4148, though any general-purpose diodes may suffice. A 0.1 µF capacitor (not shown in the schematic) may be necessary near the power pins of each IC. All components should be obtainable from standard electronics suppliers, except for the 74HCT4017 decade counter, which may not be listed. The 74HC or 74HCT series can be used interchangeably, with the primary difference being that the input switching levels of the HCT devices are compatible with worst-case TTL logic outputs. The HC device inputs operate at 50% of Vcc, which may not function correctly when driven by marginal TTL logic outputs. A standard 4017 can be substituted for the 74HCT4017, but it will have lower output current (less than 1 mA), necessitating four additional transistors to drive the LEDs. Without buffer transistors, a 10K resistor can replace the 330-ohm resistor, allowing the LEDs to remain visible, albeit dimly.This is a programmable clock timer circuit that uses individual LEDs to indicate hours and minutes. 12 LEDs 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 uF caps (not shown on schematic) may be needed near the power pins of each IC. All parts should be available from Radio Shack with the exception of the 74HCT4017 decade counter which I didn`t see listed.

You can use either 74HC or 74HCT parts, the only difference between the two is that the input switching levels of the HCT devices are compatible with worst case TTL logic outputs. The HC device inputs are set at 50% of Vcc, so they may not work when driven from marginal TTL logic outputs.

You can use a regular 4017 in place of the 74HCT4017 but the output current will much lower (less than 1 mA) and 4 additional transistors will be required to drive the LEDs. Without the buffer transistors, you can use a 10K resistor in place of the 330 and the LEDs will be visible, but very dim.

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