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Sharp IS471 Frustration

I am trying to figure out how I can be goofing up a circuit that only has three parts. I am trying to use a Sharp IS471F IR proximity detector module. Here is the data sheet:

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If I am reading the sheet correctly, the two bent leads are power (Vcc) and ground (GND) with power being the bent lead closest to the edge of the sensor. The other two leads are voltage output (Vo) and light ground (GL) with light ground being the lead nearest an edge of the sensor (and furthest from the power lead). The "outline dimensions" drawing on page 1 of the data sheet shows Vcc as pin 1 on the left side. I am guessing that the view is looking at the sensing side of the device (i.e. the sensing surface is the flat face of the device).

On page four of the data sheet, there is a schematic marked "basic circuit". It shows an LED (assumed IR since the sensitivity peak in the chart just above the schematic is around 940 nm) connected between Vcc and GL (assumed GL since the same internal schematic is on page 1 with the GND and GL location marked) and a 0.33 uf cap between Vcc and GND.

I expected that if I made this circuit and had the IR LED pointing at the smooth face of the sensor I should get a difference in voltage at Voc depending on whether I stuck something between the LED and the sensor. Nope. I tried taking readings with a voltmeter and again with a logic probe, always near 0V relative to GND.

I also expected to get a pulse indicated when I put the logic probe to GL. Nope. Constant high reading. A voltmeter shows the voltage at GL to be about 1V lower than Vcc (4.3V vs 5.38). Looking at the LED with a cheap camera that has no IR filter, I can see that the LED is on.

The frustrating thing is that I did get this to work a few months ago. I had some problems then too, but it did eventually work. I have no idea what I did different this time.

Any ideas?

Paul Pawelski

Reply to
catman
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I didn't hear you say anything about a pull up on the output Vo to VCC. Since the output, Vo, pin 2, is open collector, when an object is detected, the OC transistor grounds the signal as best it can, around .2V. When an object is detected the OC transistor opens. But if there is no pull up, there will be no high voltage either.

Try a 3.3K or so pull up. Value isn't critical, 1K to 20K ought to do the trick.

-- Randy M. Dumse

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Caution: Objects in mirror are more confused than they appear.

Reply to
RMDumse

Hmmmm. Seems to me pin 2 (Vo) already has a pull-up resistor, according to the more detailed internal schematic on page 1 of the datasheet. Or at least something that looks like a pull-up. Wouldn't hurt to try one just in case.

For the OP: Try Fig 3. to test the modulator portion of the chip. You want to see those 38 kHz pulses or else this puppy will not work. Be sure your logic probe is set to read pulses, if it has such a switch mode on it. Some have a setting (using an internal one-shot) to reject pulsing beyond a certain speed, in order to reduce the effects of transients and noise.

Note that pin 1 is the short outside lead. This is how you can readily determine the pinout. The long outside lead is pin 4. If you have connected this thing backwards, or accidentally switched 3 and 4 (the way the schematic is written this is easy to do) you could blow out the chip. The modulator LED might even stay on and not modulate in this instance.

-- Gordon

Reply to
Gordon McComb

looks ok so far...

yes... but there's a few things in this that might trick you into thinking it's not working!

First off, note the output goes high when nothing is detected and low on detection (details top of page two). Second, the sensor will pick-up IR leakage from the side.

I mis-read this first time round and struggled 'til the penny dropped.

If you have detection or IR leakage you'll see the 0V. With some sort of shield between sensor and LED you'll get your output high. I've used various methods - heat shrink, mounting on differing sides of the circuit board.

I found range to be 50-60mm at 4.7v supply and 60-70mm at 6.3v tho' this can be improved with a few mod's to the circuit.

I hope this helps.

Best regards, colin

Reply to
Colin Durrans

Everyone, thanks for the suguestions.

I tried a few things today. I found that with one IR LED on the GL line, the voltage drop was not enough for the logic probe to read the pulse. Adding a second LED or a resistor made the voltage drop enough for the logic probe to sense the pulsing of the line. I also found that a single IR LED was not sufficient to trigger the sensor when pointed directly at it from an inch away even when there was enough other load to allow the pulsing to be seen by the probe. However, two LED's pointing at it worked fine. Using three LED's I Could get the sensor to respond to light reflected off a white card 6" away.

I suspect that the with the single LED, the GL line did not even have enough load on it for the oscillator to work correctly, but I can not prove that without a scope. The reason I think that is that going from

1" travel distance to 12" travel distance (with scatter) should take MUCH more than a 3x increase in light output (more than 12^2) while going from 1 to 3 LED's is less than a 3x increase since the current I measured through the LED's went down some as more were added (the constant voltage source apparently couldn't keep up).

I could see an output change on Vo using either the miltimeter or logic probe without pull-up resistor between it and Vcc. Went from around 5V for no detection to around 0.1V for detection.

Paul Pawelski

catman wrote:

Reply to
catman

I use that sensor on my mini-sumo Seeker 2:

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You can see the near-IR sensor I built using it:

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I have a current limiting resistor on the IR LED, and a 0.47 uF cap between pins 1 and 3. The LED has a heat-shrink shroud around it, and other than that, it just works. It reliably detects objects (like my hand) out to about 4-5" in front of the robot...

Later, Jon

-------------------------------------------------------------- Jon Hylands snipped-for-privacy@huv.com

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Project: Micro Seeker (Micro Autonomous Underwater Vehicle)
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Reply to
Jon Hylands

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