How does a pulldown resistor work?

Archiver

I have the 2p40 chip and am using a pin to use a tip120 to set off a
solenoid coil.
The problem is that when the pin is zero, something is making it to
the transistor so that the coil is still getting voltage.

Could someone point me to a web site that explains what a pulldown
and pullup resistor does, how to calculate it, and a schematic on how
to lay out the circuit?

Thanks

Archiver

In a message dated 5/9/2004 6:30:38 PM Eastern Daylight Time,
uncle_lee1@y... writes:


> Could someone point me to a web site that explains what a pulldown
> and pullup resistor does, how to calculate it, and a schematic on how
> to lay out the circuit?
>

A pulldown resistor is simply a resister, from 4.9K to 10K, connected from a
pin to ground. A pullup resistor is the same thing except it is connected to
Vcc instead of ground.

Sid Weaver
New item - New prices.........
http://www.visualmuses.com/chipcircuit/index.html





[noparse][[/noparse]Non-text portions of this message have been removed]

Archiver

--- In basicstamps@yahoogroups.com, "uncle_lee1" <uncle_lee1@y...>
wrote:
> I have the 2p40 chip and am using a pin to use a tip120 to set off
a
> solenoid coil.
> The problem is that when the pin is zero, something is making it to
> the transistor so that the coil is still getting voltage.
>
> Could someone point me to a web site that explains what a pulldown
> and pullup resistor does, how to calculate it, and a schematic on
how
> to lay out the circuit?
>
> Thanks

Hi,

when a pin is not connected, the voltage on it can float to anything
btween board ground and the positive voltage of the power supply.

This can have the effect of the pin being at an odd state.

Considder a switch on the line. if the switch is open, the input is
not forced to any condition and depending on a few things, the pin
might float high or low and actually pass the threshold of your
expected signal. Of course when the switch is closed, there is a
full and complete path.

When you get into other devices such as op-amps, the floating can
make your device do odd and unexpected things.

By connecting a very high value resistor, you can make it ground or
Vcc and not worry about it floating.

Also some devices may need a real 5 volts or a full 0.0 volts. The
control signal is often a few milivolts lesss than Vcc or higher than
ground. By adding a resistor, you can 'pull-up' the voltage to the
full value. or, pull down to ground.

It may sound like a waste of energy when you want to drive a signal
down, but have a resistor connect to the 5Volt line. but do the math
on how much power a 10k resistor will use.

Hope that basic concept makes some sense.


The seattle Robotics Society has a page on this. It is not complete,
nor is my description, but it does have some pictures.
http://www.seattlerobotics.org/encoder/mar97/basics.html


Dave


New item - New prices.........
http://www.visualmuses.com/chipcircuit/index.html