Subject:
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RE: IR Demodulator Range Finder Hack
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Newsgroups:
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lugnet.robotics.handyboard
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Date:
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Wed, 26 Nov 1997 10:16:09 GMT
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Original-From:
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David Kott <dakott@kott.%avoidspam%my.domain>
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Viewed:
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727 times
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On Tue, 25 Nov 1997, John Hatton wrote:
> that he was using a circuit to modulate the IR transmission from the
> LEDs by using a NE555 to trigger a transistor. Could anybody possibly
> mail me the circuit diagram,
Sure thing. The basic wiring for a 555 in astable mode is pretty
straightforward;
+5 volts
|-----+
+-|-----|-+
+-Res.1-------| 8 4 |
| | |
|-------------| 7 3 |------> to Base of BJ Transistor
Res. 2 | |
| +----| 6 "555" |
| | | |
+--------+----| 2 |
| | |
Cap----+-------| 1 |
| +---------+
|
Gnd
If you can figure out that horrid graphic, I applaud your patience.
To review:
Pin 1 : Ground and (neg) lead of the capacitor
Pin 2 : Tied to pin 6 and Resistor 2
Pin 3 : Your output, this goes to a drive trans. or directly to an LED
Most 555's can allegedly sink 100 mA.
Pin 4 : Tied to pin 8 and +5 Volts (up to 15 volts actually, if you
are so inclined)
Pin 5 : Not used (An input to allow frequency adjustment [VCO])
Pin 6 : Tied to pin 2 and Resistor 2
Pin 7 : Tied to Resistor 2 (Resistor 2 is therefore electrically
"between" pins 6 and 7) and Resistor 1 (Resistor 1 is
therefore electrically "between" pins 7 and 8)
Pin 8 : Tied to Resistor 1 and pin 4 and +5 volts
Ok, the kicker is determining resistor/capacitance values that yeild a
duty cycle of approximately 50% (which won't happen... see below) , and a
frequency of 40 kHz (the center frequency of the bandpass filter in the
Sharp GPUY5x series reciever module)
The design equations for this oscillator are as follows:
Frequency = 1.44 / ( (Res1 + 2*Res2) * Cap)
Time On = .693*(Res1 + Res2) * Cap
Time Off = .693*(Res2) * Cap
Ton
+----+ +----+ +-
| | | | |
_| |_| |_|
Toff
(as you can see, you can never have Ton = Toff (for a 50% duty cycle) as
Res1 would have to be equal to 0... bad news there)
For ex:
Resistor 1 = 100 K ohms
Resistor 2 = 200 K ohms
Capacitor = 2.2 MicroFarads
Frequency = 1.44 / (( 100K + 2*200K ) * 2.2 E-6)
=~ 1310 Hz.
I usually choose Res1 to be twice Res2, which yields a duty cycle of
around 75%.
The 555 is a transistor driver, comparitor and a flip flop. The capacitor
charges through Res1+Res2 and discharges through Res1. A comparitor
"digifies" the voltage across the capacitor and feeds it into a flip flop.
You can do several things to get your 40 kHz...
Keep trying different
Cap/Res value until you get the frequency you are looking for. Naturally,
this would most likely entail an O-scope... or, at least a freq. counter.
oooooooooorrrrrrr... Use potentiomenters/reostats instead of fixed
resistors and tweak them until you get your 40 kHz.
Orrrrrrrrrrrrrrr....
Pick some component values that are close (close, but too "fast", or, at
LEAST 40 kHz). Make a voltage divider with a potentiometer and connect
the variable voltage wiper contact of that pot. to pin 5 of the
555. This makes a Voltage Controlled Oscillator. Use this pot. to adjust
the output frequency of the 555. The output frequency is inversely
proportional to the voltage applied to pin 5. (Voltage goes up, frequency
goes down)
>
> The reason I am Posting this to the list rather than directly to him is
> that the address appears to be un-recognisable to my mail server.
Ack! My apologies; another victim in my configuration war with BSD
sendmail. My REAL address is dakott@alpha.delta.edu.
Win95/NT - 32 bit extensions and a graphical shell for a 16 bit patch to
an 8 bit operating system originally coded for a 4 bit microprocessor,
written by a 2 bit company that can't stand 1 bit of competition. -UGU
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Message is in Reply To:
 | | RE: IR Demodulator Range Finder Hack
|
| Thanks to David Kott for the addresses, the pdf file is a slightly updated version of the original so it will come in useful. He mentioned that he was using a circuit to modulate the IR transmission from the LEDs by using a NE555 to trigger a (...) (27 years ago, 25-Nov-97, to lugnet.robotics.handyboard)
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