Problem switching a 5V mini relay from a low power LED (1,2v 2 mA) input

[dahl.larsen@mail.dk]

I am trying to make a 5V mini relay switch when the HDMI indicator
LED
in my DVD player lights up.

I have unsuccesfully tried to trigger the mini relay (5V, 40mA) via
an
optocoupler (4N33 - infrared/darlington) "piggybagged" on the
indicator LED (1,2V 2mA), but the relay doesn't switch, only "hums"
slightly. Supplying an input of 1,5V from before the indicator LED
resistor (220ohm) generates the same result. I even tried using the
(normally unused) basis of the optocoupler as input, hoping it was
more "sensitive" - but still only "humming"


The setup works when "piggybacking" a standard 3V LED (>10mA), where
the relay swithes fine. What have I overlooked ?


Any suggestions for (simple) curcuitry I could use to trigger the 5V
relay from the 1,5V 2 mA input (I only have 5V supply power
available) ?

 

[Jan Panteltje]

I am trying to make a 5V mini relay switch when the HDMI indicator
LED
in my DVD player lights up.

I have unsuccesfully tried to trigger the mini relay (5V, 40mA) via
an
optocoupler (4N33 - infrared/darlington) "piggybagged" on the
indicator LED (1,2V 2mA), but the relay doesn't switch, only "hums"
slightly. Supplying an input of 1,5V from before the indicator LED
resistor (220ohm) generates the same result. I even tried using the
(normally unused) basis of the optocoupler as input, hoping it was
more "sensitive" - but still only "humming"


The setup works when "piggybacking" a standard 3V LED (>10mA), where
the relay swithes fine. What have I overlooked ?


Any suggestions for (simple) curcuitry I could use to trigger the 5V
relay from the 1,5V 2 mA input (I only have 5V supply power
available) ?

LEDs in parallel do not share current equally.
1.2V is enough to open a si transistor.
Do you need to be opto-isolated?

 

[Stanislaw Flatto]

I am trying to make a 5V mini relay switch when the HDMI indicator
LED
in my DVD player lights up.

I have unsuccesfully tried to trigger the mini relay (5V, 40mA) via
an
optocoupler (4N33 - infrared/darlington) "piggybagged" on the
indicator LED (1,2V 2mA), but the relay doesn't switch, only "hums"
slightly. Supplying an input of 1,5V from before the indicator LED
resistor (220ohm) generates the same result. I even tried using the
(normally unused) basis of the optocoupler as input, hoping it was
more "sensitive" - but still only "humming"


The setup works when "piggybacking" a standard 3V LED (>10mA), where
the relay swithes fine. What have I overlooked ?


Any suggestions for (simple) curcuitry I could use to trigger the 5V
relay from the 1,5V 2 mA input (I only have 5V supply power
available) ?

Numbers, numbers!
2mA<< 40mA
1.5V<< 5V
Do something about those inequalities, may start working.

Stanislaw.

 

[dahl.larsen@mail.dk]

Hi Jan,

The solution does not need to be opto isolated. I just thought it had
the most sensitive input, and I didn't have to worry about any
"feedback" currents.

I think I need another alternative; anything simple that can make a
5V, 40mA relay switch from 1,2V 2mA.

Stanislaw: I don't understand your point ?

Regards,
Ole

 

[mpm]

Hi Jan,

A comparator that has enough drive on the output?
May not be the simplest solution, however.
Plus, this suggestion comes well into a Memorial Day 6-pack...

I can't remember the part#, but it was from TI.
LMC7215? (Or whatever your favorite is.)

 

[Spehro Pefhany]

I am trying to make a 5V mini relay switch when the HDMI indicator
LED
in my DVD player lights up.

I have unsuccesfully tried to trigger the mini relay (5V, 40mA) via
an
optocoupler (4N33 - infrared/darlington) "piggybagged" on the
indicator LED (1,2V 2mA), but the relay doesn't switch, only "hums"
slightly. Supplying an input of 1,5V from before the indicator LED
resistor (220ohm) generates the same result. I even tried using the
(normally unused) basis of the optocoupler as input, hoping it was
more "sensitive" - but still only "humming"


The setup works when "piggybacking" a standard 3V LED (>10mA), where
the relay swithes fine. What have I overlooked ?


Any suggestions for (simple) curcuitry I could use to trigger the 5V
relay from the 1,5V 2 mA input (I only have 5V supply power
available) ?

Maybe it's not getting DC, but rather some short duty cycle pulses.
You could try using a CMOS 555 as a monostable and drive the relay
with that. Pull pin 2 low with the optotransistor.


Best regards,
Spehro Pefhany

 

[ehsjr]

Hi Jan,

The solution does not need to be opto isolated. I just thought it had
the most sensitive input, and I didn't have to worry about any
"feedback" currents.

I think I need another alternative; anything simple that can make a
5V, 40mA relay switch from 1,2V 2mA.

Stanislaw: I don't understand your point ?

Regards,
Ole

Jan's suggestion would be something like this:

+5 ------------------+------+
| |
[Relay] [D1]
| |a
+------+
| |
| /c
+---[220R]---| NPN
| \e
[220R] |
| |
[HDMI LED] |
| |
Gnd ---+-------------+


Most any NPN you have on hand will work.

Ed

 

[Jan Panteltje]

Hi Jan,

The solution does not need to be opto isolated. I just thought it had
the most sensitive input, and I didn't have to worry about any
"feedback" currents.

I think I need another alternative; anything simple that can make a
5V, 40mA relay switch from 1,2V 2mA.

Stanislaw: I don't understand your point ?

Regards,
Ole

Hi, I understand you _do_ have a separate 5V supply available?

There are several ways, this oen for example using your current setup:


+5V
-------|
| |----------
| k |
R diode [ \ ] relay coil
10 | a |
| |----------
--- c ----------- |
opto coupler --- b c
--- e --------------- b NPN
e
|
///

The resistor is for current limit.

An other cool way would be to siply use a LDR (CdS cell - light dependent
resistor) glued to the LED.
It perhaps will be low enough R to switch the relay when the LED is on.


If you have 5V DC on the set itself, try to fin what drives the LED, likely
a 5 or 3 V output from a micro controller or some chip, and use that voltage
to drive a transistor via resistor, or via a FET.

c ---- as above.

--- 1k ------- b

------------------e
GND

 

[Jan Panteltje]

Hi Jan,

The solution does not need to be opto isolated. I just thought it had
the most sensitive input, and I didn't have to worry about any
"feedback" currents.

I think I need another alternative; anything simple that can make a
5V, 40mA relay switch from 1,2V 2mA.

Stanislaw: I don't understand your point ?

Regards,
Ole

Jan's suggestion would be something like this:

+5 ------------------+------+
| |
[Relay] [D1]
| |a
+------+
| |
| /c
+---[220R]---| NPN
| \e
[220R] |
| |
[HDMI LED] |
| |
Gnd ---+-------------+


Most any NPN you have on hand will work.

Ed

Right
One of them.
Problem why I did not publish that is that often the LED is on the +
and pulled down via a resistor...
Then we should use a PNP

 

[Jan Panteltje]

Maybe it's not getting DC, but rather some short duty cycle pulses.
You could try using a CMOS 555 as a monostable and drive the relay
with that. Pull pin 2 low with the optotransistor.

You are so smart, yes I missed a clue 'buzzing'...

Maybe just capacitor parallel to the relay would work too?

 

[dahl.larsen@mail.dk]

Any easy way to determine if the LED is being "pulsed", before trying
something else ?

Ole

 

[Jan Panteltje]

Any easy way to determine if the LED is being "pulsed", before trying
something else ?

Ole

Sure, make room dark, move head while looking in direction of LED.
If you see many dots it is pulsed.
Capacitor will smooth it.

 

[dahl.larsen@mail.dk]

How many uF ? Don't want it to hang open forever ...

 

[Jamie]

Any easy way to determine if the LED is being "pulsed", before trying
something else ?

Ole

stick a cap across it and see if it gets brighter.
100 uf sounds good.

 

[dahl.larsen@mail.dk]

Indicator LED was indeed pulsed (which also explains the low power
measurements)
100 uF capacitor in parallel with relay solved problem

Thanks all,
Ole

 

[Jasen]

Any easy way to determine if the LED is being "pulsed", before trying
something else ?

hook it to an oscilloscope, or through a capacitor to headphones, or
through a capacitor to an AC voltmeter ...