The photogate breadboard circuit shown in Figure 1 is one of the modules of the Multi-Trigger Breadboard, v11. (Note that the photo shows only a portion of the complete breadboard.) This is a dark-activated trigger circuit, meaning that when the infrared beam of the photogate cable (not shown) is blocked, the circuit is triggered. A light-activated circuit, on the other hand, triggers when light impinges on the photodetector. While the Multi-Trigger Breadoard doesn't include a light-activated circuit, the photogate circuit can be modified to produce a light-activated trigger. These modifications are shown in Figure 2 and will be described in this article.
Circuit schematics for the dark- and light-activated circuits are shown in Figure 3. The primary difference is that the locations of the infrared phototransistor (IR PT) and the 100k variable resistor, RV1, with its associated 1k resistor, R2, are switched. Another change is that the Ready LED with its associated 1k resistor, R4, are removed, as they aren't needed for the light-activated circuit.
Here are the steps to convert the dark-activated trigger to the light-activated one. The starting point is assumed to be the breadboard circuit shown in Figure 1. We'll refer to the components using the symbols in the circuit schematics in Figure 3.
- Remove the capacitor and the variable resistor, and set them aside temporarily.
- Remove the red LED, the 1k resistor (R4) from I2 to I7, and the 470 ohm resistor (R1) from E8 to G8.. These will not be used for the light-activated trigger. Also remove the green wire from H3 to H5 and the black wire from D6 to D8.
- Move the black wire from E6 to F6 to extend from E8 to F7.
- Add the yellow wire from A2 to A6. This will be used later to connect to the phototransistor.
- Move the 1k resistor (R2) that extends from E5 to F5 to extend from I3 to I6 instead.
- Replace the capacitor between H4 and H7.
- Replace the variable resistor so that the center leg is in the 9V row and the two outer legs are in J6 and J8.
This completes the changes. In order to use the light-activated trigger, one need only add the phototransistor. Insert the longer leg in D8 and the shorter leg in D6 as shown in the photo to the right.
In order to trigger a flash unit, connect a cable from the flash unit to A10 and ground, the same as you would for the dark-activated trigger. Shining light on the phototransistor should trigger the flash unit. If not, adjust the variable resistor. This sensitivity setting depends on the ambient light level.
Note: The Dual Light Sensor incorporates both a light- and dark-activated trigger into a boxed unit. A switch toggles between the two modes of operation.