Help: IR emitter in TLC555 circuit - not working!

likerobots

Hello All,
I'm trying to learn the IR emitter/receiver concepts.
Having successfully tested RadioShack IR Emitter&Detector (part# 276-142), as well as the RadioShack IR Receiver Module (part# 276-640) with BS2 to detect the beams (using FREQOUT), I wanted to experiment with 555 timer IC. I have a TLC555 timer IC (RadioShack part# 276-1718).
I connected it based on the circuit diagram shown in page 3 of the following document: http://forums.parallax.com/attachment.php?attachmentid=42602.
When the IR LED is not connected, I am able to tune the potentiometer to 39 kHz . However, as soon as I connect the IR LED, the reading drops to 0 kHz.

I noticed that when I connect a regular LED, it flickers, and the debug message still shows 39 kHz. I also verified that when I connect the IR Emitter, the voltage across the legs drops to millivolts.

I am not sure, where I messed up. Any idea what could have gone wrong?

Thanks in advance!

Mike Green

Are you sure you have the circuit wired according to the schematic? That's usually the most common reason for "not working". Even experienced people sometimes do it. The experienced people know this and check, usually more than once. The diode should have its cathode ("flat spot") connected to ground and the anode (long lead) to the 220 ohm resistor.

likerobots

Mike,
Thanks for your reply.
I'm quite sure I connected it per the schematic. Its like this: When I pull the IR LED out of the breadboard, the debug indicates ~39 kHz. As soon as I push it back in, it shows 00000 kHz. If I replace the IR LED with a regular (green) LED, the debug still stays at ~39 kHz, and the LED flickers. And I connect both the LEDs the same way - cathode to the grnd, and anode to the 220 ohm resistor leg.

Not sure if this might give a clue, but I found that if I reverse the IR LED connections (ie, anode to grnd, and cathode to the 220 ohm resistor), the debug still indicates ~39 kHz!

Is this possible if the IR LED went bad? What may be the reason for the frequency to drop to 0 kHz as soon as the diode is connected

Confused!

Mike Green

If the LED is shorted, it will cause the Stamp pin (P7 on the diagram) to stay at ground giving you a frequency of 0 kHz, but, if it's shorted, it would shorted no matter which way you inserted it. Try hooking up just the 220 ohm resistor and LED to 5V and measure the voltage across the LED. It should be around 1.5V.

Unsoundcode

I am not an electronics expert so please correct me if I am wrong. If the voltage drop across the IR led is less than the logic threshold of the stamp then the stamp will never see the 555 freq out. A possible work around might be to place another 180 or 220 ohm resistor on the 555 number 3 pin (out) and place the IR led in series with the new resistor to vss. That should ensure the voltage at the 555 pin 3 is above the stamps logic threshold.

Jeff T.

likerobots

Jeff,
Did you mean something like the attached pic?
I will test this and let you know if it worked.


Without any changes, the voltage drop is across the IR LEDs (or, p7 and Vss) after it is plugged into the circuit is around 200 mV.
However, when the IR LED is not plugged in, the voltage between p7 and Vss is around 1.5 - 2 v. So, i know the 555 is tuned to generate 39 kHz, but as soon as the IR LED is connected, it just dies off!



Thanks!

Post Edited (likerobots) : 9/26/2006 1:24:25 AM GMT

Unsoundcode

Almost there but·I meant for the·junction of the two resistors to be at the pin of the 555.

Jeff T.