Been doing a little bit of fishing around on the net and I've found this website about the 555. It mention on here that you shouldn't use a 100uF capacitor when driving an 8R speaker and it shows the Toy Organ with a 10uF capcitor, as I have seen on many other toy organ circuits. I'm confused as to why the calculation gives me one value and every circuit I look at has a completely different value? Really keen to know about the volume control and value? I need to ideally get the board design finished and etched by Thursday night.

EDIT: John Hewes' website says that I mustn't use a speaker less than 64R and that I should use a 100uF cap?

Looking forward to hearing from some

 

Hello,

Sorry, to be late at the thread, but here is a page with a toy organ that uses a couple of resistors to change the tones:
http://www.beavisaudio.com/Library/555/555.htm
http://www.555-timer-circuits.com/toy-organ.html
And one with the posibility to adjust the tones:
http://www.next.gr/audio/musical-circuits/Mini-Organ-using-timer-555-l7596.html

Bertus

 

No worries, I have the main design now Did you see my post yesterday afternoon detailing the progress and the questions I had regarding the capacitor before the speaker, the value of the speaker, a power indicator and also a volume control and suitable value?

 

Hello,

The value of the output capacitor, will influence the reproduction of the lower tones.
The lager the capacitor, the better the lower tones will be reproduced.

Bertus

 

The value of the output capacitor, will influence the reproduction of the lower tones.
The lager the capacitor, the better the lower tones will be reproduced.

So should I ignore what the 555 timer circuits website says about the 555 getting hot? "2. The impedance of the 100u electrolytic will allow a very high current to flow and the chip will get very hot. Use 10u maximum when using 8R speaker. "

I'm just getting really confused, because the calculation gives me about 75uF for a frequency down to 260Hz and yet most circuits show it with 10uF and some places are telling me not to use higher than 10uF, but some are saying don't use less than 64R speaker and only use 100uF

EDIT: Testing the circuit using 8R and 10uF capacitor on breadboard seemed to work fine and nothing seem to get too hot. The tones aren't great, but I wasn't expecting them to be because of the circuit and speaker I was using. I can fairly accurately reproduce different tones using the values I calculated in the speadsheet that I made.

 

Hello,

The main problem with the 8 Ohms speaker, is its low impedance.
The 555 can only put out a limited current.

The LM555 is a highly stable device for generating accurate time delays or oscillation.
Additional terminals are provided for triggering or resetting if desired.
In the time delay mode of operation, the time is precisely controlled by one external re-sistor and capacitor.
For astable operation as an oscillator, the free running frequency and duty cycle are accurately
controlled with two external resistors and one capacitor.
The circuit may be triggered and reset on falling waveforms, and
the output circuit can source or sink up to 200mA or drive TTL circuits.

Bertus

 

So judging by the fact that I've seen lots of circuits using 8R and 10uF capacitor and when tested on breadboard it all work fine I should just leave it as is I guess?

The other questions were... is the way I have connected the power indicator ok? I will be using a low current LED that will consume as little as 2mA.

And finally is a 50K log pot connected as shown in my schematic ok as a volume control?

 

Hello,

A 50K pot will be much to large.
The slightest turn of the pot wil silence the speaker.
You could use a 100 Ohms / 1Watt pot in series with the speaker.
This will give a max attenuation of about 12 times.

Bertus

 

Hello everyone, there has been a few modifications to the circuit. I have scrapped the idea of using a volume control. Both design and manufacture has come on a long way since my last post. Over the weekend I was able to manufacture the board (first time using the toner transfer method!). Fairly happy with the results! Get the case made and solder the components on. After rectifying a bit of a schoolboy error and mounting the 8 pin DIL the wrong way around I got it working. Apparently the 555 timer is quite a robust chip when it comes to mounting it the wrong way round!

The casing is quite simply two pieces of perspex and the board is held between the two. These were laser cut. I am thinking of posting a completed projects thread which will detail the project, design phases, actual designs and also pictures of the finished product. I am currently waiting on a right angled through hole PCB mounted miniature slide switch. Once this arrives it can be completely assembled.

I decided to go for a 3 x AA battery pack (4.5V) as this still gave plenty of volume and would provide longer battery life. I believe that PP3 batteries are a pack of 6 x AAAA batteries, which have considerably less battery life than AAs. I was a little concerned about it operating correctly because I know that a 555 timer is supposed to operate between 4.5V and 15V I think? I removed one of the 3 batteries and shorted the battery holder to simulate using 3V and it still worked absolutely fine.

Thinking about how to make a stylus too. My current thoughts are an old pen tube, feed a piece of cable screening through it, bring it back up the outside and solder. Then use heat-shrink/insulation tape on the outside. It's crude but it should work

Any questions, comments or indeed constructive criticism is more than welcomed

 

However, if you can figure out how to have each pot set up individually, it would make it much easier.

Before each switch place a small variable resistor. This would allow you to change the resistance value of each one by a set amount without changing the overall resistance. I decided not to both with this as using the E12 range the differences between a real instrument and this were negligible. The first resistor is set to an exact value using a multimeter to get the correct frequency for the first note and then the rest go from there. I mean I'm no a musician, but it sounds fairly ok to me. In fact is you use a pot instead of a preset you can get some interesting effects by playing with it whilst playing the instrument.

One of the reasons for not having each note adjustable was because I wanted to make the circuit as simple as possible and make it easy for children to understand and build.