From: LEE Tet Yoon <leety@ih...> - 2004-07-26 14:11:50
I have multiple devices to control and giving their location, I'm probably going to need at least 2 LEDs even with a strong signal. And even this would require the LEDs to be placed at a fair range from the devices which given my room setup will be annoying. I've been thinking instead of using multiple LEDs. I've tried a serial transmitter with 2 LEDs and it seems to work. I'm thinking of making a 5 LED serial transmitter. I only need 3 LEDs for my main equipment but there are another 2 devices which I might want to control occasionally and I'm thinking it might be useful to have all assuming it works.
My plan is to make a 5 LED device based on the simple transmitter circuit at http://www.lirc.org/transmitters.html
The LEDs will be in series. As said, I've already tested this with 2 LEDs. And it seems fine. I've tried using a variable resistor between 0-5k ohm with these 2 LEDs and it seems okay.
The LEDs I'm planning to use (and used for the 2 LED device) are these http://www1.jaycar.co.nz/productView.asp?ID=ZD1945
The voltage from DTR to ground with the LED protecting diode (1N4148) in place seems to be about 11.1V.
So from my calculations, with a voltage drop of ~1.2V/led and 5 LEDs if I used a 680ohm resistor I should get about 7.5mA through the circuit. However, I'm thinking of making it a bit higher. Maybe even about double that (e.g. use a 340ohm resistor). Or alternatively I might add a 6th LED as a backup in case I ever need it (obviously lowering the resistor). It's been a while since I've done anything in electronics so my knowledge is a bit hazy so do mention if I'm doing anything stupid.
Which leads me my main question. I don't mind trying things even if they don't work, I'm just a bit worried I might fry the serial port now or in the future. I know there can be no guarantees, but honestly, how risky do you think things are? I don't really see why 1 LED with a 'large' resistor would be less risky then 5 or 6 LEDs in a series with a small resistor assuming the current drawn (and therefore power) is the same, but then my knowledge is hazy and maybe I'm missing something. And how risky exactly is using a 1 LED device?
P.S. I'm not using a detector