Understanding the behaviour of a 4026BE Decade Counter

I have a very simple circuit (see below), basically a 4026BE Decade Counter linked to a 7-Seg display; when I press a momentary button to apply +ve power to the counter pin (1) it increases the 'score' on the LED by 1. No inputs are floating, I've debounced the button and it all works fine, except....

If I power it via a 9V source, either a 9V battery or a 9V adapter, the LED displays 0 (zero) when I first apply the power (momentary switch is up); pressing the switch increments by 1. Jobs a good 'un as they say and it's exactly how I want it to be.

If I power it via a 5V regulated source - i.e. the 9V source connected to a LM7805 and its 5V output routed to the momentary switch (up) the LED displays '9' usually, sometimes random segments, when I first apply the power. Pressing the button increments by 1 and it all works fine.

The counter pin is pulled low with a resistor/capacitor combination (pulls it low and debounces the button) and I've measured 0V at that pin at power on. Reset pin is pulled low as well.

I'm looking for some insights in why this might be happening: why might a lower voltage be causing output on some pins of the 4026B? Could it be because the Display Enable pin is permanently connected to source, thus 9V or 5V? I could power the circuit via 9V and be done, or use 5V and just press the reset button to set to 0, but I think I'll be missing a learning opportunity here (I'm very new to electronics.) Also, the rest of what I'm building will require 5V so I thought I'd try and keep it the same.

Thanks,

Andrew

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DAB over 6 years ago

Can you reload the picture.
It does not show up in my browser.

DAB
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Andrew J over 6 years ago in reply to DAB

Strange as it shows in mine. I've pasted it again below but it might be better as an attachment: I can't see how to do that??
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DAB over 6 years ago in reply to Andrew J

I can see it in your reply.

Thanks
DAB
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DAB over 6 years ago in reply to Andrew J

As I recall, you need to pull each output line of the 4026 up to Vcc or it will not work.
Add some 10k pull up resistors or the chip cannot drive the display.

DAB
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Gough Lui over 6 years ago

Generally, as far as I'm aware, there's no guaranteed power-on state for such a counter? The difference may just boil down to an internal race condition with the IC's internal transistors - different voltages will change their behaviour.

I think it might be possible to have the reset line held low long enough post-power-up to allow for it to start at zero if you use a capacitor and/or resistor combination although I've never tried this myself.

- Gough
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Andrew J over 6 years ago in reply to DAB

All the outputs are connected to the 7-seg display; no inputs are floating. As I say, it does all work, there's just an initial power-up difference between 9V and 5V. I'm not sure where I can add 10K resistors really.
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Andrew J over 6 years ago in reply to Gough Lui

Thanks for your input guys. Gough: putting a capacitor (only 0.1 uF) linking pin 16 and pin 15 brings up 15 just long enough to keep the counter stable and it initialises to zero. Just what I was looking for.

Cheers

Andrew
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jw0752 over 6 years ago in reply to Andrew J

Hi Andrew;

If you haven't already done it you might try a .01 uF between the Pin 16 and Pin 2. This is called a decoupling capacitor and it can stabilize the voltage and help with this type of problem sometimes.
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Andrew J over 6 years ago in reply to jw0752

Hi John,

I tried that with no change: the LED still showed 9 at power on. Also, with the capacitor between pin 16 and -ve (effectively the same.) However, it did prompt me to go back to my text to read about decoupling capacitors to find that it only talked about coupling capacitors! One quick Google later and I see what you are saying, so thanks for chipping in.

I also tried with a 0.01uF and 100uF capacitor in parallel to the circuit but that didn't help either. Putting a 0.1uF capacitor between pin 16 (+ve) and pin 15 (reset) to temporarily pull 15 up keeps the LED at zero and allows the button to increment the count (and the reset button to reset to zero.) Interestingly, a 0.01uF capacitor, at the same position, kept the LED at zero but didn't allow incrementing with the button press: it must be sinking the +ve pulse faster than the chip can recognise it - although the capacitor should be charged by that point so I'm not sure really.

Thanks

Andrew
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jw0752 over 6 years ago in reply to Andrew J

Hi Andrew,

Sometimes the only fix is to do a bunch of empirical experimenting like you have done and hopefully get the right combination.

John
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