Solving Basic 555 IC Questions and LED Array/Buzzer Setup

[MRR]

I finally got around to experimenting with a 555 IC. Despite my post from several months ago, I decided to not use the schematic offered to me for use of a normally closed switch (great circuit, though; works perfectly) and will instead use a NOT gate (I still am constrained with a normally closed switch, but I have other things that will require it to be considered opened. As such, I will be using several NAND gates).

I was testing a basic 555 monostable last night, and ran into something that confuses me.

I want the output to last for one second. A setup that I had with a ceramic capacitor and a resistor (~ 600 ohm) lasted for 0.5 seconds. My understanding is that two resistors in series adds the values together, so another resistor of the same value should double the output period. However, another resistor of the same value increased the period by at least ten seconds (I reset after that, so I do not know the actual period). I then used just the first resistor and another one of only 16 ohm. This lasted a little bit longer. When I ran the math, it looked like I needed about 1,000 ohm. However, if I used a single 1.2k resistor (or a few smaller resistors in series), anything over 1k never turned off, and anything under 1k was shorter than 1.0 seconds. (NOTE: I'm not entirely certain where the cutoff was).

Is there some basic concept in 555 ICs or electronics in general that I am missing? Since, until last night, I never even knew that some capacitors are polarized, feel free to give me any "obvious" suggestions.


Issue #2. I need my circuit to light an LED array and also to set off a buzzer. Another post suggested that the LEDs and [STRIKE]buzzer[/STRIKE] speaker be on different circuits (that suggestion was for 555 astable). I don't have the buzzer to experiment with yet. Would I also need a different circuit to control a buzzer or can I wire it to the same output as the LEDs?

 

[anorlunda]

Is there some basic concept in 555 ICs or electronics in general that I am missing? Since, until last night, I never even knew that some capacitors are polarized, feel free to give me any "obvious" suggestions.

It sounds like you need to start with some basics about electric circuits and components. Use of digital devices like the 555 are more advanced. There are many circuit tutorials and video courses available online. Select one and give it a try.

ue #2. I need my circuit to light an LED array and also to set off a buzzer. Another post suggested that the LEDs and [STRIKE]buzzer[/STRIKE] speaker be on different circuits (that suggestion was for 555 astable). I don't have the buzzer to experiment with yet. Would I also need a different circuit to control a buzzer or can I wire it to the same output as the LEDs?

Yes, some buzzers are inductive loads and they might cause the LEDs to flicker. You can drive both buzzer and LED from the same logical output, but the two circuits should be isolated. An opto isolater is a simple way to accomplish that.

 

[osilmag]

What is the value of your capacitor?

 

[DaveE]

You may have noise in your circuit that is causing erratic timing (or not, there could be a lot of issues here). These timers aren't great at making long time delays for a few different reasons. You might consider a timer IC that is intended for long delays, like the ICM7242, or CD4541. These ICs have counters built in.

 

[trurle]

I want the output to last for one second. A setup that I had with a ceramic capacitor and a resistor (~ 600 ohm) lasted for 0.5 seconds. My understanding is that two resistors in series adds the values together, so another resistor of the same value should double the output period. However, another resistor of the same value increased the period by at least ten seconds (I reset after that, so I do not know the actual period). I then used just the first resistor and another one of only 16 ohm. This lasted a little bit longer. When I ran the math, it looked like I needed about 1,000 ohm. However, if I used a single 1.2k resistor (or a few smaller resistors in series), anything over 1k never turned off, and anything under 1k was shorter than 1.0 seconds. (NOTE: I'm not entirely certain where the cutoff was).

You are certainly pushing beyond design limits of 74x5555. IC with larger counter may be more reliable for 1s delay.
Also, you should check if R and C at RS input (pin 1) are conform to specs. 74x5555 behavior with only one external resistor is poorly defined.

 

[davenn]

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