Re: Help Getting Started - Simple DC Circuit

Mark Jerde wrote:
(I apologize for posting this basic circuit question. 25 years ago in college I had to know E=IR etc. Just having problems getting started...)

I'm trying to "invent" a mechanical device. I've been having some alignment problems with it and this afternoon it occured to me some simple electronics would help a lot. But it has been many years since I've done anything more complicated with electronics than change batteries in the household smoke detectors. ;-) I'm looking at the Jameco web site, seeing if I can find enough info to play with E=IR but there are just too many options & choices...

I want to go to my neighborhood Radio Shack and buy
   - 30 ea SPST NO switches
   - 30 ea green LEDs
   - 30 (?) ea resistors to limit current to the LEDs
   - A circuit board to solder the LEDs & resistors to
   - A battery case (e.g. 4 "D" cells) or 9v clip
I have an electronics soldering iron and plenty of wire.

The goal: When everything is lined up right on my mechanical device, all the switches will be closed and all 30 LEDs will be glowing. Then I'll unplug the battery, as the machine is ok once all the switches are closed. (E.g., no long-term lighting requirement.)

How can I get started with this? For the battery which value to use, 1.5, 3, 6 or 9 volts? Does each LED need its own resistor or is one resistor enough? (I don't care how bright the LEDs are so long as they are visible.)

Thanks.

   -- Mark



Radio Shack is far too expensive for toggle switches, as you
discovered.  You can get them for 40 cents each in lost of 10
from Allelectronics - http://www.allelectronics.com/
catalog # MTS-75PC

You can add the 31st LED and a simple circuit to tell you
when the other 30 LEDs are lit. That way, you need glance at
only 1 LED to see if all the others are on instead of needing
to look at all 30 of them. You'll need 30 diodes - catalog #
1N914TR (100 for $2.00) an NPN transistor (any NPN would work -
catalog # PN2222A is a suggestion) and a couple of resistors.
The single LED can serve as a "run" light. When it is lit, the
battery must not be disconnected. When it goes out, the battery
can be disconnected.  (You could use it as the basis of an
automatic battery disconnecting circuit if you want.)
Here's the circuit:

+3 ---+------------------} }----+--------------------+
      |                         |                    |
    [LED1]                    [LEDn]              [LED31]
      |                         |                    |
     [R1]                      [Rn]                [R31]
      |     ->|-                |     ->|-           |
      +---[Diode1]---+          +---[DiodeN]---+     |
      |              |          |              |     |
   [Switch1]         |       [SwitchN]         |     |
      |              |          |              |     |
     Gnd             |         Gnd             |     |
                     |                         |     |
                     +---} }-------------------+     |
                                               |     |
                                             [4.7K]  |
                                               |     /c
                                               +----|   NPN
                                               |     \e
                                            [100K]   |
                                               |     |
                                               +-----+
                                               |
                                              Gnd

Use 100 ohm resistors for R1 through R31 and a 3 volt supply.
(If you don't want to use batteries, you could use catalog #
DCTX-330, which is a 3 volt 300 mA wall wart power supply.)
That will limit current through each LED to about 10 mA,
which is plenty bright enough for red LEDs. Catalog # LED-1
gets you 10 standard red LEDs for $1.00. The catalog numbers
for the resistors are 100-1/4 (100 ohm, 1/4 watt), 4.7K-1/4
and 100K-1/4  They cost 50 cents for 10 and must be ordered
in lots of 10.

Ed
.

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