# LED 101 - light when a plug is inserted

Discussion in 'Electronic Basics' started by anonymous, Oct 7, 2004.

1. ### anonymousGuest

Here's what Im hoping to accomplish:

I have a +9V lead and a ground going to a plug. I want to light the LED when
the plug is inserted. The load on the plug will vary from 0 to ~100 mA.

Is this (relatively) straightforward or am I thinking foolishly?

2. ### anonymousGuest

(d'oh)

my first thought was to run a line parallel to the 9V connected to the (+)
plug lead with a 350Ohm resistor. I don't think this would work as all the
current would go through the open line. What if I put a 90K resistor on the
open line? Seems like this would give me ~100mA to my plug and ~20mA to my
LED...

am I thinking on the right track?

4. ### anonymousGuest

I must be too ignorant to understand your explanation properly. It sounds
like this would always be on regardless of the state of the plug (putting
the LED + resistor across the load) -

On further consideration I think I have a problem in that the current draw
for each possible output device is unknown and variable. I can't rely on
100mA of current being pulled through the circuit. Some of these devices
only need 5-10mA. This would give me LEDs of variable brightness. Based on
my naive understanding it almost sounds like I need a digital circuit that
would "pull high" when any current flows through the line and then all the
way off when current stops.

5. ### Randy DayGuest

What John means is put the LED in parallel with the
circuit being powered; am I correct in assuming you
want an LED mounted, say, in the *outlet* - that
lights when something is plugged into it?

If you have a spare connection on the plug and
socket you plan to use, you can do it simply:

Socket Plug
| |
+V-------| ---|------o------ +V to circuit
| | |
o----------| ---|------o
| | |
| -V-----o-| ---|------------- -V to circuit
| | | |
|-----| |
.-. |
R | | |
| | |
'-' |
| |
| |
LED V |
- |
| |

6. ### anonymousGuest

What John means is put the LED in parallel with the

I can't change the wiring of the plugged in device. It would simply complete
the connection from +9 to ground with a varying load / current draw.

7. ### Randy DayGuest

anonymous wrote:

[snip]
Can you arrange it so a small SPST switch gets
pressed when you plug something in?

Socket or "module" Plug
.------. .-----.
| | | |
+V-o-------- | -------------
| | | | |
o--|--o| | | |
| |==== | |
o--|--o| | | |
| | | | |
-V---------- | -------------
|| '------' '-----'
||
|| R
|| ___
||-|___|-
| |
| V LED
| -
| |
----------
(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)

Can you create a small module to plug in between the
present socket and plug to do this?

If you're looking for something to sense a voltage
drop or current surge when a load is plugged in,
you're looking for something quite complex and likely
to be unreliable.

8. ### anonymousGuest

Can you create a small module to plug in between the
Ack. Ok. Ill put this one down.

TY for the input.

9. ### John FieldsGuest

---
I assumed (silly me...) that the load was in the device into which the
plug would be inserted. However, it now seems like the load is going
to be portable and connected across the plug.

Probably the easiest/cheapest way to do what you want to do would be
to put a small resistor in series with the return to -V then use a
voltage comparator to sense the drop and turn on the LED.