push-button circuit - a question

K

KTarke

Senior Member
I've already made/tested a bunch of Picaxe circuits, that use pushbuttons for various reasons.

Connected the "usual way": button between Vdd and inputpin, a 10K pulldown resistor between inputpin and ground.

In the code, handled the "bounce" with pause 25.


BUT not until today have I noticed the example circuit in Picaxe manual(s)!

There is also a 1K resistor between button an d Picaxe input.
(never used a resistor in that kind of place)

Since I do not now much of electronics designing (more of using and repairing), I would appreciate, if one of the Wiser ones would explain me:

-what is the purpose of the mentioned resistor?
-what are the benefits of using it?
-Do You really think, it is necessary?
 
C

Chavaquiah

Senior Member
Suppose an error in the program or a glitch turns the pin into a low output. The Picaxe would try to sink as much current as you could give it and soon you'd start smelling something burning (I did melt a breadboard, btw).

Is it really necessary? Sit down and wait for a Wiser one to explain how to cross a road... ;)
 
D

Dippy

Moderator
I think Chava has covered it, though not all examples show the extra resistor. e.g. Manual 3 page 26.

It's always best to 'play safe' when giving examples. Sometimes an extra component or two, sometimes a 'safer' component value.
It's called "erring on the side of caution".
Often an extra component is better than a smoking chip :)

These things can be modified as your electronics experience improves and as your knowledge of the interaction between software and hardware improves.

If you ever have a query just post your question.

EDit: What is the "vc++ pin" ?
 
D

Dippy

Moderator
To Curry.

If you are talking about the +V positive supply pin you don't have a resistor.
In some noisy circuits you could have a small value RC but usually not.

In general with chips you don't have a resistor on the power supply pin.
If you need to limit current on the I/O then you put resistors on the I/O.

If the res value was too high it would prevent operation entirely.
A smaller value res may allow operation but the voltage drop across it would vary with processing and O/p load.
This would give a varying supply voltage to PICAXE in effect - not desirable - therefore unwanted.
It would also upset the output driving capability which is not very clever.
A very small value does nothing and is entirely pointless.

So, it would range from inactive , through unpredictable behaviour , down to pointless.
Summary: Generally, leave it out.

Have a think why and where we we need resistors and how to determine the 'ideal' value for the task. When drawing a circuit consider the current flow and direction and calcuate what a resistor in a certain place will do.
 
K

KTarke

Senior Member
THANK YOU all for Your response!

Chava answered the thing, I was wondering, what could be the real need for that resistor.
Never thought about the possibility of bringing (accidentaly) the pin low, and the consequence...
Now, I still can leave the resistor out when I know the reason...:)

I did not understand Dippy's Edit: what is the "vc++ pin"?
Did not see anyone to use that expression in this thread.
I use (maybe erraticly?) Vdd as positive supply, Vss as negative supply (OR 0V if there is no negative voltage, although it is then more clear to talk about GND)
 
D

Dippy

Moderator
"I did not understand Dippy's Edit: what is the "vc++ pin"?"

- it was what Curry asked before editing the post and changing it from "vc++" to "voltage in".
This sort of thing happens now and then and often breaks the continuity of the thread.:rolleyes:
 
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