Medical timer warns when pills are due

JM Terrade, Clermont-Ferrand, France -- EDN, 5/10/2001

Some people need to take medication at precise, regular intervals. When you're in a hospital, a medical staff is present to ensure that you take your medication on time. But when you're taking medication at home, you must frequently look at the clock-a clear annoyance. When my wife was pregnant, she needed to take medication every two hours from 8 am to 10 pm. To help her time the two-hour intervals, I built the battery-powered circuit in Figure 1. The circuit derives its power from a 9V PP3 battery. All the ICs are CMOS-based; the low power consumption of the circuit allows one-week autonomy. When you press the start button, BP₁, a two-hour delay commences. During this delay, LED LD₁ flashes to indicate the delay is in progress and the battery voltage is satisfactory. After the two-hour delay elapses, the buzzer, BZ₁, emits short beeps for one minute If you don't press the start button again, LD₁ stays lit continuously to indicate that the delay has elapsed. Pressing the start button initiates a new two-hour delay cycle.

IC₁, a 555 timer, operates as an astable multivibrator and produces a rectangular waveform at its output. The period should be equal to 858.3 µsec. P₁ and P₂ permit gross and fine adjustments of the period. For the two-hour delay, P₁ yields a ±20-minute adjustment; P₂ produces a ±2-minute adjustment. For a precise adjustment, observe the Q7 output of IC₂ and trim to obtain a period of 110 msec. IC₂ is a 2¹² divider whose Q12 output has a 3.51-second period. This output connects to IC₃'s clock input. IC₃ then yields a 14,400-second period, or four hours. The circuit detects the positive edge of Q12, which occurs after two hours. Changing the connection to IC₃ to another output or trimming P₁ and P₂ allows you to set different delays. Table 1 shows the timing details for the various outputs of IC₂ and IC₃.

Figure 2 shows timing details for the first two hours after asserting start for the circuit in Figure 1. Q12 of IC₃ is at a low level, and transistor Q₁ is off. Thus, D₁and D₂ cannot conduct. To light LD₁, Q₂ and Q₃ must be on, which is the case when Q9 to Q12 of IC₂ are at a high level. This high-level condition occurs for 220 msec every 3.5 seconds. This time period might seem short, but R₁'s low value and the choice of a high-luminosity LED makes LD₁'s flashing clearly visible. Figure 3 shows timing details after the two hour time-out delay. Q12 of IC₃ assumes a high level, and Q₁ grounds Point C. As a result, D₁ conducts, and LD₁ stays continuously on, indicating that the delay has terminated. D₂ conducts when Q₄ and Q₅ are on, and the buzzer sounds. These transistors turn on when Q5 of IC₃ is at a low level and Q10 and Q12 of IC₂ are at a high level. These conditions occur for 439 msec every 1.8 seconds, for a total duration of 56 seconds. This time period might seem short, but it is long enough to create an audible beep. The start button restarts the cycle.