# 555 Synth Question

#### nickarnal

Joined Nov 28, 2011
16
Hello. I have a question about a 555 timer LFO. I want to pair up two 555 circuits and make a synthesizer that has a pulse speed knob and a pitch knob. The first circuit controls the pulse and the second controls the pitch. I attached a picture of the schematic.

Now, I want to make it to where there is a switch that will bypass the pulse circuit (left hand circuit), leaving a continuous pitch (right hand circuit). I also want to make an on/off switch. I came up with two options.

Option A - A single SPDT on/off/on switch. The middle terminal would be connected to the +9V, while the outside terminals would be connected to the input of each 555 circuit. The left position would enable both circuits and the right position would just be the pitch circuit.

Option B - An SPST on/off switch to stop power from reaching the circuit and an SPDT on/on switch to choose between just the continuous pitch circuit or both.

Would this work? I am worried that when just the pitch circuit is enabled, current will flow backwards into the pulse circuit (through pin 3 and to the ground). Would this happen?

Thanks,
Nick

#### Attachments

• 201.7 KB Views: 52

#### JingleJoe

Joined Jul 23, 2011
186
simply add a switch to pin 4 (the reset pin) of the tone making 555 timer which switches it between +V and the trigger pulse from the first oscillator.

Here is a crap text circuit diagram of what I mean

Rich (BB code):
          o--- pin 4
/
+V ---o/   o---- output of first 555 timer
For a power switch, very simply, a switch on the + power supply rail will work fine.

#### Wendy

Joined Mar 24, 2008
23,136
You have a major flaw in both 555 designs. If the pot goes to 0Ω the 555 is toast, as in the magic smoke has escaped. Pin 7 switches directly to ground, if it is connected to Vcc...

You need a minimum resistance there, say, 100Ω in series with the pot.

#### nickarnal

Joined Nov 28, 2011
16
Could you explain?

I made just the tone generator circuit on breadboard with a 1 mega ohm pot and turned the pot all the way around several times and nothing went wrong.

#### SgtWookie

Joined Jul 17, 2007
22,229
You need a minimum resistance there, say, 100Ω in series with the pot.
That should be 100Ω per volt of Vcc; since Vcc is 9v, then 910 Ohms, as that is the closest standard resistor. 1k Ohms or larger could also be used.

#### Wendy

Joined Mar 24, 2008
23,136
At that point it becomes a major part of defining the circuit, when all it is meant to be is a safe guard.

To the OP,
It is very simple, you would not want a switch to short out the battery, right? The 555 has such a switch, pin 7 is a transistor going to ground. When it is on, pin 7 is ground. Pin 7 can only take so much current, which is what Wookie and I were talking about. If your potentiometer were cranked all the way to one side pin 7 will be shorting the power supply, and the 555 will shortly died thereafter.

#### nickarnal

Joined Nov 28, 2011
16
Ok. Thank you guys. I updated the circuit with an LED and fixed the resistance problem. How does this look?

p.s. - for some reason the jpg rotates when uploading it. sorry

#### Attachments

• 176.6 KB Views: 32

#### nickarnal

Joined Nov 28, 2011
16
I just realized I drew the schematic wrong! Sorry guys. I meant to put the pot at R2 and not R1. Ignore the last post and its schematic.

Here is an updated schematic.

Does this look alright?

Also, what is the point of C2 and C4?

#### JingleJoe

Joined Jul 23, 2011
186
pin 8 of the right hand side 555 should go to +V, that way it is powered up and ready when activated by the trigger at pin 4 without this you will get that characteristic long starting pulse as the capacitor charges, every time it is activated.

P.S. the cap on pin 5 is to stop it picking up any stray voltages which would alter the frequency, it's not vital but can be useful, especially at high frequency.

#### nickarnal

Joined Nov 28, 2011
16
Ah ok. Thank you guys. I appreciate the help.

#### nickarnal

Joined Nov 28, 2011
16
ok, so i just got the chance to lay it out on breadboard and test it out and it works well! thanks guys.

i have just one more question though, how do i add a volume control? i tried running a 1Mohm pot in-between the pin 3 output and the jack but it only made the high end disappear.

#### Wendy

Joined Mar 24, 2008
23,136

#### Audioguru

Joined Dec 20, 2007
11,248
An audio synthesizer?
Have you listened to it?
Isn't it just a variable frequency buzzer that is abruptly turned on and off and on and off?

The square-wave from a buzzer is a harsh sound if you still have good hearing.
The smooth sound from a sine-wave would sound pleasant.
A triangle-wave would sound different.

#### Wendy

Joined Mar 24, 2008
23,136
<shrug>

The analogy I've heard (and really heard) is a square wave sounds like bag pipes.

A sine wave sounds like a flute.

A triangle wave if compared to a sine wave has 12% distortion, so it sounds very close to a sine wave.

A standard 555 circuit makes both a square wave and a triangle wave. You have to buffer the triangle wave though, it can not be just tapped into as if it were an output.

#### nickarnal

Joined Nov 28, 2011
16
Thank you Bill. Would you mind explaining what the capacitor does and what value it should be?

#### Wendy

Joined Mar 24, 2008
23,136
You talking about the volume control?

It is to pass AC (the audio) and block the DC. It is there for the speaker mostly, the 555 doesn't care. The speaker is 8Ω, at full volume the 555 might get a bit warm.

Last edited:

#### SgtWookie

Joined Jul 17, 2007
22,229
A small capacitor from pin 5 (CTRL) to ground helps to keep the threshold and trigger voltage levels constant. Inside the IC, there are three 5k Ohm resistors in series between Vcc and GND that establish the above voltage levels at 1/3 and 2/3 of Vcc. Resistors are inherently noisy, and the capacitor eliminates that noise.

#### Wendy

Joined Mar 24, 2008
23,136

#### Attachments

• 4.5 KB Views: 114
Last edited: