CS 20A AM-3 pressure sensor

Pinout & usage.

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

Top view (pins are not visible)

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

 4
  3
   2
     1

Pin numbers taken from Tensoval Comfort service printout. YMMV.

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

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.

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: