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Old 24th Sep 2018, 8:50 pm   #1
David G4EBT
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Default 'Sawtooth Generator' project

At Post #8 of the thread below I added a pic of a PCB I’d etched for a sawtooth generator as an add-on to the Radio Bygones Wobbulator.

https://www.vintage-radio.net/forum/...d.php?t=149574

At Post #10 Terry, VK5TM asked what magazine the article had been published in and I replied that it was a design forwarded to me by forum member ‘Colin’ (Colin Armstrong). I said that though Colin isn’t very active on the forum these days, I’d e-mail him to ask if he was content for me to post the circuit and PCB layout on the forum. I’m pleased to say that Colin has responded to say that he’s quite happy for me to do that, so I’ve appended the circuit below, along with the PCB artwork, both positive and negative, the component overlay and a scope trace of the completed project. (The size of the PCB is 8cms wide x 7cms tall).

To reiterate why Colin designed the sawtooth generator, he hadn't wanted to use his 'scope timebase, so he built the generator into the wobbulator and redesigned the front panel to add a timebase control. To quote Colin at the time: "The circuit for this used a couple of discreet transistors, a 555 timer IC and two Op Amps from a Quad LM324. I used the other two spare Op Amps in this as high impedance buffers for the output of the wobbulator. They also serve to invert the output so the ‘peak’ on the scope is up rather than down".

When I buiult mine, it refused to work with a 555 and Colin discovered that some 555s don’t readily oscillate, but by replacing the 555 with the CMOS version – 7555 it solved the problem. The 78L12 12V regulator was included on the board as it is fed from the 2 x PP3 batteries that feed the RB Wobbulator into which the sawtooth generator was installed.

Though on the PCB layout the frequency adjustment pot is a miniature preset one, as stated above, Colin modified the RB Wobbulator front panel layout to included a timebase control on the front panel rather than as a pre-set on the PCB, so three terminal pins were soldered to the PCB in place of the pre-set pot, with leads to the timebase pot on the front panel.

Pic1 - Circuit.
Pic2 - Positive UV mask
Pic3 - Negative UV mask
Pic 4 - Component overlay
Pic 4 - scope trace of waveform.

(I should perhaps add that I built this project back in 2013, though for clarity, I've just re-drawn the circuit as it was originally a hand-drawn sketch).

Hope that’s of interest – at least to Terry, who’d asked for more details.

Colin tells me that though he has ‘officially’ retired, he’s busier than ever nowadays, offering a service to repair, restore, upgrade and sell classic car radios:

http://agentaaudio.co.uk/index.html
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Old 24th Sep 2018, 9:20 pm   #2
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Default Re: 'Sawtooth Generator' project

The 7555 is powered via a 10k resistor (R5)?
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Old 24th Sep 2018, 10:08 pm   #3
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Default Re: 'Sawtooth Generator' project

According to David that is a CMOS 555 chip, that does not draw that much current.

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Old 25th Sep 2018, 4:59 am   #4
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Default Re: 'Sawtooth Generator' project

Thankyou David and Colin.

I've got a box of bits that I have been collecting for the purpose of building the wobbulator and I remember seeing a mention of the sawtooth generator quite some time ago.

Will add this to the data that is accumulating for the project.

Must go see what else I need.

As an aside, I saw the Wobbulator article in a Ham Radio Today magazine from the late 90's I think it was, just the other week on the American Radio History site.
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Old 25th Sep 2018, 8:53 am   #5
David G4EBT
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Default Re: 'Sawtooth Generator' project

Quote:
Originally Posted by kellys_eye View Post
The 7555 is powered via a 10k resistor (R5)?
Thanks for reading the thread and for your well observed comment expressing surprised at the 10K resistor in the supply line to pins 4&8 of the 7555 IC.

I just built it as designed, which shows a 10K resistor (R5) to pins 4 & 8, and a further 10k (R4) from there to pins 6&7.

When the originally specified 555 IC was substituted with an LM7555 it worked fine, but I don't pretend to understand how the circuit works! I'd agree that ordinarily, pins 4 & 8 of 555 circuits as often as not receive the full supply rail Voltage, as in the circuit I've added at pic 3 of a little 555 tester I built to test a batch of 555s I'd acquired back in 2015. Similarly, the square wave section of the simple square/sine wave function generator at pic 4 below using a 555 shows pins 4 & 8 receiving the full supply Voltage.

I don't like posting indistinct, scrappy or hand-drawn circuits on the forum, which is why I re-drew the original Sawtooth Generator circuit as supplied to me by Colin, but for the sake of completeness, I've attached the original circuit below, alongside the one that I re-drew. As will be seen, the original used the U.S. symbol for fixed capacitors and values for some components that are now non-standard (.02, not .022, 500k not 470k) so I updated those aspects. I also used rectangles for resistors - not zig-zags and added the 78L12 regulator, which is on the PCB layout, but not on the original circuit.

As earlier explained, the sawtooth generator was used by Colin, myself and maybe a few other constructors as an add-on to the RF Haigh Wobbulator design originally published in Radio Bygones #82 in April/May 2003, and some time later, in the BVWS Bulletin using long since unobtainable TOKO coils. Obviously it could be used with other Wobbulator designs if desired.
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Old 25th Sep 2018, 9:31 pm   #6
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Default Re: 'Sawtooth Generator' project

Suspect C4 is 470uF (or just possibly 470nF)?
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Old 26th Sep 2018, 2:30 am   #7
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Default Re: 'Sawtooth Generator' project

I knew I had seen this some where before.

Here is the pre-555 version - C4 is 470µF.

For the 555 version, R5 is 10R not 10k (10k works because the cmos 555 has such a low current draw compared to the normal 555).

Couldn't get the ohm sign to work even though µ works.

If I can get to the boxes with the whole article, I'll post the relevant part here.
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Old 26th Sep 2018, 9:59 am   #8
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Default Re: 'Sawtooth Generator' project

Arrr That's more logical ~! i was wondering about the 10 k resistor also the 555 is very frequency sensitive to what voltage its run on 10 ohm now that's more like it .
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Old 26th Sep 2018, 5:10 pm   #9
David G4EBT
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Default Re: 'Sawtooth Generator' project

Quote:
Originally Posted by buggies View Post
Suspect C4 is 470uF (or just possibly 470nF)?
Quite correct!

Well spotted - my mistake. As is clearly shown in the component overlay at pic 4 of my first post above, it's a 470 µF axial cap.

Thanks for spotting it! I've corrected the circuit, which is attached.
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Old 26th Sep 2018, 6:34 pm   #10
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Default Re: 'Sawtooth Generator' project

Oops! C4 is now reverse polarity David
No offence intended nice of you to have taken the time to redraw and post

Ps. I know you pay great attention to detail
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Old 26th Sep 2018, 9:47 pm   #11
David G4EBT
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Default Re: 'Sawtooth Generator' project

I'm afraid that I've had rather too many 'intellectual interludes' (AKA 'senior moments') with this thread. Please file the circuits to date under 'burn before reading' and when I'm not asleep at the wheel tomorrow, I'll re-correct the correction!

Every village has its idiot!
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Old 26th Sep 2018, 9:52 pm   #12
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Default Re: 'Sawtooth Generator' project

We all have days like this from time to time
I just wish I was as good at wood work as you are
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Old 27th Sep 2018, 1:12 am   #13
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Default Re: 'Sawtooth Generator' project

Here is another basic circuit from my audio sweep generator.
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Old 27th Sep 2018, 8:28 am   #14
David G4EBT
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Default Re: 'Sawtooth Generator' project

Right, here I go again - 're-corrected corrected' diagram attached.

Much more of this malarkey and I'll have to add an allergen warning to my posts: 'May contain errors and omissions'!
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Old 27th Sep 2018, 11:34 pm   #15
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Default Re: 'Sawtooth Generator' project

Not attempting to hijack the thread here but seeing as I'm laid up post-surgery and bored out of my mind, I saw this and though I'd have a go at making an even simpler sweep generator as that retriggering seems very complex. Also I'm a cheapskate and didn't have any LM324's in or CMOS 555's which disappointed me.

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Got this far. Excuse the codeine inspired artwork.

Could do with an opamp buffer on the output. Output voltage is about 2/3 VCC so use a 78L09 on a 12V supply and bob is indeed your uncle. Scales from 100mHz (milli not mega) to about 200KHz. Change the 560 ohm resistor to control sweep rate. Change capacitor to control range. Concept is about the same as the original one. I minimised the design from the original one and ended up here after an hour or so. And it worked!

Could do with an output buffer.

(also excuse the 5V in the diagram - bench power supply is too heavy to lug around so I used a phone charger)

Last edited by MrBungle; 27th Sep 2018 at 11:41 pm.
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