This countdown can count in regressive counting from 99.00 minutes until 00.00.Only the minutes are adjusted, not the seconds.
The circuit is based on a microcontroller PIC 16F873A. This is how the countdown works:
When we turn on the circuit we see 00.00 on the displays and the point next to the tens of minutes will be illuminated on the display (this display point can be exchanged with the LED diode, connecting it between the R4 resistor and the negative power pole).
This point indicates us that we have entered the minute set up menu.
Pressing S2 we can fix the minutes that we want. If we press S1 the countdown will start the backwards counting from the memorized minutes.
The S3 switch serves to advance the minutes five by five.
The countdown starts to count, from the minutes we have fixed, in the regressive counting until 00.00.
When we reach 00.00 the buzzer connected to the RA5 PIN of the 16F873A microcontroller PIC will start to beep.
To stop this sound we are to press S2 and the buzzer will stop beeping.
When the counting reaches 00.00 it will not stop, it will start the count up, that is, it’ll start to count the seconds, the minutes and the hours in the ascending order.
The buzzer will not stop beeping until we press S2.
This means that, when we press S2 to stop the buzzer, this’ll also stop our count up and we shall know how much have we passed from 00.00.
If we stop the countdown before we reach 00.00, the counter will stop, but we won’t be able to reinitiate the counting from this point, we’ll have to press S1 again to readjust the minutes.
When we press S1 it’ll remind us of our last counting data. The same thing will happen when we switch off the circuit.
When we turn the circuit on, the last countdown will stay memorized and it’ll be visualised on 7 segment displays.
The program we have created for this countdown works with a microcontroller PIC, model 16F873A. This PIC reads the seconds, the minutes and the hours of the integrated circuit DS1307.
This integrated circuit DS1307 is a real time clock which counts the seconds, the minutes and the hours in an ascending order, not the descending one, and that is why the program we’ve made inverts the DS1307’s count. The precision in the maximum time we can program (99.00) is absolutely exact.
The 16F873A connection with the DS1307 integrated circuit is made by an I2c bus with the RC6 and RC7 ports from microcontrollers to the pins 5 and 6 of the integrated circuit DS1307.
The 16F873A microcontroller data is represented by the integrated circuit decoder BCD on 7 segment CD4011, on common cathode four 7 segment displays.
We have connected a resistor to the PIN 16 of PIC, and we have connected a transistor to resistor which will alert the buzzer and it works when we put a 5V voltage to its extreme ends (if you can’t find the 5V one, the 6V one will also do the trick). The 1K trimmer serves to lower the buzzer’s volume, if it’s too high.
The circuit is powered with 5 volts of direct current and its consumption will depend on the type of the selected display. The one we’ve used for this assembly is a low consumption display “kingbright sc52-11srwa” and the whole circuit power consumption is not higher than 80mA.
The pictures of the different displays of our countdown.
When we turn on the circuit we can adjust the minutes of the countdown. The point of the tens of the minutes is illuminated. If we wish to start counting we press S1.
The countdown functioning and counting until 00.00.
Once the counting reaches 00.00 the buzzer starts to beep and the countdown becomes count up, until we stop it by pressing S2, in that moment the buzzer will stop beeping.
In the non-volatile PIC memory the last countdown will be saved. If we stop the counting or disconnect the circuit, and then turn the circuit on again, the last countdown will appear during the first visualisation.
The .hex code, the image and the .sch schema is compressed, to extract it, you should enter a password: www.kemisa.es
If you do not know how to program microcontrollers click here.
If you don´t know how to make a printed circuit boards click here.