91Views25Replies

Author Options:

Why is my 555 timer metronome circuit so variable? Answered

I constructed this metronome circuit using a IC555 timer chip (see image below) and it worked fine on the breadboard. I wanted 2 ticks a second (2Hz) so used a trimpot to set that but was getting wild fluctuations with either the tempo increasing or really fast ticks even with the trimpot at maximum resistance.

So I changed out the trimpot to a 30KR resistor and that seemed to work, but after doing the circuit nine times, it has also proved highly variable and I don't know why.

I'm running the circuit on 6V (2x CR2032) and my resistors are rated at 1% tolerance.

Any help please?

25 Replies

user
steveastrouk (author)2016-06-15

The classic method is to have 555 timer running 100X faster than you want, and then divide the result with a /100 circuit - your timing parts become much more stable

Select as Best AnswerUndo Best Answer

user
chiok (author)steveastrouk2016-06-15

Interesting idea, do you have a link on how to make the /100 circuit please?

Select as Best AnswerUndo Best Answer

user
steveastrouk (author)chiok2016-06-15

Here's a link with useful information.

http://www.electronics-tutorials.ws/counter/count_2.html

Select as Best AnswerUndo Best Answer

user
iceng (author)2016-06-15

Did you ever see if adding that stabilizer o.01 uF cap to pin 5 helped your worst phenolic circuit board ??

BTW do you clean around the socket pins with an alcohol Q-tip ?

Select as Best AnswerUndo Best Answer

user
chiok (author)iceng2016-06-15

I'll see if I can find one to try out, the smallest one I have is a 0.1uF cap right now. I haven't cleaned around the pins. I'm using a socket soldered with flux to the board and then putting the 555 in, trying to be careful not to touch it. This whole thing is driving me crazy to be honest, I thought it was going to be such a simple job!

Select as Best AnswerUndo Best Answer

user
iceng (author)chiok2016-06-15

o.1uF works as well.

Phenolic pref-board can retain more solder flux conducting dirt then fiberglass.

Ergo the solvent Q-tip cleaner.

Your wiring is slightly different boar to board and it is never a good idea to leave an unconnected input pin to an op-amp.

Select as Best AnswerUndo Best Answer

user
chiok (author)iceng2016-06-15

Thank you for all your replies by the way, I really appreciate sage advice on these matters.

Select as Best AnswerUndo Best Answer

user
iceng (author)2016-06-14

The first pulse is always longer.

Small temperature changes affect electrolytic capacitance changing the timing un-constant.

The 9v battery will discharge (lower voltage) changing the charge rate.

Select as Best AnswerUndo Best Answer

user
iceng (author)iceng2016-06-14

250K pot is more then the recommended 150K, or use a CMOS 555 to work stable with a higher R.

Use a polycarbonte more temp stable capacitor and a strong battery.

Select as Best AnswerUndo Best Answer

user
chiok (author)iceng2016-06-14

Unfortunately, polyphenylene sulphide (the apparent replacement for polycarbonate capacitors) only go up to 1uF from my supplier, can't get 22uF. I'm also limited on battery size as this needs to fit in a thin form factor, hence using 2x CR2032. Will changing the NE555 to a LMC555 make all the difference?

Select as Best AnswerUndo Best Answer

user
-max- (author)chiok2016-06-14

Electrolytic capacitors are the worst! They can have tolerances as bad as 80%!!! To put that into perspective, a capacitor with a nominal rating of 100uF capacitor may have a capacitance of anything between 20uF and 180uF!!! And the capacitance can change with temperature, pressure, even with the applied voltage (for other capacitors, I'm looking at you multilayer ceramic caps!). You should instead use a ceramic, film, or anything other than electrolytic capacitors for timing.

.

Make sure that you are not touching (or even getting your fingers too close) to any of the sensitive components while the circuit is operating, as doing so can affect the frequency of the ticks quite significantly. Shielding them and using a ground plane on the PCB will help keep external interference (and your grubby little fingers) out of the equation.

.

And like Icing already said, all the electronic parts, the resistors, 555 timers, capacitors, everything... has a tolerance to it and will lead to deviations in the frequency. That just has to be trimmed out manually on each one. Welcome to the wonderful practical world of analog circuits! :)

Select as Best AnswerUndo Best Answer

user
chiok (author)-max-2016-06-15

Could you suggest a better material for capacitors? I'm thinking that they must be my issue now.

Select as Best AnswerUndo Best Answer

user
iceng (author)iceng2016-06-14

Trimpots are notoriously unstable near the end of both limits.

A better trim pot would be to use a 15% pot in series with 85% fixed resistor.

Select as Best AnswerUndo Best Answer

user
chiok (author)iceng2016-06-14

I thought the trimpot was the source of my variability, which is why I switched it out for straight film resistors at the required resistance to give me the 2Hz tick. But just the fact that I make 9 of the same circuit and no two are the same is baffling me. Even more so that the breadboard circuit works absolutely fine every time.

Select as Best AnswerUndo Best Answer

user
iceng (author)chiok2016-06-14

That's easy each 555 (in fact all logic gates) have a unique slightly divergent voltage trip points.

Try this,...... exchange some of the 555s with the breadboard, but first put some nail polish on your std 555.

Select as Best AnswerUndo Best Answer

user
chiok (author)iceng2016-06-14

I swapped out the 555 from the breadboard and one from a faulty circuit and the ticks remain the same, i.e. breadboard is correct and circuit is off. Repeated this with a few of the boards. So it must be the capacitors? I'm wondering if they may have been damaged when I solder them to the board? I bend the legs so the cap lies flat to the board, could that crack the dielectric? Too much heat applied? Even though I don't think so.

Select as Best AnswerUndo Best Answer

user
iceng (author)chiok2016-06-14

That's utterly amazing !

I would not expect that. Your obviously skilled enough to not make wiring errors and soldering a cap only heats it until cool by now .

Before you are going to have to try each component.

Add a o.01uF ceramic disc from pin 5 to ground. That will stabalize the modulation input.

Select as Best AnswerUndo Best Answer

user
iceng (author)iceng2016-06-14

Here's the layout and common is the ground.

Select as Best AnswerUndo Best Answer

user
iceng (author)iceng2016-06-14

You may have to click on the pic to see the whole image..

Select as Best AnswerUndo Best Answer

user
iceng (author)iceng2016-06-14

Regrettably there will always be a need for a trim adjust.

Even 25 mechanical metronomes cannot be relied upon without extra ordinary contraptions, as the famous Myth_Busters video shows.

An 8 pin tiny micro with a crystal ocs is better as a metronome, and two momentary push buttons for faster / slower replace the pot..

Select as Best AnswerUndo Best Answer

user
rickharris (author)2016-06-15

accurate timing with the 555 is very difficult. Even changes in temperature will effect the timing.

If you need to accuracy try using Audacity,

http://www.audacityteam.org/

A free audio processing recorder that can produce a click track to your specification. You may also find a use for the other facilities.

Select as Best AnswerUndo Best Answer

user
Josehf Murchison (author)2016-06-14

I like the way you ask a question, it is easy to understand.

Every component is made to a tolerance.

In general 5% for most resistors, 10% for most capacitors.

Add their tolerance to the 555 ICs tolerance combined with a lose pot and weak wiring, the metronome will vary in performance. A degree of variation is expected.

Erratic signal like tic, tic, pause, tic, tic, pause, bad component, wire, or IC.

Check your joints, they can look good and still be weak.

Soldering can damage an IC that is why I use IC sockets and it makes replacing a defective IC easy.

Select as Best AnswerUndo Best Answer

user
-max- (author)2016-06-14

The 555 timer, while a very useful and versatile chip, would be hard to use in this way because of nature of the circuit. Because of the way the 555 works, the capacitor needs to charge from 0 volts all the way up to 2/3rds of the supply voltage before you hear the first tick but once the capacitor get to that 2/3rds point it starts to discharge until it reaches 1/3rd of the supplied voltage. Hence why as icing said, first pulse takes longer.

.

It is not possible (with that circuit) to adjust the frequency and duty cycle separately. You need to select both resistors so that you get as close to 50% duty cycle as possible, so that all the "ticks" are equally spaced apart. Adjusting just one or the other resistor will change the ratio of the resistances and change the duty cycle. However if you adjust the value of the capacitor (adjustable capacitor) then then how fast it charges and discharges will be equally affected and only the frequency will change.

Select as Best AnswerUndo Best Answer

user
iceng (author)-max-2016-06-14

You may like this Max.

And you may have to click the pic to see the whole image.

Select as Best AnswerUndo Best Answer

user
-max- (author)iceng2016-06-14

Yeah, I am familiar with that too, and I like that circuit better. Choosing Ra and Rb to be the same value should allow 50% duty cycle at least according to the electronics app I have, but both need to be adjusted simultaneously to maintain a 50% duty cycle. I guess that can be achieved with one of those fancy dual potentiometers. I did not think about that.

Select as Best AnswerUndo Best Answer