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Need help with Infrared Detector circuit

Hi -

I'm a digital electronics guy with what is probably a simple analog electronics problem. Please help educate the clueless (me).

I'm designing a simple circuit that uses two infrared detector/emitter pairs as triggers. I've got the basics working - I've got +5v going into the two IR detectors which pass the voltage through as long as the light path from the emitters is uninterupted. So far so good.

I envision the two IR detectors as simple switches: when IR is present they turn on (a digital 1 bit), when it is interupted they turn off (a digital zero bit). I've taken the output of these two detectors and connected them to a NAND gate. My goal is to have the NAND output go high whenever EITHER IR detector is shielded from the IR source. Like I said - I'm confident of the digital part of this circuit however the IR detector LEDs don't behave as I expected.

First I tried using a CMOS 4011 gate. This caused the IR detector output to be held high even when the IR source was interupted. Weird...

Just for grins I tried using a TTL 7400 gate. This worked a little better in the sense that the IR detectors work properly - they turn off when the IR source is removed (as they should) but the NAND gate does not trigger.

My hunch is that I am not understanding something about the relationship between the output of the IR detector (a diode) and the digital gate (transistors). Obviously the output of the IR LED is more complex than a simple on/off voltage.

So far this is the worlds simplest circuit - there is only one resistor to trim the voltage down to an appropriate level for the IR transmitter. Other than that everything else is digital gates.

Any suggestions how I can get the output of the IR detectors to act as proper inputs to the digital side of the circuit?

For extra credit - where did I go wrong? Why isn't the +5 I see on the output side of the IR detector sufficient to trigger the gate. Remember, I'm a digital kindof guy so use small words (pun intended).

Thanks in advance! GuitarManDave

Reply to
davepublic1
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I'd start with a voltmeter. Pull-up resistor?

Reply to
Charles Schuler

snipped-for-privacy@trahans.net wrote in news:1173643112.690392.76100 @t69g2000cwt.googlegroups.com:

snip

without knowing what you are using for detectors, they are commonly photo transitors with leads for collector and emitter. you should ground the emitter and connect the collector (assuming NPN) to a 4.7 k resistor that is connected to +5 V. Take the output from the collector giving a "low" when the trasnsitor is illuminated and a "high" when not. You might try that to start if you have not destroyed the detectors by connecing them directly accross the 5 volt supply.

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Reply to
me

Hi, Dave. I'm really sorry you didn't include the part numbers on your IR detectors and emitters. That would have made an answer easy.

There are many different possibilities here. One is that you've got a simple IR photodiode and a phototransistor which is made to be sensitive to light at the emitter's wavelength. If that's the case, it's simply a matter of two resistors, like this (view in fixed font or M$ Notepad):

| | VCC | + | VCC | | + | | | .-. | | | |10K 1/4 | .-. | | CD4001 |180| | '-' __ | | | | .--| \\ | '-' o-------o | )o- | | | '--|__/ | | |/ | V ~ | | - ~ |>

| | | | | | | === === | GND GND (created by AACircuit v1.28.6 beta 04/19/05

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With a 5V supply, the resistor values shown should work fairly well.

The second option is that you've got a hybrid remote control emitter and detector pair. In those, there's circuitry to modulate the light output at the emitter to 38.1KHz (or some similar frequency). The detector is then optimized to pick up that frequency, which gives it a lot of immunity from false triggering by ambient light (like a TV remote control). For these, there usually isn't a resistor for the emitter -- just a power supply hookup. The detector usually has an open collector type output which requires a pullup resistor at the output (usually 10K or so).

But there are many other different possibilities. Rather than have us guess which one you're using, how about posting again with the part numbers, and where you got them? Any links you might have to manufacturer info on the parts would also be helpful.

Good luck Chris

Reply to
Chris

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