User: Administrator/caliper

Caliper info

Caliper port

• Ground
• Clock
• Data_in
• Vcaliper (1.5V)

Caliper data format

• Data is 24bits, sent in 6 nibs
• sent every ~250ms,
• pause between nibs: ~800us
• Clock period: ~600us
• Total packet time: ~18ms

Bits	24	23	22	21		20	19	18	17
inches	1=+0.5mil	data[0](LSB)	data[1]	data[2]		data[3]	data[4]	data[5]	data[6]
mm	data[0](LSB)	data[1]	data[2]	data[3]		data[4]	data[5]	data[6]	data[7]
Bits	16	15	14	13		12	11	10	9
inches	data[7]	data[8]	data[9]	data[10]		data[11]	data[12]	data[13]	data[14](MSB)
mm	data[8]	data[9]	data[10]	data[11]		data[12]	data[13]	data[14]	data[15](MSB)
Bits	8	7	6	5		4	3	2	1
inch	data[15]	NA	NA	NA		0=(-)/1=(+)	NA	NA	0=mm/1=in
mm	NA	NA	NA	NA		1=(-)/0=(+)	NA	NA	0=mm/1=in

bit 1, is the first bit sent

Code

Operation

data acquisition

• if no change in clock for 1ms
• if clock == 0
  • count=0
  • clock_previous = 0
  • while (count < 24)
    • if clock == 0 and clock_previous == 1 (ie falling edge clock)
      • data[count] = data_in
      • count = count + 1
    • clock_previous = clock
    • sleep(10us)

data conversion

• if data[23]
  • if data[20]
    • value(inch) = [ bin2dec(data[16:1])/1000 + data[20]*0.5 ]
    • value(mm) = 25.4 * value(inch)
  • else (not data[20])
    • value(inch) = - [ bin2dec(data[16:1])/1000 + data[20]*0.5 ]
    • value(mm) = 25.4 * value(inch)
• else (not data[23])
  • if data[20]
    • value(mm) = bin2dec(data[15:0])/100
    • value(inch) = value(mm) / 25.4
  • else (not data[20])
    • value(mm) = bin2dec(data[15:0])/100
    • value(inch) = value(mm) / 25.4

• clock and input inverted because of transistor voltage level shift
• data is acquired on falling edge of clock

Circuit

based on this circuit schematic

Parts list

Power circuit with soft power switch

• USB lithium charger board
• USB lithium polymer battery
• Q1: 1N3906 (PNP transistor)
• Q2: PN2222 (NPN transistor)
• D6: Any small signal diode
• R1, R2: 10kΩ resistor
• R3: 4.7kΩ resistor
• C1: 10μF (probably can be different)
• C2: 100μF (probably can be smaller)

Caliper power regulator

• D4, D5: Any small signal diode
• R4: 4.7kΩ resistor

Caliper signal voltage shifter

• Q4, Q5: PN2222 (NPN transistor)
• R4, R5: 10kΩ resistor

MCU and display

• Generic Arduino Mini Pro
• I2C OLED display

Circuit Notes

Soft power switch=

• Voltage drop between V_batt and V_out is non ideal, but Arduino still operates fine.
  • V_{ec_q1(on)} = 0.8 - 0.3V (mosfet would be much better but i don't have any with small packages)
  • so V_o = 3.4 - 2.8V depending on lithium charge ie. 4.2 - 3.0V
• On button switch must be held for about a second, otherwise circuit doesn't turn on completely
  • This is because C₂ needs to charge
• C₂ needs to be large because when MCU shutdown pin does not pull down low long enough to completely power off and ends up resetting, then MCU shutdown pin goes high again, leaving everything on. With C₂ included, the MCU discharges C₂ turning off Q₂ and keeps it off long enough to ensure MCU is shutdown completely
  • May be able to be less that 100uF, that's just what i used

Caliper voltage supply

• diodes gives a regulated voltage of about 1.1 - 1.2V, This is lower than ideal, and not a great regulator. But is good enough an causes no problems.

Ref

• https://www.instructables.com/Hacked-Digital-Vernier-Caliper-Using-Arduino/