Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!tut.cis.ohio-state.edu!ucbvax!hplabs!hpda!hpcupt1!dclaar
From: dclaar@hpcupt1.HP.COM (Doug Claar)
Newsgroups: sci.electronics
Subject: Re: IR remote controls
Message-ID: <6220015@hpcupt1.HP.COM>
Date: 10 Apr 90 17:18:10 GMT
References: <4241@mit-caf.MIT.EDU>
Organization: Hewlett Packard, Cupertino
Lines: 27

Now, the obvious followup question: How do I construct a simple 40kHz IR LED
flasher which I can turn on and off with a TTL signal (such as a parallel
output line from a computer)? Yes, I admit it, I'm a total novice WRT the

-Yet Another Hardware Hacker WannaBe.

----------
Well, if you know of a sci.electronics archive, you can look for: "powering
50ma LED from TTL logic?" which is the notes string that I started to ask
the same thing. As I noted in the basenote, the ugly way is to find a
digital device that sinks enough current, and use it. The AS640 worked
for me: it sinks 64ma, and inverts, so that high=on: TTL can sink 'lots'
of current, but not source it. This means that you have to hook up the
LED 'backwards' in that when the output is high, the LED is off.
Since the inverter output is high when the input is low, it works out
that low = LED off.

             diode   resistor    ground
+----+        
|    |        | /|
|'640|o-------|< |---/\/\/\/\/\---+
|    |        | \|                |  
+----+                          -----       
                                 ---        
                                  -         
Of course, this is really gross, and the transistor methods in the
response are much more elegant, but it worked for the prototype.