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How can i make this counter stop until I press the start button again

RoyCone

New Member
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This circuit is from make electronics esperiment 19 page 185. It automaticaly counts from 0 to 9 and is supposed to stop when pressing the stop button and waits until the start button is pressed again to continue counting.

It works but the stop button only works while being pressed which makes the start button useless.

Now, the top 555 is a bistable timer, it's supposed to be off and i turn it on by pressing the stop button and off by pressing the start. It's connected to the stop counting pin 2 of the 4026 chip. If the pin 2 is low then the chip starts counting, so that explains why it starts as soon as the power is on, but if i press stop then it's supposed to go high and stay "stopped", why does it starts as soon as I let go of the button?

I have a logic probe that i have just used for the first time and i've put it on the pin 2, it's low when counting and when i press the stop button it goes high, but doesn't stay high. It turns low in the same amount of time as the counting. Is there a connection with the lower 555 chip that turns it low automatically? Or is it that the upper 555 chip is in monostable mode, but this is bistable right? It's suppose to stay on.

How can I make the counter stop by pressing the stop button and make it wait until I press the start button?
 
Last edited:

RoyCone

New Member
Is there a resistor in series with the LED to limit its current?

Remove the LED and measure the output voltage of the 555 after you've pushed the start button, and after you've pressed the stop button.
Okay so the voltage on pin 2 from the upper 555 timer is 7.7 volt when the button is pressed and 2.8 mVolt when the button is depressed.
There is a 330 ohms resistance with the led, but keep in mind that, for the led, i unplugged the wire from pin 2 of the 4026 counter, then placed a resistance and an led which I then connect to groung. I did that to know if the upper 555 timer was functionning as a bistable timer and it is. I did not placed a led in between the 555 output and the pin 2 of the 4026 counter
 

crutschow

Well-Known Member
Most Helpful Member
Okay so the voltage on pin 2 from the upper 555 timer is 7.7 volt when the button is pressed and 2.8 mVolt when the button is depressed.
You mean the "Stop" button and the "Start" button?

Does the 555 output stay with the voltage when the button is released, until the opposite button is pressed?
 

RoyCone

New Member
You mean the "Stop" button and the "Start" button?

Does the 555 output stay with the voltage when the button is released, until the opposite button is pressed?
The stop button. And no, the output drops to 2.2 mV about 5 second after the stop button is released, about the same time it takes as the lower 555 pulse takes.
 

crutschow

Well-Known Member
Most Helpful Member
The stop button. And no, the output drops to 2.2 mV about 5 second after the stop button is released, about the same time it takes as the lower 555 pulse takes.
I cannot see how the 555 can do that in your posted circuit.
Have you tried a different 555 chip in that location?
 

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