Voltage regulator for IR Receiver plugged on serial port

Hi,

I'm trying to build an IR Receiver for PC (plugged on serial port). To
build this, I need a voltage regulator LP2950CZ. The voltage provided by
the serial port is between 5V and 15V and the IR receiver need 5V. I
need then a regulator with a little difference between input voltage and
output voltage. Badly, I can't find the LP2950CZ, I have then to replace
it with another regulator. I'm a newbie in electronic so I don't know
with wich regulator I can replace it. Someone told me that I can use
LM2940-05 or LM4805 regulator, can someone confirm it ? Can I just
replace it or should I change some capacities/resistances ?

Here is the schema I want to build :

http://home.t-online.de/home/tb_electronic/vdr/lirc/lirc_rx.html

LP2950 datasheet :
http://www.national.com/ds/LP/LP2950.pdf

LM2940 datasheet :
http://www.robozes.com/robot_pi/componentes/LM2940.pdf

LM4805 datasheet :
http://www.usmicrowaves.com/ldoreg/L4805.htm

Thanks a lot,

Laurent

 

Digikey has them in stock in several package sizes:

http://www.digikey.com/

They sell in single quantities as long as you make a total
purchase, including other items, of $25.00.

 

Schema says a 78L05 may be substituted for the LP2950. The former is a
5v 100ma regulator in a TO99 case, if memory serves. Cheap and readily
available.

 

Schema says a 78L05 may be substituted for the LP2950. The former is a

But my laptop serial port provice only 5,6V, so I can't use 78L05 (I
think it need 7V as input to provide 5V). Also I want to make a receiver
that works on any PC ...

 

The only thing that needs 5V is the IC1 receiver. Therefore all you
need is a resistor between C1 and C2 (delete the regulator), and a
5.1V zener diode across C2, cathode to positive. The resistor should
be low enough to supply the current, plus a mA or so more. I would
try a 3.3k to begin with. Measure the currewnt going to IC1 with a
DMM and go from that value.

[snip]

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I think you're confused. The ground of the receiver circuit connects
to the TXD, which should be a negative voltage on your laptop. The
total, both positive and negative, should give you 10V or more across
the circuit.


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The TSOP17 datasheet shows 5 ma - which brings up a question
for you. Could he use LEDs in place of the zener? If I get
a chance, I'll experiment tomorrow - but you may already have
done extensive experiments with this. I'm assuming Vf of
1.6 for garden variety red leds - am I on target?

 

I'd look at the specs for the IC1 receiver. It it's okay with more or
less than 5V, then it might be okay. If it's like the regular TTL,
which wants +-5% then maybe a zener would be a better choice.

Red LEDs seem to be about 2V nowadays. But they may vary
considerably.

I'd experiment too. The problem with today's PCs is that the MoBo
furnishes all the serial and parallel signals. So if you foul up a
chip, it could be a major problem. :0)

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Thanks for the reply.

Today I got a chance to play around with this a little.
I found that the Vf on several LEDs varied a bit. These
are "grab bag" LEDs - a bag of 100 various sizes, colors
etc. for a few bucks. I only looked at 5 of them for Vf.
I intend to do some more selecting - but here's what I
found, using three of them in series with a 330 ohm
resistor:
Supply voltage: 12** (see below)
Load 1K
Vf no load 5.63
Vf full load 5.41
I load 5.4 mA
I LEDs ~17.? mA (forgot to write it down)

Test Circuit:
Supply----/\/\/\/-------+------<-Iload->-+
330 |<----+ |
LED | |
| | /
LED Vf \
| | / 1K
LED | \
| | /
^ | |
Ileds | |
v | |
| V |
Supply------------------+-----+----------+

(Meters are shown as Vf, Iload and Ileds)

**The supply was set around 12 v on the above - but I did
not use a digital meter to read it. I was more interested
in no load to full load comparison, which comes in at a little
better than 5%. I tried various currents through the LED
string by varying the supply voltage, but the best I could get
for no load to full load was a delta of about .22 (Vf) volts,
so I don't think shunt regulation with LEDS alone can be
improved much for this circuit.

That said, the .2 volt variation is well within acceptable
variation as shown on the data sheet, which shows basic
characteristics of the supply voltage of 4.5 to 5.5 volts.
Further, it shows typical I as .4 mA minimum to 1.5 mA max,
which will reduce the no load to full load variation. That
will be part of my next experiments.

What remains to be seen is if I have any LEDS that combine to
give a Vf of closer to 5 volts. From what you posted, and from
my initial experiment, I doubt I'll get significantly closer.
I may try 2 LEDS + 2 diodes to see what that does. I want to
look at line regulation as well as load regulation, and I want
to use a typical maximum load of 1.5 mA versus the 5.4 mA max
I used today. I want to try some different colors, too.
But whatever the results, I don't see the LED "regulator"
as a general replacement for a 5.1 zener any time soon.

 

Nice experiment. The big If is will the serial port put out enough
current.

Probably a simpler way to get 5V from the PC is to steal some of it
from the game port, or even the USB cable.

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wrote:



Today's results below - 3 different supply voltages,
new circuit - substituted small signal diode for one
of the LEDs, changed the LEDs to green, Rload = 2.91K

Supply voltage: 5.66 10.00 13.64
Vf no load 4.61 5.14 5.49
Vf full load 4.51 5.08 5.42
I load 1.54 mA 1.73 1.87
I LEDs full load 2.0 mA 13.3 25.0
I LEDs no load 3.2 14.8 26.7

Summary
The LEDs and diode circuit will regulate to supply the
IR chip specs over a supply range anywhere from 5.66 V
minimum to 13.64 volts maximum, under all load
conditions (0 to 1.50 mA, no load - full load) The
"sweet spot" - 10 volts - matches the OP's schematic,
and provides excellent regulation, no load to full
load.

The best selection turned out to be two green LEDs
and a small signal (1N914) diode.

While this would work for the OP's needs, a zener
is better, if for no other reason than reduced
parts count.




Test Circuit:
Supply----/\/\/\/-------+------<-Iload->-+
^ 330 |<----+ |
| LED | |
| | | /
| LED Vf \
| | | / 2.91K
Vs DIODE | \
| | | /
| ^ | |
| Ileds | |
| v | |
v | v |
Supply------------------+-----+----------+

(Meters are shown as Vs, Vf, Iload and Ileds)