Buidling this circuit Drawdio (again)

Buidling this circuit "Drawdio" (again)

I was trying to build this circuit. http://ladyada.net/make/drawdio/drawdiosch.png

I THOUGHT I had it together, but the only sound it makes is when I break/ make the connection continuously. I did notice that on the 10K resistor, there is little (>1ohm) resistance on it, which I suppose is fine, because its on a transistor in there?

Other than having someone more competent inspect the circuit, what mistakes might one readily make which would cause that?

Oh and thanks for any help.


Science Advisor
Re: Buidling this circuit "Drawdio" (again)

The battery voltage on that circuit is shown incorrectly. It should be shown with the negative end grounded and the positive end going to pins 4 and 8 of the chip. You might need a new chip if the voltage has been wrong.


Pin 3 should not go directly to the bases of the transistors. Try at least 4.7 K there.

The frequency should be about 3.4 KHz which is probably too high. If you put a 0.0047 uF capacitor at C instead of the 680 pF, the frequency would be about 500 Hz.

Here is a calculator for working out the frequency:

Check the 10 K resistor again. It really should measure 10 K, out of circuit.
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Re: Buidling this circuit "Drawdio" (again)

To be safe I should mention this: I am using a 1.5 volt with a 551. Do I still need a resistor?

Being the cautious type, I like to build very simple circuits to test components. I have a soldered 555 tester, and a bread boarded tone generator to test my speaker. Hence, I would like to build a tester for the 551. However, when I looked up the behaviour of R/C circuits, wikipidia threw differential equations at me which I know nothing of. I was going to use that calculator to design one; does the voltage have any effect on the fill rate?


Science Advisor
Re: Buidling this circuit "Drawdio" (again)

I found a data sheet for the 551:

The equations in it look like the ones for the 555. So, the 555 calculator will probably give you ballpark figures for the frequency.

Yes, you need a resistor. At least 1 K.
As a general rule, you never put semiconductor junctions directly across a power supply, even in series and even 1.5 volts.

The first step would be to get it working on some frequency and then modify the values for whatever frequency you want.

If your 555 tester has a socket for the chip, you could put this 551 in it to see if it is still working. They work with a supply from 1 to 15 volts and the pin connections are the same.
Re: Buidling this circuit "Drawdio" (again)

HaHa! It was working the whole time - I was just gripping the wires on the ends so lightly the output was nill!

Oh and thanks for the advice. That really helps to understand the 555, in my mind. Besides, I learned how to test transistors in the process.

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