Pills Reminder

Description: A Pills Reminder is a device that operates a flashing LED (and/or a beeper) at fixed hour intervals. A variety of time intervals is available with this circuit, specifically 4, 6, 8, 12, 24, and 48 hours. Initially, the hour interval must be selected by switching SW1 to the desired value, followed by applying power using SW2. Once the chosen delay has elapsed, the LED will begin to flash at 2Hz, meaning it will flash twice per second. This status will persist until pushbutton P1 is pressed; at that point, the LED will turn off, but the circuit will continue counting, and the LED will flash again when the same hour interval is reached. A notable feature of this circuit, which is often absent in similar devices, is that the internal counter remains unaffected when P1 is pressed, allowing for improved time interval precision. For example, if the time interval is set to 24 hours and the device is started at 8 o'clock, the LED will flash at 8 o'clock the next day. If the user notices the flashes at 8:10 and presses P1, the counter will not reset, and the LED will continue to flash at 8 o'clock the following day, regardless of when P1 was pressed. The circuit's clock is composed of a stable oscillator built around two inverters within IC1 and a watch crystal oscillating at 32.768kHz. This frequency is divided by 16384 by the internal flip-flop chain of IC1, resulting in a stable 2Hz clock frequency available at pin #3 of this IC. IC2 counter and IC3A, a 4-input AND gate, are configured to divide the 2Hz clock by 3600, providing a pulse every 30 minutes at the clock input of IC5. The division factor of IC5 is controlled by IC3B and the position of SW1A and B, selecting from the six fixed time intervals of 4, 6, 8, 12, 24, and 48 hours. The set-reset flip-flop formed by IC6B and IC6C is activated through IC4C whenever a low-to-high transition occurs at the pin of IC5 selected by the SW1B cursor. IC6A and C4 also enable setting the flip-flop during a high-to-low transition at the SW1B cursor. When the flip-flop is set, IC6D is activated, and the 2Hz frequency from pin #3 of IC1 is applied to pin #13 of IC6D, causing the LED to flash. The flip-flop can be reset using P1. A master reset occurs automatically upon power-up via C6 and R7. In instances where the watch crystal is difficult to find or considered expensive, an alternative clock generator circuit is available, which directly oscillates at 2Hz, thus eliminating the need for divider ICs. A CMOS 7555 Timer IC generates a stable 2Hz square wave, which must be precisely set using two trimmers. R10 should be adjusted first for coarse tuning, followed by R11 for fine tuning. Achieving an accurate 2Hz frequency for this oscillator requires patience and the use of a clock, chronometer, or preferably, a digital frequency meter capable of measuring low frequencies. After proper setup, this oscillator demonstrates stable performance, unaffected by battery voltage variations, with an accuracy of approximately ±30 seconds per 24-hour interval. For maximum time precision, a digital frequency meter can be utilized to measure and adjust a 5-50pF 50V Ceramic Trimmer Capacitor in place of C2, ensuring it reads exactly 32.768kHz when connected to pin #9 of IC1. Additionally, a piezo sounder (incorporating a 3kHz oscillator) can be connected to provide both visual and audible alerts. It should be wired across pin #11 of IC6D and negative ground, ensuring correct polarity. If the visual alert is not required, D1 and R6 should be removed.A Pills Reminder is a device that operates a flashing LED (and/or a beeper) at a fixed hour interval. A choice of time-intervals as wide as possible is available with this circuit, namely 4, 6, 8, 12, 24 and 48 hours.

At first you must choose the hour interval by switching SW1 to the desired value, then apply power by means of SW2. After the hour delay chosen has elapsed the LED will start flashing at 2Hz, i. e. two times per second. This status will last until pushbutton P1 is pressed: then the LED will turn off, but the circuit will continue its counting and the LED will flash again when the same hour interval as before is reached. A noteworthy feature of this circuit, usually not found in similar devices, is that the internal counter is not reset when P1 is pressed: this allows a better time-interval precision.

Let us explain this feature with an example: suppose you have set the time interval to 24 hours and started the Pills Reminder at 8 o`clock. Next day, at 8 o`clock the LED will start flashing, but you, for some reason, notice the flashes at 8:10 and press P1 to stop the LED.

With most devices of this kind, the counter will be reset, causing the LED to start flashing next day at 8:10 o`clock. This will not happen with this circuit and the LED will start flashing next day always precisely at 8 o`clock even if you pressed P1 at 9 or 10 o`clock.

The clock of the circuit is made of a stable oscillator built around two inverters embedded into IC1 and a Watch crystal oscillating at 32. 768kHz. This frequency is divided by 16384 by the internal flip-flop chain of IC1 and a 2Hz very stable clock frequency is available at pin #3 of this IC.

IC2 counter and IC3A 4 input AND gate are wired in order to divide by 3600 the 2Hz clock, therefore, a pulse every 30 minutes is available at the clock input of IC5. The division factor of this IC is controlled by IC3B and the position of SW1A and B, selecting from six time-intervals fixed to 4, 6, 8, 12, 24 and 48 hours.

The set-reset flip-flop formed by IC6B and IC6C is set through IC4C each time a low to high transition is present at the pin of IC5 selected by SW1B cursor. IC6A and C4 provide to set the flip-flop also when a high to low transition is present at SW1B cursor.

When the flip-flop is set, IC6D is enabled and the 2Hz frequency available at pin #3 of IC1 is applied to pin #13 of IC6D causing the flashing LED operation. The flip-flop can then be reset by means of P1. A master reset is automatically done at switch on by means of C6 and R7. Sometimes, the Watch crystal can be difficult to locate, or could be considered too expensive. For those willing to avoid the use of a Watch crystal and to accept less time accuracy, an alternative clock generator circuit is provided, directly oscillating at 2Hz, thus avoiding the use of divider ICs.

A CMos 7555 Timer IC generates a stable 2Hz square wave, whose frequency must be accurately set by means of two trimmers. R10 must be adjusted first for coarse tuning, then R11 for fine tuning. Setting precisely the 2Hz frequency of this oscillator is a rather difficult task, and can be done with great patience and the aid of a clock, a chronometer or, best, a digital frequency meter capable of measuring very low frequencies.

In any case, after an accurate setup, this oscillator showed a very stable performance, not affected by battery voltage variations and an accuracy of about ±30 seconds per 24 hours interval. Wanting the utmost time precision and if a digital frequency meter is available, a 5-50pF 50V Ceramic Trimmer Capacitor can be used in place of C2.

It must be adjusted in order to read exactly 32. 768kHz on the meter display with the input probe connected to pin #9 of IC1. A Piezo sounder (incorporating a 3KHz oscillator) can be added to provide a visual plus audible alert. It must be wired across pin #11 of IC6D and negative ground, respecting polarities. Remove D1 and R6 if the visual alert is not needed.