Help Needed: Working Out Resistor Size for Potential Divider

[Steph191290]

i am currently doing my as level coursework and for part of it i need to work out what size resistor to use in a potential divider but i have no idea how anyone that could help i would be really grateful,,

thanx
steph xx

 

[Kurdt]

It depends what one wants to do with the potential divider. That is, what is the input voltage and the output voltage that you want?

 

[Steph191290]

umm.. well we have to put 9 volts in but I am not sure what voltage out,, we are making a sensor,, we are designing the ciruit so when the light level fall too low a bulb will switch on

 

[Kurdt]

Well one part of the potential divider will be an LDR. You'll need to know some values from the LDR and how the NOT gate operates before you can work out what value of resistor you wish to place in the potential divider. The equation for the voltage output of a potential divider is given in your AS textbooks as:

$$V_{out} =\frac{R_2}{R_1+R_2} V_{in}$$

 

[Steph191290]

thank you i have the values from the ldr,, that is the equation i needed i just couldn't remember it thanks for your help.. xxx

 

[bsimmo]

P.S. I would place a Variable Resistor (VR) in place of a fixed resistance, so it can be altered to suit the requirments. You'll still need to work out your VR range though, but you'll gain the advantage of making adjustments to the amount of light needed.