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====== CS 20A AM-3 pressure sensor ======

Pinout & usage.

===== Intro =====

These sensors are pretty popular in blood pressure meters. No datasheet exists, but reverse-engineering took only several hours.

===== Pinout =====

Top view (pins are not visible)
<code>

           + calibration
     CS-20A   03
     AM-3   5 E 

 4
  3
   2
     1
</code>
Pin numbers taken from Tensoval Comfort service printout. YMMV.

Shielding: GND or N/C\\
4: GND\\
3: GND\\
2: TTL_OUT\\
1: Vcc +5V

===== Usage =====

Upon connecting to GND and +5V, strobe signal appears on TTL_OUT. Frequency varies with pressure. rtl-sdr tuned to 24 MHz can pick it easily as alias. Arduino at 16 MHz cannot directly measure the frequency using interrupt handler. OTOH, a 86CH29BF @ 16 MHz can do it on AIN1/ECIN pin. I have not reverse-engineered the firmware nor tried some exact measurements with frequency dividers. It seems to be several MHz.

===== Internal schematics =====

Once the shielding is dismantled, a tiny board with 74HC02A (4 NOR gates) appears. They are configured as NOTs, as they have one pin grounded. Here is the schematics:

{{ :cs-20a.png |}}

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