28 LED Clock Timer
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.
Using the 4017 to drive LEDs with transistor buffers is shown in the "10 Channel
LED Sequencer" at the top of this page.
Time Interval Relay Circuit
Clock circuit
Title: 28 LED Clock Timer
electronic circuit
Source: unknown
Published on: 2007-07-21
Reads: 1465
Print version:
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