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Old 11-24-2013, 09:35 PM
GunsNTulips GunsNTulips is offline
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Default Monostable 555 not timing out correctly

I'm trying to set up a simple monostable 555 timer to give me a voltage for a specific time after a trigger is pressed. This is actually meant as a hack to those musical christmas cards, so that I can press a button and play the song through. To test the circuit out on a breadboard I am using an LED with a 100 Ohm resistor off of 3.

I am following the diagram on Wikipedia precisely. I have a 1 microfarad capacitor (PMG105 2K) and a 1 MOhm resistor. Off of terminal 5 I have a 10 nF (103Z) capacitor to ground.

This should give me a time out of about 1 second. Instead, the timeout is inconsistent. It's never just one second, and it increases each time the button is pressed often, not timing out at all.
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Old 11-24-2013, 09:44 PM
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Could you post your schematic?
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Old 11-24-2013, 09:52 PM
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You know that the trigger should be a negative going pulse...right? And do you have the switch debounced?
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Old 11-24-2013, 09:54 PM
GunsNTulips GunsNTulips is offline
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I can link to it. I really am a newb to electronics, but I have a blast mucking about with this stuff.

http://upload.wikimedia.org/wikipedi...Monostable.svg

Also, I'm connecting to 3 AA in series, so it should be 4.5 volts. I've measured the voltage from the output to ground at about 3.1 volts when it is on.
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Old 11-24-2013, 09:56 PM
GunsNTulips GunsNTulips is offline
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The switch button works properly. It just triggers while it is pressed. The trigger goes to the ground.
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Old 11-24-2013, 10:00 PM
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Quote:
Originally Posted by GunsNTulips View Post
I can link to it. I really am a newb to electronics, but I have a blast mucking about with this stuff.

http://upload.wikimedia.org/wikipedi...Monostable.svg

Also, I'm connecting to 3 AA in series, so it should be 4.5 volts. I've measured the voltage from the output to ground at about 3.1 volts when it is on.
Build it like this and it will work. 4.5V is on the low edge of what a 555 needs to operate.
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Old 11-24-2013, 10:01 PM
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Do you have a 10K pull-up resistor from Pin 2 to Vcc?
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Old 11-24-2013, 10:05 PM
GunsNTulips GunsNTulips is offline
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No. Pin 2 goes to the button that goes to ground.
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Old 11-24-2013, 10:09 PM
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Add the 10k, and your timing cycle will begin from when you release the button.
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Old 11-24-2013, 10:26 PM
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Add the 10k, and your timing cycle will begin from when you release the button.
I don't think so. The 10k pull-up is required, but pin 2 needs a negative going pulse, so the timing cycle will start when the button to ground is depressed. The problem is there is bounce on the button.
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