Reversing Resistance?

Thread Starter


Joined Jun 23, 2004
Ok, I'm not really trying to reverse resistance but here's the problem I've got with an assignment question. The question itself is measuring light, so when light falls below a certain level an LED flashes.

I've got a CdS cell running into a NPN transistor (acting as a switch depending on the current coming from the CdS cell) which then triggers a 555 timing chip to control the LED flashing. Now all that works fine except for the fact it's doing the exact reverse of what it's supposed to do. ie. When there is plenty of light (ie. low resistance in the CdS cell) the LED flashes, but when there is low light it doesn't flash. Is anyone able to give me a pointer on how I could reverse this?

I'll keep reading up on it but any pointers would be appreciated.


Joined Apr 20, 2004

Your CDS cell gets more resistive as the light level falls, so your 555 will shut down. Be a good idea to put a 330 - 560 ohm resistor in series with the LED, too.


Joined Oct 11, 2004
You may be able to get rid of the transistors. Try this.

Supply the 555 V+ with 9V as normal. Connect your cell between -ve supply and Reset pin. Connect a resistor between Reset and V+.
You will need to adjust the resistor value to suit.