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This is my 3rd Fizzle Loop Synth circuit and it builds on the previous 2 which can be found here and here.

The heart of the synth is 3, 555 Timer IC's which are used to make some really interesting beeps and boops. The difference between this version and the others is; I have reduced the number of IC's down to 3 (version 2 have 4!), the rhythms and sounds you can make from this version are in my opinion are better and lastly, there is a drum sound option which gives some really cool beats.

I also have shrunk this synth into a pocket sized one. By placing some of the capacitors directly onto switches and shrinking the prototype board down, I managed to stuff all of the pots, switches and components into a small flashlight case.

You could probably use something like an altoids tin as a substitute case.

I have also been playing around with designing schematics and have included a circuit diagram which (I hope!) is easy to understand. I've also decided not to do a step by step guide to making the circuit as I don't really think it helps too much. However, if I'm wrong let me know and I'll do it in the future. What I have done instead is described some of the trickier sections and have added explanations where necessary. Such as what a Vactrol is and how to make one.

Finally - I've made a video of the synth in action so check that out to hear how it sounds.

Step 1: Parts


1. Resistors.

Use metal film ones – they are better quality and cost about the same as other ones. Also, buy them in assorted lots on eBay

- 4.7K X 2

- 3.3K X 2

- 7.5K

- 3.6K

- 1.5K

2. Capacitors

You can also buy these in assorted lots on eBay, which I suggest you do

- 100uf X 2

- 220uf

- 22uf

- 47uf

- 2.2uf

3. Making a vactrol

- 5mm white LED X 2 – eBay

- LDR (Light Dependent Resistor) X 2 - eBay

- Heat shrink (needs to be able to fit over the LED so 5 mm should be fine.

4. 10K Potentiometers X 6 – eBay

5. Potentiometer knobs X 6 - eBay

6. 555 Timer X 3 – eBay

7. 3mm LED X 2 – eBay

8. SPDT Switches X 2 – eBay.

9. 0.5W 8ohm speaker – eBay. You can use a larger one if you want to I used a small one as my case was small.

10. 3.5mm output jack socket – eBay

11. 9v Battery

12. 9V battery holder – eBay

13. Prototype Board – eBay

14. Momentary Switches X 2 – eBay

If you also want to add a amp to increase the volume – then you will also need the following parts

14. Small amp module – eBay

15. 10K pot (This is one of the 6 that I have included above)

16. Some type of case to add everything into. I used an old torch I had lying around.

KISELIN3 months ago
Oh shit... MY BAD,, after reading your ible for I guess the 3'd time I guess I've got it. Instead of using a photocoupler you use the advantage of the LDR to be kind of a "analog" signal to the next stage.. Hmm.. Still.. Is it meant to be that the "threshold" level to the next level to be the "trig" level for that??
Stil,l MY BAD, I've done some dozen of 555's and still don't get this with the tube??
You may call me "Dumd" but please "inlighten" me futher, please?
michaelo78753 months ago
I posted a comment about some books by Forrest MIMSIII Sold at Radio Shack in the 1980' this engineer wrote many books for the layman ranging from books on 555 timers to 741 op amps to digital ic of the time making everything from light wave radio's to sound effects machines. As a young engineer I was found reading his paperback books cover to cover unlocking my mind on the true infantcy of electronics and jump starting it at the same time
lonesoulsurfer (author)  michaelo78753 months ago
Yeah - he's awesome! I wish I had a copy of the book - might need to see what eBay has to offer.
After placing my comment I check EBay
Some one offers all his books on DVD at a reasonable price. Love Forrest Mims work. and I assure you I have nothing to do with his EBay listings. But I had to order the DVD which uses PDF file format.
michaelo78753 months ago
Is your Vactrol just your spin on a analog optoisolator . Typically made with a led and a photo transistor. In the 1960's we did it taking more energy using a small indecasent light bulb and a cadium sulfide cell.
lonesoulsurfer (author)  michaelo78753 months ago
exactly. Nice and simple.
michaelo78753 months ago
Check these circuits and 100's more like this in 1980's Forrest Mims III Engineers Note Book published By Radio Shack a Tandy Co. Some times still found on eBay
gobsmacked5 months ago
"Check out the image attached which shows what each knob and switch does."
I cannot for the life of me find this image or a text explanation of what they do.
Also, are you left-handed? Layout and the way you're holding it make it appear so.
lonesoulsurfer (author)  gobsmacked5 months ago
I've added an image now which has each of the components labelled. No I'm not left handed. I think though that it might seem that way due to the lack of room to attach the components on the torch body
Thanks! Forgot to mention how much I like both the circuit and your use of that case. What to put my spaghetti nests into seems to take more thought than any other portion of my builds.
lonesoulsurfer (author)  gobsmacked5 months ago
Yeah agree. I’m always hunting around for old cases to stick my builds in.
lonesoulsurfer (author)  gobsmacked5 months ago
Thanks very much!
jiangann5 months ago
great, good article, It's very comprehensive
lonesoulsurfer (author)  jiangann5 months ago
-Atlantics-5 months ago
This is WICKED, voted!
lonesoulsurfer (author)  -Atlantics-5 months ago
Legend! Thanks
wayn3w5 months ago
Nice! Have you thought about using 4093s or 40106s for generating the square waves?
lonesoulsurfer (author)  wayn3w5 months ago
Just brought 10 40106 ic for $1.50
Cool! I look forward to comparing and contrasting with you.
lonesoulsurfer (author)  wayn3w5 months ago
I've actually got a few 4093 IC's on order so I can start to play around with them.
BertH115 months ago
switch 2 and the 1k5 resistor are shorted in the schematic ????
lonesoulsurfer (author)  BertH115 months ago
That one is ok. The 1.5k resistor when activated by the momentary switch changes the pitch. When not pushed it is dormant and doesn’t effect the sound
Sorry, but that is not what the schematic shows. BertH11 is right - the schematic shows this resistor and switch shorted. Pushing the switch in the schematic won't do anything.
The fact that this push button is doing something in your circuit proves that the schematic must be wrong, sorry.
The pushbutton is (I think) normally closed. If so, it should be accros the resistor. If that were the case, it would be as you say - pushing the button would open the switch and put the resistor into the circuit, but that is NOT what the schematic shows.
Sorry, I'm not meaning to pick fault. I'm just really keen to build this and want to have the circuit right.
lonesoulsurfer (author)  MildLeeInterested5 months ago
All sorted. I have updated the schematic to thwe actual connections made on the prototype board.
Hi. Your schematic shows pins 2, 6 and 7 on IC3 all connected together. Is that right?
lonesoulsurfer (author)  MildLeeInterested5 months ago
Nope :0. good pick-up! I've updated the schematic and Fritzing files.

Thanks for the heads-up
Cool. Thanks. I'm glad that my understanding of the ciruit was good enough to see the problem. I'm planning on designing a PCB for your circuit if I do, I'll make sure the gerber files are freely available.

I cant see any changes to the schematic or Fritzing files yet. Should the IC3 connections look something like the attached diagram? There are a few different arrangments that would work to get some sort of an output, but this one should result in a pretty good square wave...

There are a couple of other things that I don't understand. Momentary switch 2 connects to a 1k5 resistor, but both of these are shorted by the connection from pin 3 on the vactrol. I think you mean that this is a normaly closed switch that shorts the 1k5 resistor, so that when you press the button, it puts the 1k5 resitor in circuit. Is that right?

Finally, you ave pin5 on IC2 and IC3 connected by another button plus a cap and resistor. Pin 5 on the 555 is the control pin and this is usually pulled to ground via a 10u cap. I can't quite see what you are doing here, as both these pins are inputs and there is no connection that puts a voltage to this part of the circuit. It's just sort of floating...
IC3 circuit fix.png
lonesoulsurfer (author)  MildLeeInterested5 months ago
That would be awesome! I've only done a few schematic designs using frizing so I'm a bit of a noob.
I've made the changes to the schematic so should be ok now. Def worth trying to set-up the pot that way - no harm in experimenting. That's pretty much what I did with most of this circuit.
In regards to the momentary switch, I had a look at the actual circuit and made the change to the schematic. This is now how I wired it up on the prototype board.
Nice call on the connections between pins 5 on IC2 and 3. This has now been rectified on the schematic.
Really appreciate your help on the corrections - this is why I love Instructables - for the amazing community!
Hopefully that's it with the mistakes but if you come across anymore let me know and I will fix.
The Pin 7 (Discharge) of 555 is a internal "open collector output" that will discharge the cap. at pin 2 via the resistor. The discharge can be used as an output allso. It act's the opposite to the pin 3 output, it is: it goes "LOW", it starts the discharge sequence immediatlly when pin 3 goes "HIGH". Without checking with the data of 555 I would recall that you can source pin 7 (Discharge) with about 100~150mA. Us it, but with care taken (Open Collector!)
Can you tell me anything about the flashlight and the diameter of the lens? I would love to find this for a project I’m working on. I’m finding similar things on eBay but the blank front of this one makes it perfect for adding knobs.
Thanks! I have been scouring ebay and etsy for the exact one you used because the lens looks like exactly the right size for a round OLED and the front is mostly featureless, which gives it a really nice smooth appearance when you mount onto it. All of the ones I've found so far are not QUITE good enough, but it's a fun hunt.
WannaDuino5 months ago
love it, but how to build it with this 555?
lonesoulsurfer (author)  WannaDuino5 months ago
haha. I'd need me 3 of those to be able to make this synth!
clark51135 months ago
That'll sure keep the evil spirits run off.
lonesoulsurfer (author)  clark51135 months ago
Most def.
ilmuri5 months ago
Mebbe an option to sync this with an external ldr to a tempo led of a drum machine?
Ham-made5 months ago
Hey lonesoulsurfer!
Rad pocket synth! Just curious as to whether you have ever integrated one of these synth circuits into a radio or boom box in the past?
Mr. Ham
lonesoulsurfer (author)  Ham-made5 months ago
Nope but I reckon you could hack one into the amp of a radio
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