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 Forum index » DIY Hardware and Software » Thomas Henry designs
What do these symbols mean?
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Hedefar



Joined: Feb 07, 2013
Posts: 15
Location: Denmark

PostPosted: Sat Feb 16, 2013 7:25 am    Post subject: What do these symbols mean?
Subject description: I'm looking for help reading the VCO-1 schematics right
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Hello guys!

I'm getting ready to order parts for building the Thomas Henry VCO-1.
I am no veteran DIY'er and I'm getting confused by the symbols used for the different types of potentiometers in the two part schematics found here:

http://www.birthofasynth.com/Thomas_Henry/Pages/VCO-1.html#Voltage_Controlled_Oscillator:_VCO-1

There are three symbols.

1 - arrow pointing to wave.

2 - arrow pointing to wave... with a "no parking sign" next to it.

3 - arrow crossing wave diagonally... with a "no parking sign" next to it.

I'm guessing these symbols are a US standard but, oddly enough, I cannot find explanations for them via google.
So if anyone could shed some light, I would be grateful! Smile
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Hedefar



Joined: Feb 07, 2013
Posts: 15
Location: Denmark

PostPosted: Sat Feb 16, 2013 7:28 am    Post subject: Reply with quote  Mark this post and the followings unread

Oh, talking about signs, maybe the sign that means "no parking" in Denmark, doesn't mean the same in the US. But I'm talking about the circle with a line through it Wink

Thanks in advance, anyone! Smile

//Hedefar
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elmegil



Joined: Mar 20, 2012
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PostPosted: Sat Feb 16, 2013 8:04 am    Post subject: Reply with quote  Mark this post and the followings unread

Arrow pointing to wave (actually pointing to a resistor symbol Smile ) is a standard pot.

With the "no parking symbol" is a trim pot.

With the crossed arrow instead of the arrow pointing at it, it's wired as a rheostat or adjustable resistance rather than as a voltage divider. Those are more tricky because which way you wire them affects which way to turn the trimmer to increase the resistance.

Traditionally, wiring as a rheostat means you tie the wiper (middle leg, or where the arrow would go) to one of the other legs. Then you'd put the other leg on one side of the symbol and the two tied legs to the other side.

I hope that's clear.... ask more questions if it's not Smile
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Blue Hell
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PostPosted: Sat Feb 16, 2013 8:09 am    Post subject: Reply with quote  Mark this post and the followings unread

The no parking thingie means it is a trimmer, i.e. not a front panel knob. The one with the diagonal arrow is in rheostat configuration ... you do not connect the 'ends' of the pot but the wiper and one end only.

too late I see Wink

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Hedefar



Joined: Feb 07, 2013
Posts: 15
Location: Denmark

PostPosted: Sat Feb 16, 2013 8:57 am    Post subject: Reply with quote  Mark this post and the followings unread

You guys are fast!
It all makes more sense now, thanks! Smile

Meanwhile, I found a few more questions that you might be able to help me out with.

1 - When looking at sheet 2 of the schematics, it looks to me like R25 is the "Sine Symmetry Trim". However, when I read through the website, at some point it says "The second trimmer, R21, let's you adjust the symmetry of the approximation".
Is this a typo? In the schematics, R21 looks like a regular resisto to me.

2 - The tempco, R8, is supposed to be a +3500ppm/Co 2K thermistor. Might I get away with using this one instead?

3 - Q5 and Q6; the matched pair of 2N3906's. I cannot find a matched pair here in Denmark, but single ones are easy to get. How many would you suppose I need to buy, to be sure to find two, that are close enough? 10? 20? 100?

4 - I have a cheap multimeter (DT380B) with a hFE test socket. Is this all I need, to find a pair? In the manual, if says "The multimeter will show the approximate hFE value at the condition of base current 10µA and a Vce of 3V".

I'm almost ready to place the order now. I just need a helping hand from kind strangers once again Wink
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elmegil



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PostPosted: Sat Feb 16, 2013 10:35 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
1 - When looking at sheet 2 of the schematics, it looks to me like R25 is the "Sine Symmetry Trim". However, when I read through the website, at some point it says "The second trimmer, R21, let's you adjust the symmetry of the approximation".
Is this a typo? In the schematics, R21 looks like a regular resisto to me.


Yes, I think that is a typo, since R21 is right there next to R25.


Quote:

2 - The tempco, R8, is supposed to be a +3500ppm/Co 2K thermistor. Might I get away with using this one instead?


The most important characteristic is the ppm, and I can't figure out what that rating is for the one you linked to. I'm also dubious about using the encased ones vs using the resistor style. And being 2.2K, you would need to adjust some other values in the exponential converter to use it successfully.

magsmoke.com sells the "right" thermistors for a reasonable price, and there are other sources that can be used as well, they're just not common.


Quote:

3 - Q5 and Q6; the matched pair of 2N3906's. I cannot find a matched pair here in Denmark, but single ones are easy to get. How many would you suppose I need to buy, to be sure to find two, that are close enough? 10? 20? 100?


The last time I hand matched transistors, I went through about 20 to find what I considered to be good matches. 2N3906/3904 transistors are cheap, so I usually buy lots of 100.

Edit: BTW I got about 6 pairs out of this procedure that were "good enough" for my taste.

Quote:

4 - I have a cheap multimeter (DT380B) with a hFE test socket. Is this all I need, to find a pair? In the manual, if says "The multimeter will show the approximate hFE value at the condition of base current 10µA and a Vce of 3V".


I match with the following circuit:

http://home.comcast.net/~ijfritz/projects/transmat001.pdf

It has the advantage of being a differential test, so you can get MUCH closer matching than you can by measuring raw hFE. I'm also a little vague about which parameter matters more, hFE or vBE, I'm pretty sure Ian's circuit matches them by the latter. I should do the math, maybe they're related....

I wired mine up in a project box, and using a 4PDT switch I was able to use a single 6 pin socket and two switches to do the base swap described (second switch goes from NPN to PNP variation). I basically just put one transistor in each side and then rotate through all the other transistors measuring against the same one to start. Then the ones which have the same voltage difference from the master match each other.
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Blue Hell
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PostPosted: Sat Feb 16, 2013 11:31 am    Post subject: Reply with quote  Mark this post and the followings unread

Yup an NTC is not suitable .. it has a Negative (and non linear) Temperature Coefficient .. i.e. resistance goes down with temperature going up.

(the + on the ppm spec indicates it needs be a positive temperature coefficient, and ppm itself implicates it being linear)

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Hedefar



Joined: Feb 07, 2013
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PostPosted: Sat Feb 16, 2013 1:12 pm    Post subject: Reply with quote  Mark this post and the followings unread

@elmegil
Thanks a lot for taking the time to answer me!

About the Tempco, I've been a bit reluctant to ordering from the US because the shipment might end up in customs, raising the cost a bit and adding 2-3 weeks to the delivery time.
But since they are so cheap and light, I guess they will be sent in an envelope and therefore probably not be interesting for customs.

By the way, it's not that I don't want to pay VAT... I'm just annoyed that buying a tiny component like that has to be complicated by customs. Yay, European Union! Wink
But I'll probably end up buying from Magic Smoke anyway.

Thanks for the link to the transistor matching setup and all the info about how you do it. I'll look into that!

@Blue Hell
Ah, wow, that was an important detail.

You've been a great help guys, thanks! Smile
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