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Making a photogate with a wide sensing area

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Dago

Thu Nov 10 2011, 11:30AM

Registered Member #538 Joined: Sun Feb 18 2007, 08:33PM
Location: Finland
Posts: 181

I'm making a chronograph to be used with bows. I thought it was gonna be easy but from playing around with some IR leds and phototransistors it seems it's not that simple.

The problem is that an arrow is very narrow but I'd need a fairly wide area from which I could sense it. I could not get the phototransistor to work sensitively enough to detect a reflection from the arrow (or using a passive reflector like commercial ones). I CAN get it to work by putting the IR led adjacent to the phototransistor and I can get it to work from pretty far away (like 30cm which is ok) but the area is very narrow, probably impossible to shoot through. How could I get a much wider area from which I can detect an arrow? What if I'd use one LED adjacent to an array of several phototransistors in parallel (forming a row)? What about PIN diodes, would they be more sensitive, maybe sensitive enough to detect reflection from the arrow (I bet carbon fiber doesn't reflect IR too well)?

I'm using the phototransistors in a darlington connection with a 2N3904 to get enough sensitivity.

Forty

Thu Nov 10 2011, 05:35PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

I think using multiple phototransistors essentially in parallel would be an easy solution. then whenever one of them has their light blocked out, it will send a signal. I'd have the phototransistors on the opposite side as the LEDs because it would likely be easier to tune and you wouldn't have to rely on the reflection off of the arrow.

Dago

Tue Nov 15 2011, 09:14AM

Registered Member #538 Joined: Sun Feb 18 2007, 08:33PM
Location: Finland
Posts: 181

Forty wrote ...

I think using multiple phototransistors essentially in parallel would be an easy solution. then whenever one of them has their light blocked out, it will send a signal. I'd have the phototransistors on the opposite side as the LEDs because it would likely be easier to tune and you wouldn't have to rely on the reflection off of the arrow.

That's my best bet I guess. I'd really like to try to use PIN diodes (or maybe even avalanche photodiodes) and try to see if I could capture the reflection from the arrow. I bet it should be possible because at least APDs can apparently detect even single photons (at best). The mechanical construction would be very simple, with the "regular" photogate I'd have to construct some kind of actual gate to house the LEDs and phototransistors.

The only thing is that I'm unable to find any example _photogate_ circuits or any similar application with PIN diodes or APDs. I can find lots of stuff about using PIN diodes for RF stuff and APDs for single photon counting and what not but nothing "basic". Can anyone point me to the right way with using PIN diodes or APDs in a photogate kind of situation?

Steve Conner

Tue Nov 15 2011, 09:35AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

PINs and APDs work similarly to regular photodiodes. The regular and PIN types both speed up when a reverse bias voltage is applied, as it reduces the junction capacitance. But PINs start with a lower junction capacitance because of the "I" layer in the sandwich, hence they go faster in practice.

APDs also have the reverse bias voltage, but it's very high, 50-100V, just below the breakdown voltage of the junction. This greatly increases the gain of the device, because each incoming photon causes an avalanche of electrons. It's the solid-state version of a photomultiplier tube.

The bias voltage has to be temperature compensated, or the APD has to be temperature controlled, because the breakdown voltage falls with temperature. I personally have fired up an expensive Perkin-Elmer one off a bunch of bench power supplies in series, but it was an uneasy feeling.

Your problem is not one of absolute speed or sensitivity of the detector. It's detecting a small change of intensity within an accurate time window in the presence of ambient light. That is an information theory problem concerned with SNR, bandwidths, integration times, and so on, and can be looked at independent of whatever detector you choose to use.

Example considerations:
The APD would probably be blinded by whatever ambient IR made it through the filter.
If you used modulated light with a lock-in amp, the integration time of the lock-in would spoil the accuracy of the timing. Unless you used a higher modulating frequency and a faster integration time, then the SNR would be worse. Etc, etc.

My money would be on a diode laser with a line generating head to illuminate the arrow, and a photodiode with a collimator to detect the reflection. If that didn't give enough SNR I would modulate the laser at 1 or 5MHz, and use a tuned amplifier in the receiver.

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