Countdown Timer

Discussion in 'General Electronics Chat' started by 06awaism, Feb 23, 2013.

  1. 06awaism

    Thread Starter New Member

    Feb 2, 2013
    Hi, I am attaching a schematic below for a time which which just counts up to 99 automatically one the SW2 is enabled. The speed of the timer is adjustable via VR1. The SW2 starts/stops the timer, SW1 resets the timer back to 00 and SW3 enable the decimal point so instead of 11, it would be 1.1. I have used the IC4093B NAND Gates as my tone generator so the timer beeps every second.

    Is it possible if I can achieve this following result following on from the schematic I have attached?
    The result I would like to achieve is that, if I add another potentiometer, I can change how many seconds I would like for countdown. So for example, I would like a player to have 30 seconds, I change the potentiometer manually to display 30 seconds on the Dual 7 segment display (Like shown in the attachment), and once I press the push to make switch (I can add another one in the circuit) the countdown starts. Now, I am not sure whether it will countdown backwards like 30...29...28 using the IC4026B in the schematic or if possible, it will reset to 00 and then automatically countdown from 00 to 30 upwards. But any of these two are preferable. Thank You.

    Here is a video I found on YouTube that has a very similar function but I don't want to use Arduino or any programming stuff as I am not allowed:-


    The credits of this schematic goes to ScottWang and Sheldons as I had major help building up the schematic above -
    Last edited: Feb 23, 2013
  2. sheldons

    Active Member

    Oct 26, 2011
    Looking at your circuit its still incorrect,you should be using the clock generator for the 4026 and gating for your tone generator as shown ,youve got extra oscillators in there that you dont need and a couple of the connections shown in the diagram are wrong.
    your circuit can be altered to work as you require but depending how accurate you want the timing you will need a stable 1hz clk and a bit of redesign to get things running
    heres the original thread
    06awaism likes this.
  3. GopherT

    AAC Fanatic!

    Nov 23, 2012
    That option will get complicated (lots of wires). Your cd4026 chip is not pre-settable and has no latch. To do what you want, You will need,
    - potentiometer connected to an analog to digital converter (adc0808) to digitize your input
    - two cd4510 to latch the output of the ADC chip these 4510 chips accept your clock pulse to count up or down with a binary-converted-to-decimal(bdc) output (one chip for 10s and one for ones digit)
    - two cd4511 to convert the bcd signal to 7-segment display and drive your LEDs.
    - You will need a toggle switch to bypass the 555 timer while you preset each digit,
    - You will need a momentary switch to latch the potentiometer value of each digit.

    1) A cd4029 may be able to to the same thing as the 4010 chip (not sure).
    2) for accuracy, you will need two adc and potentiometer sets, one for each digit. You should only use the 4 MSB pins on each adc so they are not overly sensitive. Also, connect a fixed resistor to the potentiometer positive side to prevent impossibe inputs for the cd4510. That is, you dont want to cause errors by latching a binary value of 1110 which is a 14 (14 cannto be displayed as a single decimal digit). You will have to make sure the potentiometer will only input values from zero to 9/15th of supply voltage to get valures of 0 to 9 for each digit).
    2a) If you are clever and make a 4 line buss for the input latch, you can get away with one adc and one potentiometer, you will just latch each digit value separately with the two momentary switches. Easier to explain after reading Note 2 above.

    Learning to program a pic or arduino will be different but you will still run into all of the same issues with bcd and driving multiple digits. They will just have to be solved in software instead of hardware.
    06awaism likes this.
  4. 06awaism

    Thread Starter New Member

    Feb 2, 2013
    Sorry, I think I might have made a mistake but I have updated the picture and the attachment. They show the correct schematic and simulate accordingly. So what do I do next? Thanks Again!

    Thanks for your help Sir! Well, this is my first term of doing A-Levels electronics and I am being introduced to a lot of stuff. And still learning them. Is it possible if you can make a diagram or schematic please. I am not so good at reading text and following it. And as far as the wires are concerned, don't worry about it. Thank You Very Much though.
  5. MrChips


    Oct 2, 2009
    Wow! When did they start teaching electronics at A-Levels?
  6. 06awaism

    Thread Starter New Member

    Feb 2, 2013
    I have no idea! Although, we as a whole class, are facing few problems as the syllabus we are being taught, we don't have enough resources to support it and teachers available (hence why I am asking you guys for help). There is only one teacher in the whole college who can teach A-Levels Electronic. For more details, you can have a look at my college website for the course details.