Will this work? (electronic flaslight circuit)

jeroen · Post by **jeroen** » Sat Mar 25, 2006 3:46 pm

I just made a little flasing light circuit. Not for real building so without values and stuff. But more for learning electronics. So I was wondering will the basics work?

Image: <a href="http://img92.imageshack.us/my.php?image ... cht5vc.jpg" target="_blank"><img src="http://img92.imageshack.us/img92/7600/k ... 5vc.th.jpg" border="0" alt="Free Image Hosting at www.ImageShack.us" /></a>

Edit: made a little adjustment.

daguuy · Post by **daguuy** » Sat Mar 25, 2006 4:26 pm

aside from the battery being reverse polarity and me not knowing what the boxes are, it might. what are the box things?

blackbox_dev · Post by **blackbox_dev** » Sat Mar 25, 2006 4:28 pm

I believe those boxes are polarized caps...

jeroen · Post by **jeroen** » Sat Mar 25, 2006 4:29 pm

iam7805 wrote:I believe those boxes are polarized caps...

They are.

And did I reverse the polarity? (thanks for nothing that because i'm stil learnin and stuff. Also can someone comferm I switched them because i'm confused cause of another website)

blackbox_dev · Post by **blackbox_dev** » Sat Mar 25, 2006 7:58 pm

Yes, I think the polarity of the battery is switched. But you could protect the capacitor from blowing with a little diode...

A.J. Franzman · Post by **A.J. Franzman** » Sat Mar 25, 2006 8:02 pm

I think you need a current-limiting resistor in series with the LED. It seems to me that the one in series with the transistor base is for timing control, not current limiting (maybe it could do both, but that would depend on the transistor's gain, so not a very reliable way to do it since transistor gain is usually not very precisely spec'd.)

jeroen · Post by **jeroen** » Sun Mar 26, 2006 2:51 am

Ok. But aside from currrent exc. Will it go on and off? (I'm really into the basics here)

And/\/\ The transistor and resistor are indeed for timing not limiting. (It needs too go on and off)

codeman · Post by **codeman** » Sun Mar 26, 2006 12:16 pm

interesting circuit...but i don't think it would even turn on. It would flash for a instant when the battery is first connected because it would be coupled through the capacitor, but after that the transistor would be off. The base of the transistor needs to be driven with current to turn it on, and DC current won't pass through the capacitor you have there. Were you trying to get a delay before the LED would turn on or something?

daguuy · Post by **daguuy** » Sun Mar 26, 2006 12:43 pm

could somebody explain how you get current to pass through a capacitor? i never really understood that

jeroen · Post by **jeroen** » Sun Mar 26, 2006 12:59 pm

codeman wrote:interesting circuit...but i don't think it would even turn on. It would flash for a instant when the battery is first connected because it would be coupled through the capacitor, but after that the transistor would be off. The base of the transistor needs to be driven with current to turn it on, and DC current won't pass through the capacitor you have there. Were you trying to get a delay before the LED would turn on or something?

Well it was supposed to turn on and off.

codeman · Post by **codeman** » Sun Mar 26, 2006 2:22 pm

the relationship between voltage and current for a capacitor is I = C*dV/dt. What it means is that current will only flow through a capacitor if the voltage across it is varying with respect to time, like the AC voltage in the wall. The only time that the voltage in that circuit is varying is when the voltage is first applied. So when it is first turned on it would flash for an instant and you might not even be able to see it.

Post by **gannon** » Sun Mar 26, 2006 2:46 pm

It will blink at the rate it takes for the capacitor to charge. For a fast blink with a longer wait between blinks solder on a pulldown resistor to the transistor base.

codeman · Post by **codeman** » Sun Mar 26, 2006 2:58 pm

gannon wrote:It will blink at the rate it takes for the capacitor to charge. For a fast blink with a longer wait between blinks solder on a pulldown resistor to the transistor base.

um...no it won't. Assuming that the black square with the + sign is a capacitor, that will prevent the transistor from turning on, meaning that there is no complete circuit for current to flow through the LED. Even if the transistor would turn on it would never oscillate, so it would never blink repeatedly. If you wanted it to oscillate you'd need a schmitt trigger or something like it.

Post by **gannon** » Sun Mar 26, 2006 3:08 pm

From what I understand, the capacitor will hold back the voltage untill it is full, then discharge into the base of the transistor. It's been awhile since I've worked with that kind of circuit though.

timmeh87 · Post by **timmeh87** » Sun Mar 26, 2006 3:13 pm

are you sure it wouldnt work? listen to this logic:

assuming the black thing as a capacitor, and the white box isnt anything.

you turn on the circuit and the capacitor charges. but, when it gets charged a little, it opens the transistor and lets itself drain. but that closes the transistor, and it starts to charge again.

---

current never actually flows through a capacitor. when it is charging or discharing, current flows into it (or out of it), which gives the illusion of current flowing through it for a breif instant.