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Step 4Building the Laser Sensor
So, there's these nifty things I found at Radio Shack called Photoresistors. Sometimes they're called Light Sensitive Resistors (or LSR). They change resistance based on the amount of light they see. Different photoresistors have different values, so unless you are lucky enough to be using the exact same ones as me, I'd suggest you measure their high and low resistance. I'll tell you how in a second, but first things first. Let's use one of these guys to make a sensor.
First, find a ball point pen. You know, the kind you steal from hotel rooms? The kind you used for spit wads in elementary school? Yeah, those. Disassemble the pen and throw away the cap and the ink cartridge. This leaves you with the tube and the little plug at the end. Take the plug out because this is where the photoresistor is going.
Straighten the legs of the photoresistor and slide it into the tube about 1/2 inch or so. Bend the leads of the photoresistor around the edge of the tube. Jam the plug back into its place, pinning the two leads between the side of the tube and the plug.
Congratulations! You've just made a photosensor.
A few notes... First, the pen doesn't need to be black, but if it is not, then wind a bit of electrical tape around the tube. In fact, even if its black, wind some electrical tape around the tube. The idea is that only light that comes in from the end of the tube will reach the photoresistor. White pens, in particular, bleed light through the sides of the tube. Gotta put a stop to that because it'll cause false readings later on. Also, this is where if you have a laser that is too powerful, it'll burn out your photoresistor. Stick to cheap laser pointers and you'll be fine. Once this thing works reliably, I'm planning on experimenting with shorter tube lengths. Having a 5" tube as a sensor isn't terribly flexible. With some tweaking, I'd like to get it under 1" and in the camera..er...laser head.
Now, this next part is important and I hope you have an ohm-meter handy.
Grab your ohm-meter and hook it up to the leads of the photocell. We're going to take readings on the resistance of the photoresistor in complete darkness and in laser lit conditions.
First, darkness. Rather than put your finger over the end of the sensor (your skin actually bleeds a crazy amount of light), tape it up and throw it in a drawer. Take your ohm-meter reading. It should be a very high number, so make sure your meter is set correctly. My photocell exceeded 2,000,000 Ohms in complete darkness, which topped out my meter, so I just called it 2MOhms. Write it down! Rdark = 2MOhms
Next, grab your laser camera and shine the laser into the open end of the sensor. Take your reading as the lowest resistance measured. Its going to be pretty darned low, so just get close. My reading was around 100Ohms. Write it down! Rlaser = 100Ohms
Why am I doing this? Good question, but I can't tell you yet, you're going to have to read the next step. I'll give you a hint, voltage divider.
First, find a ball point pen. You know, the kind you steal from hotel rooms? The kind you used for spit wads in elementary school? Yeah, those. Disassemble the pen and throw away the cap and the ink cartridge. This leaves you with the tube and the little plug at the end. Take the plug out because this is where the photoresistor is going.
Straighten the legs of the photoresistor and slide it into the tube about 1/2 inch or so. Bend the leads of the photoresistor around the edge of the tube. Jam the plug back into its place, pinning the two leads between the side of the tube and the plug.
Congratulations! You've just made a photosensor.
A few notes... First, the pen doesn't need to be black, but if it is not, then wind a bit of electrical tape around the tube. In fact, even if its black, wind some electrical tape around the tube. The idea is that only light that comes in from the end of the tube will reach the photoresistor. White pens, in particular, bleed light through the sides of the tube. Gotta put a stop to that because it'll cause false readings later on. Also, this is where if you have a laser that is too powerful, it'll burn out your photoresistor. Stick to cheap laser pointers and you'll be fine. Once this thing works reliably, I'm planning on experimenting with shorter tube lengths. Having a 5" tube as a sensor isn't terribly flexible. With some tweaking, I'd like to get it under 1" and in the camera..er...laser head.
Now, this next part is important and I hope you have an ohm-meter handy.
Grab your ohm-meter and hook it up to the leads of the photocell. We're going to take readings on the resistance of the photoresistor in complete darkness and in laser lit conditions.
First, darkness. Rather than put your finger over the end of the sensor (your skin actually bleeds a crazy amount of light), tape it up and throw it in a drawer. Take your ohm-meter reading. It should be a very high number, so make sure your meter is set correctly. My photocell exceeded 2,000,000 Ohms in complete darkness, which topped out my meter, so I just called it 2MOhms. Write it down! Rdark = 2MOhms
Next, grab your laser camera and shine the laser into the open end of the sensor. Take your reading as the lowest resistance measured. Its going to be pretty darned low, so just get close. My reading was around 100Ohms. Write it down! Rlaser = 100Ohms
Why am I doing this? Good question, but I can't tell you yet, you're going to have to read the next step. I'll give you a hint, voltage divider.
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Aug 13, 2011. 2:51 PMsanosuke_kursch
says:
Actually it's LDR (light dependent resistor), and I think a photo-diode would've been better in this case, the LDR is quite slower than the photo-diode (or even a photo-transistor).
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