Hi everyone,
I've been working on an 8-step tone sequencer which uses a 4017 IC to signal eight separate audio-tone-generating 555s to turn on in series, each one producing an adjustable tone. A bright, different-colored LED lights up during each tone for an audio-visual effect. The speed of the sequence is adjustable by varying R2 - it's a 10K resistor right now which makes each step have about a half-second duration but I will replace with a variable resistor to vary the sequencing speed.

From what I've seen, the usual way to make such a circuit is to use only one audio 555, and have its frequency/tone adjusted by varying the voltage applied to its #5 (Control) pin. I tried this but it only provided a modest range of possible tones. I don't know if I was doing something wrong. So I decided to try this more cumbersome route and have eight separate 555s, each powered on in sequence (in my schematic I only have enough space to show the first three). I was happy to find that the range of tones each 555s generates is now infinitely adjustable by varying the resistance between pins 6 and 7. So I went with this design. It is powered by 6V battery power - four AA cells for now, I will switch to C or D if it goes through batteries too quickly.

It works as I had hoped - except for an anomaly: at high volumes (turning up the variable resistor upstream of the speaker), some of the steps/tones became slightly (but noticeably) shorter in duration; the sequence was no longer producing an even tempo. But when I made the resistance between pins 6 and 7 the same for all eight audio 555s, the timing became even again. But the whole point of this project is to work properly while producing a whole range of adjustable tones. Does anyone know the reason for this anomaly and the solution? I tried connecting a 220uF smoothing cap between Vcc and ground - I was surprised it changed the sound a little but did not solve my problem.

 

ROUGH sketch of 8 selectable tone sequencer. A 4022 supplies stepping rate.& 555 supplies the tone as selected by R1-8 & R1-8'.
R1 controls C charge time, R1' controls discharge time. 50 % duty is maintained for all tones if R' is properly selected or just make them the same.

 

Sorry for my delay in getting back here. Thanks for the alternate circuit suggestion...but I must be honest that I'm still pretty much a newb and I'm not familiar with a 4022 IC or two of the schematic symbols in that circuit, or how it works. For now I really would like to stick with my 4017 and 555 design and get it to work properly. Just by looking, can anyone see anything wrong with my circuit that would explain why the steps become slightly (but noticeably) uneven at high volumes?

I thought maybe the transistor taking up too much current at high volumes and not leaving enough for the 4017 to function properly - but I still have the same problem when I use a benchtop DC power supply. I thought maybe reverse voltage feeding back into the outputs of the 4017 - but placing diodes on the 4017 outputs didn't help.