Here IC1 is working as frequency dividing circuit. In this internal oscillator stage is made using resistor and capacitor which can be connected to pin 11,10 and 9. A capacitor is connected to pin 9 and pin 11 and 10 is connected with a resistor. Most importantly, pin 10 is connected to a variable resistor so that we can select different frequency range. We are also using a fixed resistor so that while adjusting pot pin 10 values may not fall to zero. With the components explained, alarm can be adjusted for 15 minutes at the output. When power is on the pulse at the junction R3 and C2 resets the counter and counting starts. And when counter reaches bit 14(Q13), pin 3 goes high so that the buzzer connected with the transistor turns on. We have provided a reset switch S1 at pin 12 which when press momentarily will reset all counter and oscillation starts over again.
If you want to change the time period of oscillator you can do it just by changing the value of component R1, R2 and VR1.One thing we must take care that value of resistor connected to pin 10 including variable resistor should be 10 times less than resistor connected to pin 11. We have connected to pin 12 reset input to ground with a capacitor and resistor because when power is applied it initially behaviors as a short and prevents the IC from getting a ground potential through the resistor. And with the few milliseconds the capacitor charges and makes the pin potential to ground level. This ensures that the IC initiates counting from zero so that we will get regular time period making the result more reliable.
During testing of circuit you can connect the output at Q10 and Q13 means at pin 15 and pin 12 .It gives lesser division and corresponding less time cycle of operation. It is tedious to wait that the circuit is working or not , so you can also make hit and trial by connecting different value of R1 ,R2, VR1 and you will get the output soon.