# 555 Tutorial Questions

Discussion in 'General Electronics Chat' started by CoachKalk, Sep 24, 2011.

1. ### CoachKalk Thread Starter Member

Sep 20, 2011
139
2
I am a total newbie that is trying to get a quick education for several projects I am going to try to jump into. I have spent many hours reading over the 555 timer tutorial and have printed and completed the worksheets/experiments as well.

Based on the last couple of days, I just wanted to summarize what I have done and what I think is going on so I can be corrected before continuing down the wrong road. Warning - I will probably use incorrect terms, but hopefully you can follow.

The one-shot set-up can provide a pulse for a set time that can be determined by the user based on R and C. After the time is up, output goes low again. I am not sure if/how this function will work with what I am looking to do, but I wanted to do see how it worked just in case.

I also went through the astable worksheet. This is where I want to describe how I understand what is going on and you get to tell me where/if I am messed up. The 2 and 6 pins are connected and based on the V they see, decide which "path" to take based on the 1/3 (Pin 2) or 2/3 (Pin 6) of the supply voltage. The C "loads" and "drains" to cause the different times for high and low. Although I had to use slightly different components for my circuit, I was suprised to see how close my calculations came to my measured results.

I had also found an experiment using the 555 as a Schmitt Trigger. So, I jumped in and did that as well. I was able to use the pot. to raise/drop the voltage the 2/6 pins saw to flip (high/low) between 2 LED's. However, this set-up did not include an external capacitor. Pins 5 and 7 were not used.

My final experiment was to replace the pot. with a photocell. I was able to trigger the 2/6 flip by covering/uncovering the photocell. I did have to adjust the additional R so the effect of the covered photocell made the 1/3 to 2/3 trigger. I have several projects that I want to use photocells for so I was happy to get this to work.

Now potential stupid questions.
Is it overkill to use the 555 to just flip based on a photocell?
Is there a better IC to use?
Assuming I have not messed up the "front" side of the photocell experiment, I have the 555 outputing to the red and green LED's. Obviously, I have other plans for the 555 to control. Ideally, I plan to use the output to stop a timer and start a flashing light.

Am I going down the correct path? As long as the output voltage (Supply - 1.7) is sufficient for the "next" circuit at the output, I can just keep going?

I apologize for the length of the post, but I wanted to give some info on what I have done so you can give suggestions. I do not have any of my own circuits to show becuase I have just used the circuits in the tutorial and experiment sections of this site.

Any feedback would be appreciated.

2. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
Welcome to the forums, CK.

That's kind of our specialty.
Good for you. Try to keep your projects small and fairly simple at first. Once you get a good understanding on how things are working and experience regular successes, then you can go a bit more complicated. Try to not stretch it too far; as it's pretty easy to "go overboad." Trying to build complex circuits when you're just starting out can set you up for failure, which we don't want to happen.

Great!

We'll try to nudge you gently in the right direction when you go astray.

There are practical limits, but that's basically true. 555 timers don't do so well with time periods over a half-hour or so due to things that are difficult to calculate, like capacitor tolerance and leakage rates.

That kind of circuit is handy if you just want to have a single time delay from some kind of event, like maybe an egg timer.

The threshold (pin 6) is the 2/3 point, trigger (pin 2) is the 1/3 point. The threshold is adjustable via CTRL, pin 5.

The correct terms are "charges" and "discharges".
A "load" is something that by design dissipates power to accomplish some sort of useful work.
A "drain" is a terminal on a MOSFET; which is a type of transistor.

Actually, it's the RC time. The resistors control the rate at which the timing capacitor charges and discharges; so it's a function of both resistance and capacitance.

It's always nice when things work as expected.

That's perfectly fine. However, when pots get used a good bit, they will get "dirty" and may have a rapidly varying output. You can use a small capacitor from the wiper to ground, say 100pF to 10nF (0.01uF) to keep the pot relatively "quiet".

Good. Keep in mind that under bright light, a photocell will have VERY low resistance. You should put an additional resistor in series with the photocell to "take the heat", otherwise you might burn the photocell up. 100 Ohms per volt of Vcc should protect it pretty well.

Not at all. You can get a 555 timer just about anywhere and they are cheap; why not use it?
It really depends on the exact application. 555 timers are very useful over a very broad range of applications, and are very useful 'tools' to have in your electronic "belt". However, if all you have is a hammer, every problem starts looking pretty much like a nail.

Well, it helps to embed links to the exact schematic or web page that you are talking about; otherwise the effort required to help you increases considerably, and delays a useful response.

Like I said, I'm not sure which experiment you're referring to.

3. ### CoachKalk Thread Starter Member

Sep 20, 2011
139
2
Sgt,

Thanks for your feedback. I knew I would have some terminology wrong.

I did have a 2nd resistor when I tried the photocell - the experiments had it all worked out and I just tried to keep the changes to a minimum.

As far as what I have planned for the "output" ... Hmmmm ...

I have been following the 7 segment display thread but have not spent too much time trying to figure out if that is out of my league as a beginner. Honestly, I would never have any reason to go beyond 10 minutes for duration and if anything I may need tenths of seconds (timing races and games). I do not remember seeing anything discussing less than second increments. I would just shamelessly try to copy some of the projects already posted here.

I have looked into some flashing lights/sirens that I would like to go off. I know you guys do not like to talk in generalities and I know somewhere in the hundreds of threads I have scanned I remember someone mentioning a freeware/trial software for drawing circuits. Would you happen to have any suggestions for that type of thing?

Steve

4. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
Nothing wrong with copying stuff. It's best to try to follow projects that have plenty of explanation behind them - however, some of the threads on here go on for many, many pages and never get resolved for one reason or another - sometimes, it's best to look at the last few pages first to see if it's worth reading all the way through.

As far as circuit simulation, I've primarily been using LTSpice; it's good and free to download. There is a support group on Yahoo! groups; "LTSpice Users Group", that has a board where you can ask questions, and a files area for downloading models and symbols.

Cadsoftusa.com has Eagle schematic capture and PCB layout available in a somewhat limited freeware mode for hobbyists; schematics are limited to 1 page and PCBs are limited to 3"x4" and 2 layers - but you can do a lot with that. If you decide to try it, you should go through the Sparkfun Eagle tutorial (Google is your friend) or you'll be lost. Eagle is quite powerful, once you get over the learning curve. If you don't go through some tutorials first, you will spend a lot of time floundering around being frustrated. Ask me how I know this.

There's ExpressSCH and ExpressPCB, but I find them limited. They're handy if you want to buy PCB's from the PCBexpress company, but they are rather high priced - and the pcb file is in a PCBexpress proprietary format (other companies can't use them). However, you CAN use the files to do laser transfers for making your own boards.

Mar 24, 2008
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6. ### CoachKalk Thread Starter Member

Sep 20, 2011
139
2
Bill,

Actually, I am pretty sure most of the tutorials I have been looking through have indeed been from you. I did not acknowledge the people before, other than mentioning that the worksheets/experiments were from this site. Perhaps someone else did the experiment write ups, but make no mistake the tutorials were an EXCELLENT resource for me.

Actually, I thought it was you involved in the 7 segment led display thread as well. In general, I have found this site incredably useful and welcoming for the newbie. Perhaps to welcoming because my kids are now making fun of me for trying out all of the experiments.

Oh well ... I can take it.

7. ### SgtWookie Expert

Jul 17, 2007
22,183
1,728
Challenge the kids to try to build them as well.

8. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
What age range are you teaching? If you have any special experiments written up I do requests.

What part of the world are you too? I am one of the folks that like to support schools, especially kids.