Many a time it happens we forget to switch off the gas, electric oven, stove or motor while watching TV or taking on the phone or while talking to a friend in your neighborhood. To solve this problem we have described a circuit below which alarms you after the preset time interval. It will give a loud musical alarm and reminds you that you to off your machine or gas. It uses NE555 which is a well known multi-vibrator IC. Here 555 timer IC is wired as an a-stable oscillator which provide a clock pulse to CD4017 IC. CD4017 which is a medium speed Johnson Counter that works at 10MHz and has 10 decoded outputs. CD4017 ICs are widely used in frequency dividers, binary counters, and divide by N counters and register design applications.
Fig. 1: Prototype of Reminder Alarm Circuit on Breadboard
When we apply power supply, we will get continuous pulses from 555 timer output pin 3 (as it is wired in a-stable mode) and it is connected to pin 14 of IC2. After receiving the clock input CD4017 starts its counter from zero and it advances one by one each time pin 14 goes high. Like first we get output from pin 3 that is Q0 then from pin 2 that is Q1 and so on.
As we have connected the alarm circuit with the first output so when we switch on the circuit we will receive the musical sound and after that output of other pin goes high like Q1 (pin2), Q2 (pin 4)….Q9 (Pin 11). This can be seen with the help of LED which glow and stop to tell you that counter is in progress. Then again after 2.5minutes Q0 output become high and you will get the alarm.
Therefore after every 2.5 minutes you will get an alert. You can increase or decrease the time interval by changing the value of resistor R1, R2 and capacitor C1. You can also obtain output from any of the pin from (Q0 to Q9). And if you want to reset the time period in between you can do it, just press on switch S1 it will clear the counter and counting starts from initial stage.
We have used a zener diode with UM66 IC because excess voltage more than 3.3V will destroy the IC. And transistor T1 is used to amplify the musical sound so that it can be heard from distance.