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Glasshopper explains L.E.D.s and 555 timers

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#21 MNL



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Posted 18 October 2006 - 08:44 AM

I understand using the 555 to PWM (pulse width modulate) the LEDs to control brightness... but I'm not sure I agree that it saves so much power (makes batteries last longer). It uses 6-15mA all the time...depending on your voltage, this is almost the same as one always-on LED.

For easy brightness control (PWM), or a simple 24 hour timer with multiple on/offs a PIC might save more power. A 16F pic at 32.768khz uses uAs, rather than mAs.

BUT, and thats a might big butt, why use lights in a stealth cab? I've never given shrooms any more light than they get when I pick and water every few days. Its never been a problem.

Regardless, automation is a ton of fun, heh? Everyone likes a tricked out mod. I came back to the hobby to try out some new ventilation systems with Co2 and 02 sensor feedback loops. Fellow geeks might check out the Co2 sensor here : http://www.futurlec....s_Sensors.shtml .

#22 Hippie3



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Posted 26 November 2006 - 01:19 PM

you out there ?
i can't fix the links, can you repost ?

#23 Guest_Glasshopper_*

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Posted 26 November 2006 - 02:17 PM

Ok, so far we learned how to hook up miltiple LEDs. Then we learned how to run a 555 in astable mode, now it's time to put the both lessons together.

As I stated in the 555 post it's output current is too low to light more than one LED. So how are we going to light our array with one?
Simple really we will use a transistor which is simply a device used to have a smaller current control a larger current. Actualy we are going to use a special kind of transistor...

A MOSFET, this stands for Metal Oxide Silicone Field Effect Transistor, and we are going to use it because it's gate operates on voltage not on current like the base operation of a normal transistor (yes this is english)
Without getting into the intricacies and construction of a MOSFET, this simply this means that it consumes far less power to operate than a normal transistor to run.
This equates to a longer battery life as well as total control over the LEDs.

A MOSFET looks just like a normal transistor and comes in all the standard TO-92, TO-220, TO-3 cases etc.(Transistor Outline)...

This is the schematic symbol for a MOSFET...

This is how you would use a MOSFET by itself to control the brightness of a LED...

By turning the 1M potentiometer (pot) we bias the gate voltage more towards positive voltage or ground.
This varies the power allowed through the transistor, adjusting the voltage on the LED. And yes you could just use a similar resistor setup directly without the MOSFET but because more current would flow through the resistors a lot more power would be turned to heat (wasted).

And this is how we will be hooking our circut up..

The pot here changes the frequency AND the duty cycle of the 555 timer witch in turn controls the MOSFET

Because of this the gate current of the mosfet if so low (a few millliamps) you can of course hook up dozens of them in parallel...

if you want more strings than a few dozen you will need to buffer the 555's output using a transistor or MOSFET.

Now go get yourself a breadboard and start playing with those 555s.
When you get the circuit like you want buy yourself a prefboard that is wired just like your breadboard (Radio shack has them) and start soldering!!

in my next post we will look at the 555 in monostable mode and describe how you can use it to turn on a fan or other device for a pre determined time at regular intervals.
Stay tuned kiddies!

Attached Thumbnails

  • variable_brightness_MOSFET_.gif
  • multiple_LEDs_and_MOSFETs.gif
  • mosfet_schematic.gif
  • 555_MOSFET.gif
  • mosfet.gif

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