On 21 Oct 2011 at 18:57, Yigit Turgut wrote:

> LDR is light dependent resistor. When I set RB4 high to feed LDR,
> current it drains will change constantly and this may cause
> instability in overall operation due to overdrive of LDR. It has dark
> resistance of 4 mega ohms and bright resistance of 5k ohms.

Like we have been asking, what "instability" fears do you have?

If you has a single LDR between +5V (or PIC output providing +5V) and groun=
d=20
then max current drain is: 5V/5kOhms =3D 1mA, or power is 5V^2/5kOhms =3D 5=
mW

Is that too much current for your power supply? It is not too much current =
for most=20
PIC I/O port lines, except perhaps RB4... as previsouly mentioned have you=
=20
checked to make sure RB4 is not an "open drain" output for your particular =
PIC?=20
You can not source current from an open drain output (cannot provide +VE ou=
tput=20
voltage or current), you can only sink current (when turned on current, fro=
m a=20
source external to the I/O pin, flows into the PIC I/O pin to GND).

But anyway, you can't measure current directly with a PIC I/O line. Voltage=
 othe=20
other hand you can measure with a PIC ADC (or some other tricks that are sl=
ightly=20
more complicated). So then you need fixed value resistor in series with you=
r LDR,=20
thus giving a voltage divider: the voltage at the node of the LDR and fixed=
 resistor=20
will vary with varying resistance of the LDR.

--=20
Brent Brown, Electronic Design Solutions
16 English Street, St Andrews,
Hamilton 3200, New Zealand
Ph: +64 7 849 0069
Fax: +64 7 849 0071
Cell: +64 27 433 4069
eMail:  brent.brown@clear.net.nz



--=20
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