View Full Version : led photoresistor off / on process

Alexander Dumas
09-18-2006, 08:36 PM
Hi. I'm a newbie to electronics..

I'm fiddling with some LED's and have gotten stuck.·

I've got a working circuit built using a 9volt battery + 470 resistor + red LED + a transmitter + photoresistor;

I·would like to modify the design but don't know how to do the following:

1) I can turn·OFF the light by putting a finger over the photoresistor.· However, I would like it to work the opposite way. If I put my finger over it, I would like the light to turn ON.

2) How can I add a timed sequence to this. Let's say the light goes·on or off·but I want it to automatically turn on/off after 5 minutes...

I'd be curious to know how to do this without programming. Seems like the solution should be just mechanical. I've been reading about capacitors and 555 chips - but I don't know how to implement the solution.

Chris Savage
09-18-2006, 10:02 PM
This thread is being moved from the·BASIC Stamp·Forum to the·Sandbox Forum.

Chris Savage
Parallax Tech Support

09-18-2006, 11:13 PM

Have you looked at some of the books by Forrest Mims? He wrote several experimenters books for Radio Shack with lots of 555 circuits, as well as some good applications of plain logic, which is what you're doing .

Or, just google "555 timer circuits".


Tom Sisk


Alexander Dumas
09-19-2006, 12:06 AM
Thanks for answering... I haven't... I'll try and go to the library and find some...

In the meantime, I'm going through a Dummies book on electronics...

If I understand correctly what I am reading about capacitors, it would seem a simple solution to get the blinking effect. By putting a capacitor near the hot wire, will it just "fill up" and eventually release current downstream? If so, when the current is released, the LED downstream should light up. Once the current has based through, the LED should then turn off...

Is this right?

If so, then I just need to know what size capacitor will work... One for a 5 sec delay and one for a 60 sec delay.