555 Based Tremolo Snapping Sound

[Box_Stuffer]

The circuit has so many issues. 

Thanks a bunch!!  I will explore these suggestions. Actually, when I wired up my board I swapped the 10uf cap on the 1 & 8 pins of the 386 with a 2k resistor to keep it from distorting and I skipped the 10k "pre-gain" pot and instead used a 100k audio pot as the output volume control.

[Rob Strand]

Thanks a bunch!!  I will explore these suggestions. Actually, when I wired up my board I swapped the 10uf cap on the 1 & 8 pins of the 386 with a 2k resistor to keep it from distorting and I skipped the 10k "pre-gain" pot and instead used a 100k audio pot as the output volume control.

See how you go pulling the cap. I think it will be fine.

FYI, it's not entirely correct to put a resistor across pins 1 and 8 as it screws up the DC bias.   The correct way to do it is to put the 2k resistor in series with a cap.   You can see an example of this in the LM386 datasheet under the "amplifier with gain=50".   The gain of 50 example uses 1.2k but 2k is fine, you can use whatever value you want to tune the gain.

[Box_Stuffer]

So it's been a little while, but I finally got a new board wired up just like the revised plan that Rob made for me - including cmos 555 IC.





It made a drastic difference and it is actually dead silent when sitting idle. Unfortunately, as soon as you pluck a note or strum a chord , the snapping comes through again. It is only audible while the signal is going through.

I hate to give up on this project after so much effort. Are there any other possibilities?! What about shielding of components or wires? I do not have tantalum caps, which are recommended. Would that make a difference?

[Rob Strand]

I hate to give up on this project after so much effort. Are there any other possibilities?! What about shielding of components or wires? I do not have tantalum caps, which are recommended. Would that make a difference?

You have cleaned up the power supply and prevent junk coming through the power and the grounds - all good.

Maybe the modulator is chopping too abruptly and that's causing a completely different form of "tick" to come through.    You might need to slow down the edges of the modulator.    As a crude mod you could try a 1uF or 10uF cap across the LED or across the base and emitter of the switching transistor.   There's other places to put the cap  but the positions mentioned might give a hint that the edges are the cause.

[R.G.]

Maybe the modulator is chopping too abruptly and that's causing a completely different form of "tick" to come through.    You might need to slow down the edges of the modulator.    As a crude mod you could try a 1uF or 10uF cap across the LED or across the base and emitter of the switching transistor.   There's other places to put the cap  but the positions mentioned might give a hint that the edges are the cause.

Good call. Abruptly changing an audio waveform can cause an audible tick. Some electronic organs back in the days before everything-in-one-chip and sampled waveforms used this as a way to simulate organ-pipe chiff.

The fly in the ointment is that LDRs are usually slow, so slow that they don't chop waveforms, and it's a property of the LDR. The only way to get fast ones is to buy LDRs that are concocted to be "fast". Fast for an LDR tends to be on the order of tens of milliiseconds, and that's usually too slow to cause waveform abruptness.

I think there is something to try involving driving the BJT less hard and more slowly. A BC547 will have a substantial current gain, so driving its base from the output of a 555 with a 1K is really blasting the base. It only has to pull down ~9ma max, so the base drive could be cut as low as 900uA for a saturation to a gain of ten, and actually probably much less is needed. The base drive resistor could be increased to 10K for sure, and probably as much as 47K and still turn the LED on and off enough. I might try changing to two 4.7K's or 10K's in series instead of the base-drive 1K and then using a cap from the middle of the two resistors to either ground or the collector of the transistor.  That would ramp the transistor up and down a little; might be enough to stop any abruptness ---- if the LDR is fast enough to be abrupt.

There still may be something odd hiding in there. I would be agitating for changing the 555 to a cmos type if it was not being quiet other than with signal. Shoot-through on the output transistors in a  bipolar 555 is nasty.

[ElectricDruid]

RG, they've done the CMOS 555:

I finally got a new board wired up just like the revised plan that Rob made for me - including cmos 555 IC.

[R.G.]

Ooops. Missed that. Thanks for catching.

[Box_Stuffer]

Maybe the modulator is chopping too abruptly and that's causing a completely different form of "tick" to come through.    You might need to slow down the edges of the modulator.    As a crude mod you could try a 1uF or 10uF cap across the LED or across the base and emitter of the switching transistor.   There's other places to put the cap  but the positions mentioned might give a hint that the edges are the cause.

I think there is something to try involving driving the BJT less hard and more slowly. A BC547 will have a substantial current gain, so driving its base from the output of a 555 with a 1K is really blasting the base. It only has to pull down ~9ma max, so the base drive could be cut as low as 900uA for a saturation to a gain of ten, and actually probably much less is needed. The base drive resistor could be increased to 10K for sure, and probably as much as 47K and still turn the LED on and off enough. I might try changing to two 4.7K's or 10K's in series instead of the base-drive 1K and then using a cap from the middle of the two resistors to either ground or the collector of the transistor.  That would ramp the transistor up and down a little; might be enough to stop any abruptness ---- if the LDR is fast enough to be abrupt.

Right now I have a branch off from the collector and I am running another LED as a rate indicator. Will that still be possible if I raise the resistor value to the base?

[Box_Stuffer]

What would happen if I use a mini lamp instead of an LED? I have thought about trying it, but I have to find the right sized matching sockets and the bulbs. I may order some anyway because I have seen some really simple lamp-based compressor circuits I would like to try.

[sinthmart]

What would happen if I use a mini lamp instead of an LED?

It's unlikely that the LED "clicks." And it clicks like a load.
Something clicks inside 555.

And this doesn't seem to be a nutritional problem, but something else.
Possibly as radiation, influencing neighboring elements, although this seems like a stupid assumption.
(a radio transmitter is hidden inside 555!!)))