# Is this a correct 7555 circuit?

Discussion in 'The Projects Forum' started by Fenris, Jun 21, 2009.

1. ### Fenris Thread Starter Active Member

Oct 21, 2007
288
2
Hi All

I am drawing up a vero board LED matrix oscilloscope from a schematic found on the web. But the 7555 based clock circuit doesn't look right. I have added where I think the +VE supply should go if it is a working iteration of such a clock. But it just doesn't look right to me but I may be wrong........in all likely hood Could some one just say yay or nay please.

regards

Fenris

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2. ### SgtWookie Expert

Jul 17, 2007
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Yay.

I'd increase R10 to around 2k though.

Mar 24, 2008
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4. ### Fenris Thread Starter Active Member

Oct 21, 2007
288
2
Thank you both I have never seen this variation and couldn't find anything that looked exactly the same. I managed to work out the +12V line though Although I guess it's kind of obvious you don't want a variable resistor between the supply and pins 4/8........ isn't it?

thanks again

regards

Fenris

5. ### SgtWookie Expert

Jul 17, 2007
22,183
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Fenris,

R10 actually didn't need to be increased; discharge current will be a maximum of 8mA. However, R11 needed to be added; otherwise if you set the 1M pot to 0 Ohms, the discharge pin of the 555 would be trying to short out the power supply, which would result in smoke.

7555 timers wired as astable multivibrators (like your schematic is) charge the timing capacitor (C3, C4 or C5) to 2/3 Vcc, and then discharge to 1/3 Vcc. Since 2/3 Vcc is in your case 12 x 2/3 = 8v, the maximum current through the discharge pin will be 8v/1000 Ohms = 8mA.

Now if R9 (the 1M pot) were set to 0 Ohms, there would still be R11 in the current path. Since R11 is 4K3 Ohms, maximum current would be 12v/4300 Ohms = 2.79mA, for a maximum total discharge pin current of 10.78mA. This is well within the capabilities of the 7555 timer.

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6. ### Fenris Thread Starter Active Member

Oct 21, 2007
288
2
Thank you Sgt much appreciated! This circuit is the clock which drives the 4017 pair which in turn drive a trio of 4049's which in turn drive the column lines of the matrix. Similarly the row lines are driven by a pair of 3914's which then drive a trio of 4049's. From the pictures on the guys site this circuit seems to have a very good presentation of audio signals

Heres the site addy. I did find that the guy had messed the pin ID of pin 4 and pin 8 of the LF353. Pin 8 was marked as pin 11 and the +ve and -ve was the wrong way round as well.

http://www.geocities.com/rlaude2000/ledscope.htm

The LF needs a dual rail supply the rest of the circuit is single rail.

regards

Fenris