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Newbie working on SE555P and 4026 Decade Counter project..needs help

Other Parts Discussed in Thread: SE555

Hello Everyone,

First of all, thanks for all the great posts and responses that have helped me get this far. I've been reading on the forums here for a while, just not chiming in to get some assistance and hopefully get to the point where I too can contribute to the community.

I'm trying to create a simple stop-watch style circuit using the SE555P timer and a couple 4026 Decade Counters - to drive two 7-segment LED displays to show the seconds only (counting up to 99 seconds).

I've done a tone of reading, researching and have come up with this circuit for the SE555P, in astable multivibrator format.. but, for some reason.. when I'm connecting it to the 4026 I am NOT getting a consistent  second pulse. I will get a reading of 1,2, 4, 1.. and then just randomly changes between those numbers. Obviously I could have an issue with the 4026 connections that I've made - but they are all based on the spec docs for the IC and the LED...


Could really use some help, at first, just to get the 555 dialed in so I know it's generating the required pulse frequency and then I can move onto the other parts of the chain.


Here is my goal : To create a stopwatch that starts at the push of a button, stops at the 2nd push and clears/restarts at the 3rd,.. so there is only one NO button connected to init the count - not shown in my attached diagram as I'm just trying to get the freq. right.

Would love your feedback on any changes that need to be made to the circuit. Where to interface the momentary button to start/stop the counter and any values that you see should be changed

Thanks again!

HERE IS A LINK TO MY 555 DIAGRAM: https://www.dropbox.com/s/qrilc5rcareate8/555-timer.jpg

  • Jesse,

    Be sure to put bypass caps (0.1uF is good) across power pins for SE555 and CD4026. Be sure that no input pins of the CD4026 are open. I am also moving this post to the logic forum.

  • Hi Ron,

    Thanks for the reply and moving the post to the best forum.

    Bear with me, as I'm new to this.. to add the bypass caps (0.1uf as you recommended) where would I place them? You said 'across the power pins' - but I was only aware of the one pin I'm using for power (Pin 8). Which pins should I use the caps on and should I place them in series to power or ?

    Thanks again!

  • Jesse,

    The capacitor goes from VCC to ground near each IC. The SE555 is noisy, meaning ICC current peaks during the output switching. The bypass capacitor provides the current needed and keeps the VCC line cleaner. 

     

  • Ron,

    Thank you for the clarification. I've added the cap to my diagram - from Vcc to ground.
     

    Are there any other changes I need to make to maintain a 1hz astable multivibrator state?

    Would you recommend the lower value Ra,Rb values (8.2k and 68k) or the higher values (86k and 47k) from the data at the bottom of my diagram?

    I plan on using a Positive Trigger to start the 555 circuit. Obviously this is the first draft at this, but it looks like I need to make some changes to make that work. Any suggestions or links to online examples can study?

    Thank you

  • Jesse,

    The SE555 can clock close to 1Hz but it needs a variable resistor for fine tuning. By the time you get to a 99 second display. It might have taken 98 or 100 seconds to get there. 1Hz+/-1%. That is free running mode, if you stop the SE555 (reset), the first pulse on start up will be longer.

    Getting start, stop, clear action from one button will be a 'fun' challenge that I won't try. 

    If you prefer a crystal accuracy and software control, we have this inexpensive MSP430 kit.

    http://www.ti.com/tool/msp-exp430g2?keyMatch=msp430%20kit&tisearch=Search-EN#0

  • Understood... I'm ok with it being 'close enough' for this project (eg. .5 second accuracy or so.. once tuned in with a trim pot). I'm not ready for 'software' projects this side of Arduino LOL.. but, I did see a design that used a crystal oscillator (is that the correct term for it?) tuned to 1hz as the trigger for a timer.

    How would YOU recommend someone go about making a simple, inexpensive timer circuit like this? Am I way off base wanting this to work as simply as I think it should?

    Thanks Ron!

  • Would it make more sense, to overcome the issue of the first pulse being somewhat longer than the following, if I used another 555 as a monostable gate that triggers the astable 555? This would allow the astable to remain 'on'.. eager to hear your suggestions for a solution.


    Thanks

  • Jesse,

    I think a 1 second mono-stable that triggers itself should work. A small delay RC from pin 3 to 2 might (or might not) be needed for it to trigger itself properly and provide a good clock to the counter.