Re: Help Getting Started - Simple DC Circuit
- From: Jasen Betts <jasen@xxxxxxxxxxxxxxx>
- Date: Mon, 14 Nov 2005 09:30:14 -0000
On 2005-11-12, ehsjr <ehsjr@xxxxxxxxxxxxxxxx> wrote:
> 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
you're pushing the envelope there...
with a 3.0V supply and a 1.7V led drop (typical for green LEDs) and a 0.6V
drop in the diode and in the be junction of the transiistor there's only 0.1V
left through 4.7K that's about 20uA at 0.6v the 100K will pass 6uA leaving 14
to flow into the base of the transistor.
unless that pn2222 has an hfE around 500 that last LED could be pretty dim.
and that's assuming 0.6V Vbe is sufficient
sticking a 1K resistor in parallel with each LEDn-Rn pair would be one
way to fix that.
Bye.
Jasen
.
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