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.

 

Could you post your schematic?

 

You know that the trigger should be a negative going pulse...right? And do you have the switch debounced?

 

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/wikipedia/commons/1/19/555_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.

 

The switch button works properly. It just triggers while it is pressed. The trigger goes to the ground.

 

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/wikipedia/commons/1/19/555_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.

 

Do you have a 10K pull-up resistor from Pin 2 to Vcc?

 

No. Pin 2 goes to the button that goes to ground.

 

Add the 10k, and your timing cycle will begin from when you release the button.

 

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.

 

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.

That's normal; the 555 output cannot get much closer to the supply voltage than that. It gets closer to ground on the low end.

 

I figured the voltage was normal, but I thought the information could be relevant. Tracecom's diagram works like a charm, but that circuit is going to be bitch to etch. I wish I understood what all the extra stuff did. I'm hoping the Circuit Analysis course I'm taking my group through will teach me.

Now I will need to get the voltage and timing right for the greeting card circuit and shrink it down so I can wear it to win my ugly sweater competition.

 

R4 and C4 can be eliminated and circuit will still work. C2 and C3 are often left off as well, but I always include them.

 

By eliminate, do you mean short or break?

 

#2 pin must be high when it is not low, this is not a default.

Also, when pin 2 is low the 555 will time internally, but the output will be stuck at high.

 

I see, so some of the elements added in tracecom's design were meant to bring 2 back to Vcc. Is that right.

Looking at it, by eliminating R4 and C4, Tracecom must mean break R4 and short C4.

 

Post deleted, contained wrong information.

 

Post deleted.

 

Negative. While Reset (pin 2) is low, the discharge transistor (pin 7) is turned on, so the timing network is held in the low (discharged) state, and the timing cycle begins after the rising edge of the Reset pulse (pin 2).

The reset pin on a 555 is pin 4, not pin 2; pin 2 is the trigger pin, and is activated by a negative-going pulse.

 

I got the Trig~ and Reset functions confused.