Sunday Reminder

The circuit in the attached figure works in a cycle of seven days a week. You can use the switch S1 in the figure to select a day as a reminder within seven days to remember the specific work, events or other important things that should be done on that day. The reminder method of the circuit is that LED1 keeps flashing.

This circuit uses a photoresistor (LDR1) to detect the rising of the sun. When the sun rises, the ambient light intensity reduces the resistance of LDR1, causing the level of the input terminal (13) of the NAND gate N4 to also decrease. Since the other pin (12) of N4 is at a high level, the output pin 6 of the NAND gate N10 becomes low, causing the counter IC3 to count once. At night, the resistance of LDR1 increases, causing the N4 (13) pin to reset to a high level, resulting in the N10 pin 6 reset to a high level, and IC3 waits for another trigger. Every day, IC3 counts once according to the above method. Using this counting method, IC3 starts counting once every morning until seven days later.

On the morning of the seventh day, all inputs of AND gate N9 become high level, and the output of N9 turns to low level. The output of NAND gate N11 turns the low level output by N9 to high level. The high level is used to trigger the pulse generator composed of NAND gates N7 and N5, so that it generates a narrow pulse. The narrow pulse triggers the trigger composed of N8 and N6, and the result is that the high level generated by the trigger will cause the multivibrator (no stable state) composed of N3 gate to oscillate. The oscillation frequency of the multivibrator is 2Hz, and it drives T1 to work through N2 gate, making LED1 flash and light up, as a reminder of Sunday. LED1 keeps flashing and light up, and it stops flashing only when the reset switch S2 is pressed.

The relevant logical functions of the circuit are as follows:

Once the enable terminal (input pin ⑧) of the NAND gate N3 is at a low level, the output of the multivibrator remains at a high level. At this time, the NAND gate N2 transforms the high level into a low level. That is, the output of N2 is at a low level. The transistor T1 is no longer turned on, so the LED1 no longer flashes (the multivibrator is in a blocked state).

The (11) pin R of the counter IC3 is the reset terminal (high level reset), which is connected to the output terminal of N11. On the seventh day, when the ⑩ pin of N11 becomes high level, IC3 will be reset. Once IC3 is reset, its output becomes low level, waiting for the counting work of the next week.

For example, if you need to remind you of the last 4 days of next week, first cover the light of sensor LDR1 with your hand, then press switch S1 three times. At this time, your reminder will start counting from the 4th day.

Install the circuit in the attached figure on a common printed circuit board (PCB), and then put the PCB board in a small box. When using the reminder, first turn on the power supply, then LDE1 will light up and flash. Then press the reset switch S2 to stop LED1 from flashing. Cover the light of LDR1 with your hand and press switch S1 several times until LED1 starts flashing again. At this time, the counter IC3 starts counting from 0, and immediately press the reset switch S2 again to stop LED1 from flashing.

After the above settings are completed, the counter IC3 will start counting from the second day after the settings are completed, and after six days, LED1 will start to flash and work.

Note: When the reminder is working properly, the photoresistor (sensor) LDR1 must be placed in the brightest place in the room to ensure that LDR1 works normally according to sunrise and sunset (the resistance of LDR1 becomes very low at sunrise and very high at sunset).