Schematic Resistors

[Jon128]

I just built a tvbgone recently and am very impressed.

However, there is a problem with the schematic. The 120 ohm resistors are wayyy too small! (It still works fine because the current is limited by the output pin)

To get 100 mA in the LED you only need a 3.3k resistor. The base current would be (3 - 0.7) / 3.3k = 0.7 mA, times the current gain (150) the collecter current would be 105 mA. I tested this with pspice and on a breadboard.

You could get away with more current by using a 1k or 2k resistor.

[maltman23]

Yeah, true, but if you use 47 ohms you get lots of current through the IR emitters, and the range is even further. The tradeoff is that there is no way to program using an ISP if the resistors are small.

I tried a bunch of different values empirically on my lab bench, with an IR detector at the other end of the bench measuring the intensity of IR it receives, and using 47 ohm resistors significantly increased the brightness over 120 ohms.

I've been using 47 ohms in mine for the last month, with the same batteries, and nothing has blown, and the batteries are still good (and I do lots (LOTS!) of field testing).

[Jon128]

Interesting, I guess the current is limited by the output pin and the resistor causes a voltage drop. So the resistor is only there to split the current evenly

It would be a good experiment to add an emiter follower to buffer the voltage from the pin, then use the proper sized resistor to limit the current. I don't have any extra room on my board though.

[adafruit]

the resistor values go against pretty much everything i learned about properly biasing transistors. still, mitch did the testing and he said it works better so thats what we went with

[maltman23]

I've never really been very orthodox in much of anything I do.

I am pumping as much current through the IR emitters as I can. I'm relying on the internal resistance of the batteries to limit the current enough to not fry anything. I haven't actually done the math, but I've been field-testing with these parts in this configuration for a few months, and nothing's blown yet. And quite a few TVs have turned off from quite a far distance.

Mitch.

[caladan]

It's strange, that there was change between 47R and 120R... It shouldn't be so.
Even if collector current is 0.5A, the B=100, base current should be like 5mA, so R should be 470R or something like this...
Well, I will test this on my own. Now I used transistors with small current, but will try to use "bigger" ones.

[adafruit]

I've never really been very orthodox in much of anything I do.

I am pumping as much current through the IR emitters as I can. I'm relying on the internal resistance of the batteries to limit the current enough to not fry anything. I haven't actually done the math, but I've been field-testing with these parts in this configuration for a few months, and nothing's blown yet. And quite a few TVs have turned off from quite a far distance. .

hmm... i dont think its the internal batt resistance thats limiting it here. the microcontroller pins are very well current limited (to the extent that you could put an LED on there without a resistor and it would be just fine) somewhere between 20ma-25ma

[gmg]

I wondered about that too.
Atmel datasheet says 40 ma max at each I/O pin.
And I'd think you'd saturate the 3904 with far less than what 120 ohms provides.

[caladan]

But it may turn on/off slower, with bigger resistors. This may be the case. It depends on Cbe...

[maltman23]

Well, my bench tests show that the light hitting an IR receiver from several feet away is brighter using four 47 ohm resistors than when I used four 120 ohm resistors. Try it out, and see how it works for you.

Another thing I've tried (and I'm doing this with the TV-B-Gone Pro SHP) is to use an emitter follower on the output of the micro, and then I use it to drive 8 other transistors (these 8 are set up like the TV-B-Gone Kit). This works really well!

Mitch.