Long Timing with a 555 IC

Question

I am not an engineer. I am an old retiree who took up woodworking as a hobby and recently took an interest in making whirligigs. My third whirligig had a nautical theme, with a sailboat and a buoy which rocks when the wind blows. This time I decided to put a blinking LED on the buoy and started watching videos on YouTube to learn about LED's and solar charging circuits. I soon had a steadily blinking LED on my buoy, but wanted it to have an on-off-off-off-on-off-off-off, etc. pattern, so soon found Karen's videos on the 555 IC, and now have the desired blink pattern on my buoy. Now I want sound.

I picked up a few of those greeting card type MP3 player circuits and have gathered some suitable nautical sound files ranging in length of from about five seconds to about a minute. The MP3 players are triggered by a momentary push button switch, which I want to replace with a pulse timed to play the file, wait a while, and play it again. I need a circuit to generate those pulses.

The 555 IC claims "timing from microseconds through hours", but I've only seen demonstrations of fast blinking or flashing LED's, and I've seen the formulas and the nomograph in the data sheet, but just can't figure out where or how to start to get the necessary time delays. Do I start with a capacitor and the desired delay in microseconds and figure the R values from there? I'm really lost. Am I on the right track at all?

I'm very new on this site, and this is my first post. I hope it's appropriate and in the right place. This is a very busy web site, and I get confused easily.

Message was edited by: John O'Neil Sorry for the long delay in getting back to this, but we had a major wind and rain storm the beginning of the week and the electricity has been off since Tuesday night. I'm going to have to start over reading the replies and studying the new information, and I have some catching up to do with other things, so it will be another day or so before I get back again. I think I should have mentioned that I'm talking about the astable mode of the 555, so my specific issues are the values of the two resistors and the capacitor, frequency, and the duty cycle. Thanks again!

Sorry for the long delay in getting back to this, but we had a major wind and rain storm the beginning of the week and the electricity has been off since Tuesday night. I'm going to have to start over reading the replies and studying the new information, and I have some catching up to do with other things, so it will be another day or so before I get back again. I think I should have mentioned that I'm talking about the astable mode of the 555, so my specific issues are the values of the two resistors and the capacitor, frequency, and the duty cycle.

Thanks again!

dougw · Accepted Answer

There is a graph in the following link (labelled Monostable Nomograph) that allows you to pick any combination of resistance and capacitance that achieves the time you want:

https://www.electronics-tutorials.ws/waveforms/555_timer.html

Time = 1.1 x R x C

C = T / (1.1 x R)

For example if you want 52 seconds, and you pick R to be 1MOhm, then C is about 47 uf.

Jan Cumps · Answer

If you need to tweek the duty cycle (to get 3 offs for 1 on), you need a bit of extra components. I have found speedy examples , no slow ones. Checking if I have a 555 in my bin ...

Jan Cumps · Answer

So I did not find an NE555 but LTSpice has a model. I've replicated this design from All About Circuits (the article discusses several options); R3 and R4 are the 500K potentiometer of their schematic. This simulation could be used to find a good combinations of Rs and Cs, if you don't want to do the maths.

Jan Cumps · Answer

Changing R3 to 100K and R4 to 400K gives a very different duty cycle. With a 100µ capacitor, 150K and 330K (E12 resistors somewhat close to on-off-off-off):

jc2048 · Answer

I've got some LM555, so I thought I'd try your last circuit (the 150k/330k one). I couldn't find a 100uF capacitor, so I'm using two 47uF tants in parallel. I'm using a 6V supply for this, like in your simulation. The waveforms look like this (yellow is the output, blue is the voltage on the capacitor) The first timing interval is longer because the capacitor has to climb from zero rather than bobbing up and down between the two thresholds. With the bipolar part [the CMOS variants may well be different], the upper output level doesn't go to the rail and moves around quite a bit with varying loads, and that affects the timing quite a bit. The original device was intended to have TTL output levels on 5V, so the low level is a saturated transistor, but the high will be some other kind of arrangement under the supply rail. On the above waveform, you can see it moving under the influence of the charging current. I've not seen the timing done like this from the output before, but then all the material on 555s I have predates the CMOS devices. If I change your 150k to a 68k, I get this which is closer to the mark-space ratio you have, though the period is shorter One thing to be wary of, if the resistor values go too high, is capacitor leakage. If the charging current gets to be of the same order as the capacitor leakage plus the input bias currents, then obviously the capacitor won't then charge.

Long Timing with a 555 IC

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