Sensor Color
How do I make a simple color sensor?
Hello, I need to make a sensor color that distiguish at least three colors with the least quantity of parts as possible, but something that works. For example, a different LED for the three colors. If you can sketch the circuit, it would be the best, what I'm trying to say it's that I don't want just simple words telling me what to do, at least I want a circuit. And if you've had some experience using those kind of sensors, any problems, etc, I'd like to know it.
If you have more than one option in the way of making the circuit, I'll look trough them carefully.
You cannot expect other people to design circuits for you. Enough to describe the setup and give a few hints, which is already a lot of experience and effort that you get for nothing.
First it does depend on the actual object to be measured, and the purpose of the measurement. Lets say it is a sheet of coloured paper, which means we can illuminate it with leds of different colours, and measure the reflected light. In other cases it may be the light transmitted through the object, or radiated from the object.
We need a method of showing the colour. The actual method I am suggesting is to use a RGB system. Now you need a co-ordinate system to describe the colour.
For simplicity let us use three leds as colour sources, red, green and blue. We have to accept wavelengths provided. They should all be the same viewing angle. They are all directed at the same point on the paper. Chose the ultra-bright type, the highest number of millicandelas that you can find. Get the data sheet for each one.
Power supply.
Use a 9V DC regulated wall adaptor (plug pack). The current - greater than 100mA - almost any will do. It is possible to use a 12V battery, but an 8V or 9V x 3 terminal regulator device is still needed. A second plug pack is for the negative rail needed for the op-amp.
There are various ways to achieve this. You need approximately +8V, 0V, -8V supply, so two supplies connected so. The negative rail is only a few mA, the positive rail is a few tens of mA, so almost any regulated supply will do.
The light source
The three leds have to be switched separately, so only one is on at a time. Three separate small on/off switches or a 3 position switch. The current will be 10-15mA, and should be regulated. Use the led forward voltage (Vf) and a "ballast" resistor from a regulated 8V supply to do this. The Vf and so resistor is different for each led. Use (9V - Vf) to get the voltage across the resistor. Calculate the resistor value for each led as R=V/I.
The detector
This is a silicon photodiode. The best way to get this is a single small solar cell, say the size of a postage stamp. Perhaps from an old solar powered calculator. Arrange it so the reflected light of the leds will hit the active surface.
Measuring.
The solar cell must have a load resistor. This is determined experimentally. The resistor is connected directly across the output of the cell, and adjust its value so the voltage across the resistor is approx. 20-25 mV for the highest light signal reflected from the LED when using brilliant white paper. This is so we can get a linear light response from the photo detector. It is measuring the current which is proportional to light. The next step is an amplifier circuit. This will convert the 0-20mV signal to a 0-2V signal. That means a voltage gain of approximately 100.
See the wiki link below for a non-inverting amplifier using an op-amp. Almost any op-amp will do. Use 100K for the Rf and 1K for the Rg. This gives a gain of 101, close enough.
Readout.
The simplest is to use the DMM. You could buy a panel meter or cheap DMM dedicated to the job.
It can be connected to an Analogue to digital converter in turn connected to a microprocessor or PC, so the whole thing can be displayed and controlled from the computer. A bigger effort required. I don't know what level you are at.
Calibration
You use a reference white sheet to adjust the gain of each channel to the same sensitivity. That means a factor you can multiply the reading by so all will be the same result = 100% for reference white. Select the weakest channel reading, call that 100%. Adjust the other two to suit. This adjustment can be a calculation when processing the result, or it could be the switch for the leds has 2 poles. One pole controls the LED off/on, the other the gain of the amplifier, so all channels read the same (100%) with reference white.
Using it.
To compare different coloured objects just read the ratios of red/green/blue and express them as percentages of the reference white. There are other measuring systems using this but yours will not be directly referenced to them because the wavelengths used are different.
A better system would use a white light source with colour filter set that matches each other to get the colour ratios. However the leds give a far simpler and cheaper starting point, even if some colours will be measured incorrectly because the leds are just single colours, not bands of colours.
The second link shows a red/green/blue photo sensors, with three filters built in. This is a better way, but may be more expensive than you hope. It can have a white lamp or even ambient lighting, depending on the purpose.
The third link details a colorimeter design. This is for a full understanding.
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