==== YL-40 A/D D/A Converter Module ====
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The YL-40 is a cheap analogue I/O device based on the PCF8591 I2C interface chip.
There is at least one variant of this board not named YL-40 on but instead named
QiFEi on the reverse.

These devices combine the silicon chip with a potentiometer, light dependant resistor,
thermistor and two LEDs.

I have three YL-40 devices and one QiFEi. On two of the YL-40s the earth connection to the thermistor is missing
on the PCB, with this fault the NTC (channel 1) will always return a value of 255.
=== Pinout ===

<code>
    SCL SDA GND VCC

  O/P PWR
  LED LED LDR NTC P4
              POT P5
                  P6
                    
AOUT AIN0 AIN1 AIN2 AIN3

P4=NTC ENABLE
P5=LDR ENABLE
P6=POT ENABLE
</code>

The LDR and NTC part numbers are unknown but the following characteristics were
found.

^Board   ^LDR @ 10lx ^NTC @ 25'C  ^ Earth fault ^ Jumpers |
|YL-40 #1| 10K       | 4K7 / 3977 | No          | RED     |    
|YL-40 #2| 5K        | 6K8 / 3977 | Yes         | BLACK   |
|YL-40 #3| 10K       | 10K / 3977 | Yes         | NONE    |
|QiFEi   | 10K       | 4K7        | No          | BLACK   |

As we can see, the components vary, plus there is a massive variation
in response for the LDRs and they may be considered unique.

The LDRs are likely to be the GL/PGM 5516 (~5K) and 5528 (~10K).

=== PCF8591 ===

^Control ^Register   ^Function   ^ Device        |
|0        | channel 0 |Read ADC 0| LDR           |
|1        | channel 1 |Read ADC 1| NTC THERMISTOR|
|2        | channel 2 |Read ADC 2| NC            |
|3        | channel 3 |Read ADC 3| POTENTIOMETER |
|+64      | analog OE |DAC output| O/P LED       |

The O/P LED will begin to turn on with the output set
to 140 or more with VCC at 3v3.

=== RPi ===

Set up I2C as per the [[interfaces:slicepi#rpi_i2c|Slice of Pi/O]].

Set O/P to VCC and turn on LED.
<code>
i2cset -y 1 0x48 64 255
</code>

Read POT.
<code>
i2cset -y 1 0x48 3 ; i2cget -y 1 0x48 ; i2cget -y 1 0x48
</code>

The first i2cget will read and discard any previous sample. This will
be from the any previously read input channel and perhaps not the
one required.

=== Resources ===

[[http://www.vishay.com/docs/29049/ntcle100.pdf|Vishay NTC datasheet]] \\
[[http://en.nysenba.com/upfiles/file/LDR.pdf|GL series guide]] \\
[[http://akizukidenshi.com/download/ds/senba/GL55%20Series%20Photoresistor.pdf|GL55 series datasheet]] \\
[[http://www.token.com.tw/pdf/resistor/cds-resistor-pgm.pdf|PGM series datasheet]] \\