Creating a light sensing night light

[LawnMaster1]

Ok, so here's a fun one, I'm currently trying to create a battery operated nite-lite that will get brighter as the light around it gets dimmer.

Here's the plan, I have several CDS cells (the circles with a squiggly line snaking on the front that resist depending on how much light is present), and I'm planning to wire them to batteries and leds to make a cool "jewlery/acessory" for a friend.

Problem is that the CDS cells give more resistance when put in darker conditions, and less resistance when moved to brighter enviornments. Which is the exact opposite of what I'm hoping to do.

I would like it to be a gradual transition, not one that is stepped or on/off switched. How would I go about inverting the effects of the CDS cell, as commonly seen in most purchased night lights?

 

[betwixt]

With an inverter!

Doing what you ask can be achieved by simply using a transistor as an inverter. The LDR (Light Dependent Resistor) drops in resistance as more light falls on it so you want to use it to starve a normally conductive transistor of base current as it gets lighter. You put your lamp/LED in the collector where the transistor's current flow is being controlled.

Your biggest problem is not electrical, it's keeping the light you are producing out of the LDR. You will find a tendency for this type of application to find a stable point where the light level hold itself constant.

Brian.