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 oclock. Next day, at 8 oclock 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 oclock. This will not happen with this circuit and the LED will start flashing next day always precisely at 8 oclock even if you pressed P1 at 9 or 10 oclock.
Parts:R1______________10M 1/4W Resistor
R2,R3,R4_______100K 1/4W Resistors
R5,R7___________10K 1/4W Resistors
R6_______________1K 1/4W Resistor
C1,C2___________22pF 63V Ceramic Capacitors (See Notes)
C3______________22µF 25V Electrolytic Capacitor
C4,C5__________100nF 63V Polyester Capacitors
C6_______________1µF 63V Polyester, Multilayer Ceramic or Electrolytic Capacitor
IC1____________4060 14 stage ripple counter and oscillator CMos IC
IC2____________4040 12 stage ripple counter CMos IC
IC3____________4082 Dual 4 input AND gate CMos IC
IC4____________4075 Triple 3 input OR gate CMos IC
IC5____________4520 Dual binary up-counter CMos IC
IC6____________4001 Quad 2 input NOR Gate CMos IC
D1_____________5 or 10mm red LED
XTAL_________32.768 kHz Sub-miniature Watch crystal
P1_____________SPST Pushbutton
SW1____________2 poles 6 ways Rotary Switch
SW2____________SPST Toggle or Slide Switch
B1_______________9V PP3 Battery Clip for PP3 Battery
Alternative Clock Parts:
R8_______________1K 1/4W Resistor
R9_____________330K 1/4W Resistor
R10_____________20K 1/2W Cermet or Carbon Trimmer
R11______________1K 1/2W Cermet or Carbon Trimmer
C7_______________1µF 63V Polyester Capacitor
IC7____________7555 or TS555CN CMos Timer IC
Circuit Operation:
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.
Alternative Clock:
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.
Notes:
- 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.
