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Frequently Asked Questions

 

Questions about Photogates

  1. Can I use a visible light or other emitter for the photogate trigger?

  2. What's the difference between an SPG1 and an SPG2 photogate?

  3. What is the maximum separation of the emitter and detector of the SPG1 or TPG photogates?

  4. The photogate of my SPG1 doesn't trigger on the passage of a water drop. What do I do?

  5. Can a photogate trigger on the passage of a pellet or bullet?

  6. How do I test if the SPG1 photogate is working without using a flash?

  7. I notice some variation in timing with an SPG trigger even without making any changes to the delay, sensitivity, or positions of the parts of my set up. How much variation in timing is to be expected?

  8. Can the triggering logic of the SPG circuit be reversed?  That is, can the circuit be made to trigger when light shines on the detector instead of vice versa?

1. Can I use a visible light or other emitter for the photogate trigger?

The photogate trigger circuit described on this site normally uses a small infrared emitter. You can also use it with the light from a flashlight or red laser pointer. The latter requires precise alignment of the laser beam with the sensing element of the phototransistor. You may be able to make the alignment less critical by diffusing the beam in front of the detector. This may also help if the beam is too intense, making it difficult to adjust the sensitivity of the circuit. Also see item 4 below.

 

One problem with many laser pointers is that the pushbutton is momentary. If you use such a pointer, you need to clamp or tape the button down. There are sources for inexpensive laser pointers with switches that lock in the on position. For example, see this item from Advance Auto Parts.

 

2. What's the difference between an SPG1 and an SPG2 photogate?

The SPG1 is the most flexible of the two versions, because the emitter and detector can be positioned independently of each other. Objects of various sizes can pass through the SPG1 photogate. The SPG2 uses an interrupter as shown to the right. The emitter and detector are fixed in position in a plastic housing. This is convenient for triggering on drops but is limited to objects that can pass through the gate. You can use either type of photogate with the same base trigger circuit.

 

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3. What is the maximum separation of the emitter and detector of the SPG1 or TPG photogates?  I'd like to have the circuit trigger on birds breaking the beam of the photogate.

With critical alignment and sensitivity adjustment, 12 inches is possible. For larger distances, a red laser pointer can be used in place of the infrared LED. The detector is sensitive to red light as well as infrared. When using a laser, you may need to place a pinhole aperture in front of the laser so that the beam on the emitter isn't too intense.  (This tip comes from Roy Marshall.) Note also that a red spot may appear on the subject so try to arrange things so that the red spot will be on the opposite side of the subject as the camera. Alternatively, you might be able to edit the red spot out of the photograph. Also see item 1 above.

 

4. The photogate of my SPG1 doesn't trigger on the passage of a water drop. What do I do?

You may have the emitter and detector too close together. Move them farther apart and then adjust the sensitivity. (This tip comes from Antonio Bruscino.)

 

5. Can a photogate trigger on the passage of a pellet or bullet?

This is usually possible. The sensitivity of the photogate must be adjusted very critically for small and fast projectiles. A sound trigger is the most convenient way to trigger on a gun shot.

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6. How do I test if the SPG1 photogate is working without using a flash?

Connect a red LED from pin 7 of the 555 timer to the 9-volt row through a 470-Ω resistor. Note that the longer leg of the LED is positive. When the photogate is beam is unbroken, the LED should be lit. The LED goes out when something passes through the photogate. (This tip comes from Brian Rowe. Check out his splash photo here.) The newest versions of our kits (v10) incorporate this indicator LED.

 

7. I notice some variation in timing with an SPG trigger even without making any changes to the delay, sensitivity, or positions of the parts of my set up. How much variation in timing is to be expected?

Martin Waugh did some testing of the 555 timer and found that with an average delay of 113.038 msec, the times varied from 112.821 to 113.687 (113.038 +0.65/-0.21) msec. That's a little more than half a percent. By the way, check out Martin's website, Liquid Sculpture. He takes phenomenal photos of splashes.

 

8. Can the triggering logic of the SPG circuit be reversed?  That is, can the circuit be made to trigger when light shines on the detector instead of vice versa?

Yes, this is the way our light-activated trigger circuits work. If you have an SPG1, first remove the 10-kΩ potentiometer and the phototransistor from the circuit. Then connect the phototransistor where the 10-kΩ pot used to be. Connect a 100-kΩ pot in the position where the phototransistor used to be.

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