I am trying to repurpose a replica GPO 746 using a Raspberry Pi, however the rotary mechanism is rather different to that in an original 746. The circuit has 12 LDRs arranged in an array (3 columns, 4 rows), a shutter attached to the dial, and an LED above the shutter. The shutter is arranged so that when the dial is rotated, a single LDR is illuminated by the LED. The finger stop incorporates a microswitch which powers the LED. I've included a rough circuit diagram below.

The row and column wires were previously connected to an HM9102D, however I cannot reliably replicate the setup using a Raspberry Pi. Reading the GPIO pins via multiplexing seems to have inputs triggered at random, even when the LED is off.

How can I reliably read the inputs from the dial?


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ Can you define Req of LDR with Light ON/OFF? and thus make window comparators reliable for logic level detection? Can you ensure wiring does not pickup noise using twisted pair AWG 30 Magnet wire? Can you detect source/destination of noise current loop? Do you have noise filters for FL light flicker or DC ripple?? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 16 '17 at 10:46
  • \$\begingroup\$ The Charlieplexing style requires unidirectional elements of the matrix. LDR is bidirectional. \$\endgroup\$ – AltAir Apr 16 '17 at 11:28
  • \$\begingroup\$ @TonyStewart.EEsince'75 Pretty much no to all of that... \$\endgroup\$ – CyberJacob Apr 16 '17 at 11:35
  • \$\begingroup\$ Then you know what you need to learn Generally Vishay/Sharp/Infineon PD's are less sensitive (hi Z) than LDRs, but far more accurate. So start with IR or Red LED and black or clear PD (respectively) and measure I=V/R with light on /off with close distance using narrowest angle LED. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 16 '17 at 11:37
  • \$\begingroup\$ @utu2012 Sorry, got charlieplexing and multiplexing mixed up. I've corrected the post now. \$\endgroup\$ – CyberJacob Apr 16 '17 at 11:39

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