View Full Version : How do we find the correct resistance for a component

12-31-2011, 12:56 PM
This is a project of my own I want to do.
It's a laser security system.
I have programmed the circuit as the same as a pushbutton.
Here is a schematic representing the circuit.
My question is how can I figure out what resistors to put between the photoresistor
How can I determine the resistance if I'm testing this circuit in the dark with a laser ( unknown specifications)
Will it work?

Thank You.
Sorry, I speak better french than english and I'm new in electronics.

12-31-2011, 01:13 PM
Why have you got two resistors on either side of the photoresistor?

Photoresistor data?

12-31-2011, 02:13 PM
Hi , take a look at this link http://www.acroname.com/howto/photoresistor/photoresistor.html

This is just one example out of hundreds you can find with a Google search.

In this example the wire marked V is where you would connect to your Stamp pin P14. The only modification I would make would be to add a 220 Ohm resistor from P14 to the connection V, this would be a precautionary measure to prevent excess current damaging your Stamp chip

Jeff T.

Mike G
12-31-2011, 02:23 PM
I'm not sure exactly what you are trying to do but... Use an RC circuit with a known capacitance to calculate the resistance. The idea is to measure the time it takes to charge or discharge the RC circuit.


Measure Resistance and Capacitance -·Updated (2/2/10) - found on the following link.


Plus we need to know what model (part number) photoresistor you are using.

01-02-2012, 02:58 PM
Use Ohms Law To calculate

01-02-2012, 03:26 PM
I'll probably be a good idea to use a current limiting resistor on that speaker. Unless it's a very tiny audio emitter you'll likely have to drive it with a transistor or op-amp or......

The photoresistor circuit you have drawn might not work depending on what you have in mind:
P14 as an input would do nothing as the photoresistor doesn't generate a voltage
I suppose you could set P14 as an output, then current limit it to 5ma or so with the photoresistor at the lowest resistance (bright light?) Use ohm's law for this.... then you would monitor the voltage drop with another pin, or you might be able to read the state of P14.

01-02-2012, 05:02 PM
P14 should be a input, the first resistor should not be inline like that, but instead pulled up to 3.3v (or 5v if that is Vss) and a 10k value should be OK.
The second one you don't need at all.