Simple advice on schematic required

Afternoon all,

I'm trying to build the following circuit on stripboard :



Surely the polarity of the 220uF cap in the bottom left hand corner is an error isn't it?  The cap should never have the negative side connected directly to +9V and positive side connected to ground right?

Also, I forgot to get 220uF caps altogether when I placed my last order.  What is the purpose of the cap, and could I substitute 150uF or 470uF there instead?

Cheers

Rob

Yeah that seems like a typo. It should the other way around indeed. It's just an additional smoothing cap. You see these in stompboxes helping smooth 9DC wallwarts. With a battery it's completely unnecessary of course. The size is very uncritical as well. I've seen everything between 10-1000uF.

Thanks Kingston.  In fact, I plan to use this with a switch mode PSU which of course means the supply voltage should be well regulated so I'll dispense with the cap altogether I reckon.

With a battery a bypass cap is necessary for a good design.  Sometimes a 9 VDC battery have a not so good internal connections between cells.  You can run into problems if you don't use a battery bypass cap.

With 555's you need to be careful some can cause a supply "bounce" when switching.  This can/will introduce a tick to other parts of the circuit that share the supply.

It is the reset.

If you are going to use an external power supply or switch mode psu I suggest leave that cap where it is. In fact probably shunt it with 0.1uF ceramic too.

OK thanks all.  The cap stays!

Is value particularly critical here?  I am trying with 470uF and the circuit is producing notes, though the low notes are not as bassy as I would expect (this is a dub siren circuit).  Could the value of this cap affect the sound at all?

The value is uncritical.  It should not affect in such way. Basically that point is at +9V and the purpose of the cap is to keep it at +9V.  Therefore in reality the higher (to a degree) is better.

Thank you

Gus said:

With a battery a bypass cap is necessary for a good design.  Sometimes a 9 VDC battery have a not so good internal connections between cells.  You can run into problems if you don't use a battery bypass cap.

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Interesting, didn't know this. I've always been under the impression battery power is the closest thing to pure DC there is.

Batteries will have real internal resistance and inductance, so the source impedance can be supplanted by reservoir capacitors. I would expect most of the benefit from HF effects, but often larger value caps are used, to facilitate operation with battery eliminators (rectified AC).

JR

Shouldn't the 220uF cap be after the resistor, to provide a power-up reset delay of about 2.2 secs?
If the reset is not used at all, then the data sheet shows pin 4 tied direct to V+, hence the 10k resistor is superfluous.

Walrus said:

Shouldn't the 220uF cap be after the resistor, to provide a power-up reset delay of about 2.2 secs?
If the reset is not used at all, then the data sheet shows pin 4 tied direct to V+, hence the 10k resistor is superfluous.

Click to expand...

It is good practice to connect logic inputs that are permanently pulled up via a protection resistor of non critical value.