Why are my LEDs still glowing...?

A

Ade

Guest
Now, there's a question to keep you up at night....


Hi group - electronics noob here, although I've been hanging around
t'interweb (and Usenet) for many years, so apologies if this Q is in the
wrong place...


I was poking through some old Everyday Electronics mags the other day,
looking for anything interesting to stop me having to do some Real Work,
when I came across a water level sensor implemented using a uA747 dual
op-amp, a 7-segment display & some resistors. No caps, no fiendishly
complex circuitry. Great, thought I, this'll do...

So, I order the opAmp & resistors; but only the opAmp showed up by
Friday; so I built it using what I had. It works perfectly with green
LEDs, but if I put one (or more) red LEDs in there, they end up glowing
(dimly) when they should be out. My guess is it's a resistor value
problem, but I'd really appreciate some input, as I'm not entirely sure
why it's doing what it's doing...

The details, a bit of a write up, and photographs of the schematic,
circuit in action & the red led problem can all be found here:

http://meddlingmonk.blogspot.com/2010/05/electronics-or-why-didnt-i-use-
green.html

(short version: http://tinyurl.com/288uakq)

Replies here would be preferred, although you can comment on the blog
itself if you like.

--
Cheers, Ade. http://meddlingmonk.blogspot.com

"Your face reminds me of a roadkill's arsehole. Certainly not on my list
of things to kiss." - http://sleeptalkinman.blogspot.com
 
"Ade" <spam@solutionengineers.com> schreef in bericht
news:MPG.26623a7e2a8209469896f3@news.enta.net...
Now, there's a question to keep you up at night....


Hi group - electronics noob here, although I've been hanging around
t'interweb (and Usenet) for many years, so apologies if this Q is in the
wrong place...


I was poking through some old Everyday Electronics mags the other day,
looking for anything interesting to stop me having to do some Real Work,
when I came across a water level sensor implemented using a uA747 dual
op-amp, a 7-segment display & some resistors. No caps, no fiendishly
complex circuitry. Great, thought I, this'll do...

So, I order the opAmp & resistors; but only the opAmp showed up by
Friday; so I built it using what I had. It works perfectly with green
LEDs, but if I put one (or more) red LEDs in there, they end up glowing
(dimly) when they should be out. My guess is it's a resistor value
problem, but I'd really appreciate some input, as I'm not entirely sure
why it's doing what it's doing...

The details, a bit of a write up, and photographs of the schematic,
circuit in action & the red led problem can all be found here:

http://meddlingmonk.blogspot.com/2010/05/electronics-or-why-didnt-i-use-
green.html

(short version: http://tinyurl.com/288uakq)

Replies here would be preferred, although you can comment on the blog
itself if you like.

--
Cheers, Ade. http://meddlingmonk.blogspot.com

"Your face reminds me of a roadkill's arsehole. Certainly not on my list
of things to kiss." - http://sleeptalkinman.blogspot.com
Click to expand...
Well, as for the LEDs, green ones used to need a little higher voltage and
more current to produce visible light than red ones. ( More detailed, LEDs
need current to produce light but you need voltage to make current flow.)

But even red LEDs need current which can only flow with some voltage
(somewhere between 1V and 2V) on the outputs of your circuit. Which in turn
will be caused by some unwanted voltages on the inputs. This voltage can
exist due to the small leakage currents from your inputs into the huge pull
down resistors. So I expect lowering this resistors to 2M2 or even 1M is
more likely to help then going to 4M7.

petrus bitbyter
 
On 23/05/2010 5:01 AM, Ade wrote:
Now, there's a question to keep you up at night....


Hi group - electronics noob here, although I've been hanging around
t'interweb (and Usenet) for many years, so apologies if this Q is in the
wrong place...


I was poking through some old Everyday Electronics mags the other day,
looking for anything interesting to stop me having to do some Real Work,
when I came across a water level sensor implemented using a uA747 dual
op-amp, a 7-segment display& some resistors. No caps, no fiendishly
complex circuitry. Great, thought I, this'll do...

So, I order the opAmp& resistors; but only the opAmp showed up by
Friday; so I built it using what I had. It works perfectly with green
LEDs, but if I put one (or more) red LEDs in there, they end up glowing
(dimly) when they should be out. My guess is it's a resistor value
problem, but I'd really appreciate some input, as I'm not entirely sure
why it's doing what it's doing...

The details, a bit of a write up, and photographs of the schematic,
circuit in action& the red led problem can all be found here:

http://meddlingmonk.blogspot.com/2010/05/electronics-or-why-didnt-i-use-
green.html

(short version: http://tinyurl.com/288uakq)

Replies here would be preferred, although you can comment on the blog
itself if you like.
Click to expand...
The problem is that the outputs of the IC's doesn't go down to zero
volts. There is just enough voltage to let a little current flow through
the red leds. It doesn't happen with the green leds because they require
a bit more voltage before any current flows. A simple solution is to put
a standard diode in series with the red leds.
 
On Sun, 23 May 2010 08:33:18 +1000, David Eather <eather@tpg.com.au>
wrote:

On 23/05/2010 5:01 AM, Ade wrote:
Now, there's a question to keep you up at night....


Hi group - electronics noob here, although I've been hanging around
t'interweb (and Usenet) for many years, so apologies if this Q is in the
wrong place...


I was poking through some old Everyday Electronics mags the other day,
looking for anything interesting to stop me having to do some Real Work,
when I came across a water level sensor implemented using a uA747 dual
op-amp, a 7-segment display& some resistors. No caps, no fiendishly
complex circuitry. Great, thought I, this'll do...

So, I order the opAmp& resistors; but only the opAmp showed up by
Friday; so I built it using what I had. It works perfectly with green
LEDs, but if I put one (or more) red LEDs in there, they end up glowing
(dimly) when they should be out. My guess is it's a resistor value
problem, but I'd really appreciate some input, as I'm not entirely sure
why it's doing what it's doing...

The details, a bit of a write up, and photographs of the schematic,
circuit in action& the red led problem can all be found here:

http://meddlingmonk.blogspot.com/2010/05/electronics-or-why-didnt-i-use-
green.html

(short version: http://tinyurl.com/288uakq)

Replies here would be preferred, although you can comment on the blog
itself if you like.


The problem is that the outputs of the IC's doesn't go down to zero
volts. There is just enough voltage to let a little current flow through
the red leds. It doesn't happen with the green leds because they require
a bit more voltage before any current flows. A simple solution is to put
a standard diode in series with the red leds.
Click to expand...
---
And decrease the value of the current limiting resiaitor in order to
compensate for the added voltage drop.

Even easier, switch out the opamp for something which can source 10mA
or so and includes ground as common-mode.

The search is left to the student. ;)
 
In <MPG.26623a7e2a8209469896f3@news.enta.net>, Ade wrote in part:
I was poking through some old Everyday Electronics mags the other day,
looking for anything interesting to stop me having to do some Real Work,
when I came across a water level sensor implemented using a uA747 dual
op-amp, a 7-segment display & some resistors. No caps, no fiendishly
complex circuitry. Great, thought I, this'll do...

So, I order the opAmp & resistors; but only the opAmp showed up by
Friday; so I built it using what I had. It works perfectly with green
LEDs, but if I put one (or more) red LEDs in there, they end up glowing
(dimly) when they should be out. My guess is it's a resistor value
problem, but I'd really appreciate some input, as I'm not entirely sure
why it's doing what it's doing...

The details, a bit of a write up, and photographs of the schematic,
circuit in action & the red led problem can all be found here:

http://meddlingmonk.blogspot.com/2010/05/electronics-or-why-didnt-i-use-
green.html

(short version: http://tinyurl.com/288uakq)
Click to expand...
I have few ideas:

1) Others said the low output level of the op-amp may be high enough to
make a red LED glow. That sounds unlikely to me - but I would check with
a DC voltmeter. If the problem is voltage, then a adding 1N4148 in series
with the red LED should fix that, as another poster said.

2) If the output low voltage is high but at low current, then the red LED
glowing can be fixed by putting a resistor of maybe 4.7K in parallel with
the LED.

3) I suspect the inputs of this circuit could be picking up hum or radio
interference or ultrasonic interference from a nearby CRT monitor. That
can be fixed by putting .1 microfarad capacitors in parallel with R1, R2
and R4. It is sometimes recommended to add resistors in series with the
op-amp inputs when capacitors are there like that - maybe 2.2K to 4.7K.
The problem these resistors solve is the .1 uF capacitors discharging high
current through the op-amp inputs if the power supply voltage collapses
(shorting or power-off).

4) I don't see in your schematic any capacitor across the power supply
rails. It is usually recommended to put one there, at least .01
microfarad, I would say .1 microfarad. I would use 2 capacitors across
the supply rails. One would be located close to pins 7 amd 14 of IC2, and
the other would go from pin 13 of IC1 to close to the ground ends of R1,
R2 and R4. I give a slight chance that the circuit behaves better with
these capacitors.

--
- Don Klipstein (don@misty.com)
 
jfields@austininstruments.com did gone and wrote:
On Sun, 23 May 2010 08:33:18 +1000, David Eather <eather@tpg.com.au
wrote:

The details, a bit of a write up, and photographs of the schematic,
circuit in action& the red led problem can all be found here:

http://meddlingmonk.blogspot.com/2010/05/electronics-or-why-didnt-i-use-
green.html

(short version: http://tinyurl.com/288uakq)

The problem is that the outputs of the IC's doesn't go down to zero
volts. There is just enough voltage to let a little current flow through
the red leds. It doesn't happen with the green leds because they require
a bit more voltage before any current flows. A simple solution is to put
a standard diode in series with the red leds.

And decrease the value of the current limiting resiaitor in order to
compensate for the added voltage drop.
Click to expand...
In the event, the red & green LED brightness seems to be OK with the
560R resistors.

Even easier, switch out the opamp for something which can source 10mA
or so and includes ground as common-mode.
Click to expand...
Are you hinting at a CMOS device, by any chance? As I explained above,
I'm a bit of a noob when it comes to electronics; I do have a copy of
the Art of Electronics 2nd edn, which is very helpful but rapidly goes
beyond my meagre ability to soak up the knowledge...

Having said that, I've been treating it purely as a "book to read",
rather than sitting down with the soldering iron (or breadboard) &
actually trying out the concepts they are explaining... maybe that
approach will help me more.


The search is left to the student. ;)
Click to expand...
I reckon some kind of FET device; but if you feel like dropping me a
hint or two, I'd be most grateful :)


--
Cheers, Ade. http://meddlingmonk.blogspot.com

"Your face reminds me of a roadkill's arsehole. Certainly not on my list
of things to kiss." - http://sleeptalkinman.blogspot.com
 
Firstly - thanks to everyone who replied, your input is much
appreciated. Rather than replying 4 times, I'll reply once to this post,
but it's to everyone...

don@manx.misty.com did gone and wrote:
In <MPG.26623a7e2a8209469896f3@news.enta.net>, Ade wrote in part:

The details, a bit of a write up, and photographs of the schematic,
circuit in action & the red led problem can all be found here:

http://meddlingmonk.blogspot.com/2010/05/electronics-or-why-didnt-i-use-
green.html

(short version: http://tinyurl.com/288uakq)

I have few ideas:

1) Others said the low output level of the op-amp may be high enough to
make a red LED glow. That sounds unlikely to me - but I would check with
a DC voltmeter. If the problem is voltage, then a adding 1N4148 in series
with the red LED should fix that, as another poster said.
Click to expand...
That, as it happens, fixed the problem completely (no more afterglow).
By using green LEDs on "High" and "Medium", I only needed to add 1
diode, in series with the red LED.

The one downside is it's increased the overall current draw to ~12.5mA
when the red LED is lit (vs. 5.6mA when the mid LED is on, and ~9.5mA
for the "High" LED).

Calculating expected current usage & why it should be thus, is currently
far beyond me (I'm ploughing through the Art of Electronics, but haven't
got to op-amps yet).

2) If the output low voltage is high but at low current, then the red LED
glowing can be fixed by putting a resistor of maybe 4.7K in parallel with
the LED.
Click to expand...
I didn't try this one, 'cos I tried the diode first & it worked.

3) I suspect the inputs of this circuit could be picking up hum or radio
interference or ultrasonic interference from a nearby CRT monitor.
Click to expand...
There's no CRTs in here any more... There is a lot of electronic gubbins
- and, in fact, the "power supply I've been using is a Boarduino device
powered off a USB hub. I should dig out my bench supply & see if that
makes any difference. But, again, as the diode fixed the issue, is this
likely to make a difference?

That
can be fixed by putting .1 microfarad capacitors in parallel with R1, R2
and R4. It is sometimes recommended to add resistors in series with the
op-amp inputs when capacitors are there like that - maybe 2.2K to 4.7K.
The problem these resistors solve is the .1 uF capacitors discharging high
current through the op-amp inputs if the power supply voltage collapses
(shorting or power-off).

4) I don't see in your schematic any capacitor across the power supply
rails. It is usually recommended to put one there, at least .01
microfarad, I would say .1 microfarad. I would use 2 capacitors across
the supply rails. One would be located close to pins 7 amd 14 of IC2, and
the other would go from pin 13 of IC1 to close to the ground ends of R1,
R2 and R4. I give a slight chance that the circuit behaves better with
these capacitors.
Click to expand...
You're right that there's no decoupling caps - and there probably ought
to be. Is this likely to be an issue if the circuit is powered from a
steady-state DC source (i.e. a battery)?



OK, another question to add to the mix:

When in place, this circuit will be powered from a 24v battery (2x truck
batteries), stepped down to 5v via a regulator.

http://www.dimensionengineering.com/switchingregulators.htm contains a
handy (assuming it's right) formula for estimating the waste current
that a linear regulator will have to dump as heat. Assuming my circuit
draws a maximum of 19mA:

(24v-5v) * 13mA = 247mW

(or, I could hook it to just one of the batteries, and reduce the waste
to 91mW, at the risk of unbalancing the battery bank)

Does that sound about right, and will a standard 7805 regulator
dissipate 247mW without an added heatsink?


--
Cheers, Ade. http://meddlingmonk.blogspot.com

"Your face reminds me of a roadkill's arsehole. Certainly not on my list
of things to kiss." - http://sleeptalkinman.blogspot.com
 
On Sun, 23 May 2010 00:26:37 +0200, "petrus bitbyter"
<pieterkraltlaatditweg@enditookhccnet.nl> wrote:

"Ade" <spam@solutionengineers.com> schreef in bericht
news:MPG.26623a7e2a8209469896f3@news.enta.net...
Now, there's a question to keep you up at night....


Hi group - electronics noob here, although I've been hanging around
t'interweb (and Usenet) for many years, so apologies if this Q is in the
wrong place...


I was poking through some old Everyday Electronics mags the other day,
looking for anything interesting to stop me having to do some Real Work,
when I came across a water level sensor implemented using a uA747 dual
op-amp, a 7-segment display & some resistors. No caps, no fiendishly
complex circuitry. Great, thought I, this'll do...

So, I order the opAmp & resistors; but only the opAmp showed up by
Friday; so I built it using what I had. It works perfectly with green
LEDs, but if I put one (or more) red LEDs in there, they end up glowing
(dimly) when they should be out. My guess is it's a resistor value
problem, but I'd really appreciate some input, as I'm not entirely sure
why it's doing what it's doing...

The details, a bit of a write up, and photographs of the schematic,
circuit in action & the red led problem can all be found here:

http://meddlingmonk.blogspot.com/2010/05/electronics-or-why-didnt-i-use-
green.html

(short version: http://tinyurl.com/288uakq)

Replies here would be preferred, although you can comment on the blog
itself if you like.

--
Cheers, Ade. http://meddlingmonk.blogspot.com

"Your face reminds me of a roadkill's arsehole. Certainly not on my list
of things to kiss." - http://sleeptalkinman.blogspot.com

Well, as for the LEDs, green ones used to need a little higher voltage and
more current to produce visible light than red ones. ( More detailed, LEDs
need current to produce light but you need voltage to make current flow.)

But even red LEDs need current which can only flow with some voltage
(somewhere between 1V and 2V) on the outputs of your circuit. Which in turn
will be caused by some unwanted voltages on the inputs. This voltage can
exist due to the small leakage currents from your inputs into the huge pull
down resistors. So I expect lowering this resistors to 2M2 or even 1M is
more likely to help then going to 4M7.

petrus bitbyter
Click to expand...
I tested some green Agilent super-efficient LEDs, just to see when
they quit making light. Under ideal conditions, me dark adapted and
eyeball right up against the LED, I could barely make out the light at
about 700 pA. I have one soldered to a Tadiran lithium battery and a
1M resistor, sort of a night light. It should glow for at least 20
years.

They are less efficient at low current, the knee of the curve being in
the low uA ballpark.

John
 
don@manx.misty.com did gone and wrote:
In article <MPG.26634d95671d95009896f8@news.enta.net>, Ade wrote:

[diode in series]

That, as it happens, fixed the problem completely (no more afterglow).
By using green LEDs on "High" and "Medium", I only needed to add 1
diode, in series with the red LED.

Congratulations! Good!
Click to expand...
Ta :) It was a nice moment, for sure...

The one downside is it's increased the overall current draw to ~12.5mA
when the red LED is lit (vs. 5.6mA when the mid LED is on, and ~9.5mA
for the "High" LED).

Are you saying that current draw while the red LED is on increased as a
result of adding a diode in series with the red LED? (If so, can yiou
retry your measurements?)
Click to expand...
On the breadboard, it definitely affected consumption. Total draw (when
the red LED was lit) rose from ~9.5mA to ~13mA. By touching one side of
the diode with my pliers, I could reduce the current to ~7.9mA, by
touching the other side it would go back to 13mA. I decided this was
ludicrous, and so now I've built the circuit on stripboard. Max current
draw @ 5v is now ~8mA with the red LED lit - little different to when
the high green LED is on. The middle LED still uses less current,
presumably for the reasons you explain below.

Part of the new lower-consumption may be because I've made the voltage
divider with 3M3ohm resistors instead of the 1M ohm specified; only
because I don't have any 1M ohm here, and I didn't want to muck about
with 3x 3M3s on each side like I did on the breadboard. The circuit
still works perfectly - so I'm going to stick with the 3M3's I think.

Or that the red LED is using more current than either green one?

The mid LED may be using less if the 74LS02 has higher output resistance
or higher voltage drop in its output than the op-amps have.
Click to expand...
Quite possible. The '02 chip is around 30 years old, so it probably had
a higher resistance/voltage drop from new.

The green LED may have a little more voltage drop than the red one
plus
a diode.
Click to expand...
The output is very similar now with the two op-amp driven LEDs. Only the
logic-driven LED is lower. The overall brightness of each LED is pretty
similar, however, which makes no sense to me (unless the difference is
too small to make much odds.

You're right that there's no decoupling caps - and there probably
ought
to be. Is this likely to be an issue if the circuit is powered from a
steady-state DC source (i.e. a battery)?

Sometimes these caps prevent oscillations and absorb power supply noise
caused by things switching.
Click to expand...
It's no bother to add some to the stripboard version.

OK, another question to add to the mix:

When in place, this circuit will be powered from a 24v battery (2x truck
batteries), stepped down to 5v via a regulator.

http://www.dimensionengineering.com/switchingregulators.htm contains a
handy (assuming it's right) formula for estimating the waste current
that a linear regulator will have to dump as heat. Assuming my circuit
draws a maximum of 19mA:

(24v-5v) * 13mA = 247mW

That is correct.

Will this have to work if the vehicle is running? (battery voltage more
like 28V - 299 mW to dissipate)
Click to expand...
It's possible that the circuit could be accidentally left on when the
vehicle is running (it wouldn't ordinarily be used in that
circumstance). However, now that consumption is less than 9mA in all
conditions, I reckon that regulator will have an even easier job: 171mW
@ 24V or 207mW @ 28V.

(or, I could hook it to just one of the batteries, and reduce the waste
to 91mW, at the risk of unbalancing the battery bank)

Does that sound about right, and will a standard 7805 regulator
dissipate 247mW without an added heatsink?

This does sound OK to me. I would let things run for a while (at least
10 minutes) and see if a damp tissue sizzles when touched to the 7805's
heatsink tab. In the unlikely eent that occurs, a small heatsink should
easily be sufficient for 299 mW.
Click to expand...
I lightly burned myself the other day on a 7805 after it had
accidentally tried to feed a short circuit for a few seconds... I like
the sound of the wet tissue test...

--
Cheers, Ade. http://meddlingmonk.blogspot.com

"Your face reminds me of a roadkill's arsehole. Certainly not on my list
of things to kiss." - http://sleeptalkinman.blogspot.com
 
In article <MPG.26634d95671d95009896f8@news.enta.net>, Ade wrote:

Firstly - thanks to everyone who replied, your input is much
appreciated. Rather than replying 4 times, I'll reply once to this post,
but it's to everyone...

don@manx.misty.com did gone and wrote:

In <MPG.26623a7e2a8209469896f3@news.enta.net>, Ade wrote in part:

The details, a bit of a write up, and photographs of the schematic,
circuit in action & the red led problem can all be found here:

http://meddlingmonk.blogspot.com/2010/05/electronics-or-why-didnt-i-use-
green.html

(short version: http://tinyurl.com/288uakq)

I have few ideas:

1) Others said the low output level of the op-amp may be high enough to
make a red LED glow. That sounds unlikely to me - but I would check with
a DC voltmeter. If the problem is voltage, then a adding 1N4148 in series
with the red LED should fix that, as another poster said.

That, as it happens, fixed the problem completely (no more afterglow).
By using green LEDs on "High" and "Medium", I only needed to add 1
diode, in series with the red LED.
Click to expand...
Congratulations! Good!

The one downside is it's increased the overall current draw to ~12.5mA
when the red LED is lit (vs. 5.6mA when the mid LED is on, and ~9.5mA
for the "High" LED).
Click to expand...
Are you saying that current draw while the red LED is on increased as a
result of adding a diode in series with the red LED? (If so, can yiou
retry your measurements?)

Or that the red LED is using more current than either green one?

The mid LED may be using less if the 74LS02 has higher output resistance
or higher voltage drop in its output than the op-amps have.

The green LED may have a little more voltage drop than the red one plus
a diode.

Calculating expected current usage & why it should be thus, is currently
far beyond me (I'm ploughing through the Art of Electronics, but haven't
got to op-amps yet).

2) If the output low voltage is high but at low current, then the red LED
glowing can be fixed by putting a resistor of maybe 4.7K in parallel with
the LED.

I didn't try this one, 'cos I tried the diode first & it worked.


3) I suspect the inputs of this circuit could be picking up hum or radio
interference or ultrasonic interference from a nearby CRT monitor.

There's no CRTs in here any more... There is a lot of electronic gubbins
- and, in fact, the "power supply I've been using is a Boarduino device
powered off a USB hub. I should dig out my bench supply & see if that
makes any difference. But, again, as the diode fixed the issue, is this
likely to make a difference?

That
can be fixed by putting .1 microfarad capacitors in parallel with R1, R2
and R4. It is sometimes recommended to add resistors in series with the
op-amp inputs when capacitors are there like that - maybe 2.2K to 4.7K.
The problem these resistors solve is the .1 uF capacitors discharging high
current through the op-amp inputs if the power supply voltage collapses
(shorting or power-off).

4) I don't see in your schematic any capacitor across the power supply
rails. It is usually recommended to put one there, at least .01
microfarad, I would say .1 microfarad. I would use 2 capacitors across
the supply rails. One would be located close to pins 7 amd 14 of IC2, and
the other would go from pin 13 of IC1 to close to the ground ends of R1,
R2 and R4. I give a slight chance that the circuit behaves better with
these capacitors.

You're right that there's no decoupling caps - and there probably ought
to be. Is this likely to be an issue if the circuit is powered from a
steady-state DC source (i.e. a battery)?
Click to expand...
Sometimes these caps prevent oscillations and absorb power supply noise
caused by things switching.

OK, another question to add to the mix:

When in place, this circuit will be powered from a 24v battery (2x truck
batteries), stepped down to 5v via a regulator.

http://www.dimensionengineering.com/switchingregulators.htm contains a
handy (assuming it's right) formula for estimating the waste current
that a linear regulator will have to dump as heat. Assuming my circuit
draws a maximum of 19mA:

(24v-5v) * 13mA = 247mW
Click to expand...
That is correct.

Will this have to work if the vehicle is running? (battery voltage more
like 28V - 299 mW to dissipate)

(or, I could hook it to just one of the batteries, and reduce the waste
to 91mW, at the risk of unbalancing the battery bank)

Does that sound about right, and will a standard 7805 regulator
dissipate 247mW without an added heatsink?
Click to expand...
This does sound OK to me. I would let things run for a while (at least
10 minutes) and see if a damp tissue sizzles when touched to the 7805's
heatsink tab. In the unlikely eent that occurs, a small heatsink should
easily be sufficient for 299 mW.

--
- Don Klipstein (don@misty.com)
 
Ade wrote:
don@manx.misty.com did gone and wrote:

In article <MPG.26634d95671d95009896f8@news.enta.net>, Ade wrote:


[diode in series]

That, as it happens, fixed the problem completely (no more afterglow).
By using green LEDs on "High" and "Medium", I only needed to add 1
diode, in series with the red LED.

Congratulations! Good!



Ta :) It was a nice moment, for sure...


The one downside is it's increased the overall current draw to ~12.5mA
when the red LED is lit (vs. 5.6mA when the mid LED is on, and ~9.5mA
for the "High" LED).

Are you saying that current draw while the red LED is on increased as a
result of adding a diode in series with the red LED? (If so, can yiou
retry your measurements?)


On the breadboard, it definitely affected consumption. Total draw (when
the red LED was lit) rose from ~9.5mA to ~13mA. By touching one side of
the diode with my pliers, I could reduce the current to ~7.9mA, by
touching the other side it would go back to 13mA. I decided this was
ludicrous, and so now I've built the circuit on stripboard. Max current
draw @ 5v is now ~8mA with the red LED lit - little different to when
the high green LED is on. The middle LED still uses less current,
presumably for the reasons you explain below.

Part of the new lower-consumption may be because I've made the voltage
divider with 3M3ohm resistors instead of the 1M ohm specified; only
because I don't have any 1M ohm here, and I didn't want to muck about
with 3x 3M3s on each side like I did on the breadboard. The circuit
still works perfectly - so I'm going to stick with the 3M3's I think.


Or that the red LED is using more current than either green one?

The mid LED may be using less if the 74LS02 has higher output resistance
or higher voltage drop in its output than the op-amps have.


Quite possible. The '02 chip is around 30 years old, so it probably had
a higher resistance/voltage drop from new.


The green LED may have a little more voltage drop than the red one

plus

a diode.


The output is very similar now with the two op-amp driven LEDs. Only the
logic-driven LED is lower. The overall brightness of each LED is pretty
similar, however, which makes no sense to me (unless the difference is
too small to make much odds.


You're right that there's no decoupling caps - and there probably

ought

to be. Is this likely to be an issue if the circuit is powered from a
steady-state DC source (i.e. a battery)?

Sometimes these caps prevent oscillations and absorb power supply noise
caused by things switching.


It's no bother to add some to the stripboard version.


OK, another question to add to the mix:

When in place, this circuit will be powered from a 24v battery (2x truck
batteries), stepped down to 5v via a regulator.

http://www.dimensionengineering.com/switchingregulators.htm contains a
handy (assuming it's right) formula for estimating the waste current
that a linear regulator will have to dump as heat. Assuming my circuit
draws a maximum of 19mA:

(24v-5v) * 13mA = 247mW

That is correct.

Will this have to work if the vehicle is running? (battery voltage more
like 28V - 299 mW to dissipate)


It's possible that the circuit could be accidentally left on when the
vehicle is running (it wouldn't ordinarily be used in that
circumstance). However, now that consumption is less than 9mA in all
conditions, I reckon that regulator will have an even easier job: 171mW
@ 24V or 207mW @ 28V.


(or, I could hook it to just one of the batteries, and reduce the waste
to 91mW, at the risk of unbalancing the battery bank)

Does that sound about right, and will a standard 7805 regulator
dissipate 247mW without an added heatsink?

This does sound OK to me. I would let things run for a while (at least
10 minutes) and see if a damp tissue sizzles when touched to the 7805's
heatsink tab. In the unlikely eent that occurs, a small heatsink should
easily be sufficient for 299 mW.


I lightly burned myself the other day on a 7805 after it had
accidentally tried to feed a short circuit for a few seconds... I like
the sound of the wet tissue test...
Click to expand...
You can move some of the heat out of the 7805, by putting
a resistor in series between the battery and Vin of the chip.
e.g. At 9 mA, a 1K resistor will drop 9 volts, so the 7805
would dissipate (24-9-5)*.009 or ~ 90mW. The resistor would
dissipate ~ 81 mW. That said, the 7805 will handle the 171 mW
you'd get at 24V Vin and 9mA current, without the added resistor
and without a heatsink.

Ed
 
John Larkin <jjlarkin@highnotlandthistechnologypart.com> wrote:
On Sun, 23 May 2010 00:26:37 +0200, "petrus bitbyter"
pieterkraltlaatditweg@enditookhccnet.nl> wrote:


"Ade" <spam@solutionengineers.com> schreef in bericht
news:MPG.26623a7e2a8209469896f3@news.enta.net...
Now, there's a question to keep you up at night....


Hi group - electronics noob here, although I've been hanging around
t'interweb (and Usenet) for many years, so apologies if this Q is in the
wrong place...


I was poking through some old Everyday Electronics mags the other day,
looking for anything interesting to stop me having to do some Real Work,
when I came across a water level sensor implemented using a uA747 dual
op-amp, a 7-segment display & some resistors. No caps, no fiendishly
complex circuitry. Great, thought I, this'll do...

So, I order the opAmp & resistors; but only the opAmp showed up by
Friday; so I built it using what I had. It works perfectly with green
LEDs, but if I put one (or more) red LEDs in there, they end up glowing
(dimly) when they should be out. My guess is it's a resistor value
problem, but I'd really appreciate some input, as I'm not entirely sure
why it's doing what it's doing...

The details, a bit of a write up, and photographs of the schematic,
circuit in action & the red led problem can all be found here:

http://meddlingmonk.blogspot.com/2010/05/electronics-or-why-didnt-i-use-
green.html

(short version: http://tinyurl.com/288uakq)

Replies here would be preferred, although you can comment on the blog
itself if you like.

--
Cheers, Ade. http://meddlingmonk.blogspot.com

"Your face reminds me of a roadkill's arsehole. Certainly not on my list
of things to kiss." - http://sleeptalkinman.blogspot.com

Well, as for the LEDs, green ones used to need a little higher voltage and
more current to produce visible light than red ones. ( More detailed, LEDs
need current to produce light but you need voltage to make current flow.)

But even red LEDs need current which can only flow with some voltage
(somewhere between 1V and 2V) on the outputs of your circuit. Which in turn
will be caused by some unwanted voltages on the inputs. This voltage can
exist due to the small leakage currents from your inputs into the huge pull
down resistors. So I expect lowering this resistors to 2M2 or even 1M is
more likely to help then going to 4M7.

petrus bitbyter


I tested some green Agilent super-efficient LEDs, just to see when
they quit making light. Under ideal conditions, me dark adapted and
eyeball right up against the LED, I could barely make out the light at
about 700 pA. I have one soldered to a Tadiran lithium battery and a
1M resistor, sort of a night light. It should glow for at least 20
years.
Click to expand...
Ha!
 
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