Mark Jerde
Guest
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 Posted:
Tue Nov 15, 2005 1:35 am Post subject:
Re: Help Getting Started - Simple DC Circuit |
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Peter --
| Quote: | Just a thought - it is much easier to see one lit LED among many unlit
ones than it is to see one unlit LED among a bunch of lit ones, so I'd
wire the thing so that all lights are out when things are all properly
aligned. Also, if you do it this way, there is no need to remove the
battery while running the machine, and you will get a warning if
anything does slip out of alignment when it shouldn't. You would,
however, want to remove or switch off the battery while the thing is
disassembled.
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Excellent point. Thanks.
-- Mark
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ehsjr
Guest
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| Posted:
Wed Nov 16, 2005 9:35 am Post subject:
Re: Help Getting Started - Simple DC Circuit |
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Jasen Betts wrote:
| Quote: | On 2005-11-12, ehsjr <ehsjr@bellatlantic.net> 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...
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Yes. I had a "window" of only roughly 3.25 to 2.25 volts. Your
excellent suggestion (more on that later) fixes that nicely.
The trick was to get the single LED to light without pulling enough
current in the base path to make a "main" LED (one of the first 30)
glow even a tiny bit.
| Quote: |
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.
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Not quite. The diode drops about .45 volts at that very low current.
| Quote: |
unless that pn2222 has an hfE around 500 that last LED could be pretty dim.
and that's assuming 0.6V Vbe is sufficient
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I guess "pretty dim" is relative. On the breadboard, the LED
glows merrily, and is easy to see. I used 2 10K in parallel
in place of the 4.7 K, and 150 ohms in series with the "main"
LED.
500 is way too high for hfe. As I recall, a 2N2222 is good for
about 300, max. On the breadboard, without the 1K in parallel
with the main LED, base current measures ~23 ua and LED current
measures ~3.69 mA so hfe ~ 160. With the 1K in parallel with
the main LED, base current is ~280 ua and LED current is
~ 7.52 mA, so gain is ~27
| Quote: |
sticking a 1K resistor in parallel with each LEDn-Rn pair would be one
way to fix that.
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It fixes the "window" problem very nicely. The window problem
was that the single LED would glow with the battery voltage no
lower than 2.75. Below that it was too dim. And above roughly
3.25, the "main" LED would start to glow dimly. With the 1K
resistor across the main LED, the circuit works down close
to 2 volts, and will work without causing the main LED to glow
above 6 volts. 3 volts is still a viable option, but he could
use 6 volts if he changes the value of R1 through R31 to 220.
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ehsjr
Guest
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| Posted:
Thu Nov 17, 2005 1:35 am Post subject:
Re: Help Getting Started - Simple DC Circuit |
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ehsjr wrote:
Here's an updated schematic to incorporate Jason's idea
of adding a 1K resistor in parallel with each of the first
30 LEDs. It will allow the circuit to work as the batteries
drain below 2.75 volts. It will also allow you to use a higher
supply voltage if you want, like 3.6, 4.5 or 6 volts, but you
would need to change the value of R1 through R31 using the
values below:
3.6 volts use 150 ohms; 4.5 (or 4.8) volts, use 270 ohms;
6 volts, use 330 ohms.
Ed
+3 ---+-------+----------} }----+-------+------------+
| | | | |
[LED1] [1K] [LEDn] [1K] [LED31]
| | | | |
+-------+ +-------+ |
| | |
[R1] [Rn] [R31]
| ->|- | ->|- |
+---[Diode1]---+ +---[DiodeN]---+ |
| | | | |
[Switch1] | [SwitchN] | |
| | | | |
Gnd | Gnd | |
| | |
+---} }-------------------+ |
| |
[4.7K] |
| /c
+----| NPN
| \e
[100K] |
| |
+-----+
|
Gnd
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