This easy-to-make electronic alarm will let you know when it's time to do something important. It is especially helpful for housewives and people who work a lot. All you have to do is use the two thumbwheel switches (S3 and S4) to set the time in minutes and press and release the start switch. Exactly when the time you set is up, you will hear and see a signal to let you know that the time you set has passed. The device can be taken with you and works with a 9V battery. Two counter ICs CD4029 are the heart of this circuit (IC4 and IC5). These are 4 bit binary/decade counters that can be programmed up or down. They are part of the

CMOS family of digital integrated circuits. The information on them is fed in parallel to inputs P0 through P3. When the PL input is high, regardless of the clock pulse input, it is loaded into the counter. When the up/down input is high or low, IC4 and IC5 count in up/down mode. These have been set up as 4-bit binary counters in countdown mode with the B/D input low. Every time the clock pulse goes from low to high, the counter moves forward by one. Most of the time, the output terminal TC, which is used for counting, is high. It goes low when the counter reaches its maximum count (if wired in "up" mode) or its minimum count (if wired in "down" mode), and it goes high again as soon as the clock changes. As shown by the bar, the clock-enable (CE) input is an active-low input. This means that the clock pluses will only work when this input is low. IC2 (CD4013) is a dual D-type flip-flop. Each D flip-flop inside it has its own data, clock, set, and reset inputs. When the clock input goes from low to high, the data bit (which is either low or high) on the D-input is sent to the output. Set and reset are separate inputs that are turned on when these lines go high. This means that when the set input is high, the Q output is also high, no matter what the logic state of the D input is or how the clock changes. In the same way, when reset is high, it overrides everything else and forces the Q output to a low state. The IC3 (timer 555) is set up as an astable multivibrator with a waveform that repeats every minute. For accurate and stable timing, the resistors R5 and R6 should be metal- film resistors, and the capacitor C1 should be made of tantalum. IC1 (CD4011B) is a quad NAND gate with two inputs. Two of these four NAND gates, A and B, along with pull-up resistors R1 and R2, make up the de-bouncing circuit for micro-switch S1, which sends out the master reset pulse every time it is pressed and released. The other debouncing circuit for microswitch S2 is made up of the last two NAND gates (C and D) and resistors R3 and R4. When microswitch S2 is pressed and then let go, it sends out the start pulse. Here's how the circuit works: At first, microswitches S1 and S2 are in a place where both the reset and start outputs are low. First, use the two BCD switches, also called thumbwheel switches, S3 and S4, to set the time in minutes. If you choose "5" with

thumbwheel switch S3 and "6" with thumbwheel switch S4, the time delay is set to 65 minutes. The second step is to press microswitch S1 and then let go of it. A pulse that goes in the right direction resets flip-flop IC2 and loads the information about the time delay into counters IC4 and IC5. Since IC2's Q2 output is low at first, when we press and let go of S2's start switch, IC2 is "clocked," and Q2's output goes high. Reset pin 4 of timer IC3 is given power to turn it on. At this point, the time delay will start to count down. To sum up, to use the reminder device, you need to set the time after which you want to be reminded, press and release switch S1 and then do the same thing with switch S2. IC5's TC output is normally high, but when the start switch S2 is pressed, it goes low. If the time set by thumbwheel switches S4 and S5 is 65 minutes, it would only go high after 65 minutes. This is a 65-clock cycle, in essence. Keep in mind that the time period of IC3 is about one minute. This pulse at the output (TC) of IC5 clocks at CP1 of IC2, whose output goes from low to high and turns on both LED1 and piezobuzzer PZ1. The clock is turned off when the clock-enable (CE) input of IC4 goes high, which is caused by the Q1 output of IC2. LED1 and piezobuzzer PZ1 stay "on" until the system is reset through switch S5. After the system is reset, you can set the time on the device again. The counters are wired in count-down mode because that is the only way for the counter IC to finish its count cycle in the number of clock cycles set by the thumbwheel switches. The time can be changed in one-minute increments. If the clock period is changed to, say, 2 minutes, the time resolution will also be 2 minutes, but the most time delay that can be set will go from 99 minutes to 198 minutes. With a time resolution of one clock cycle, this circuit can only delay time by 99 clock cycles at most.