Dual ctrl an dig mux

PCB Design for Hardware Control of Confocal.nl AION in Bypass Mode

Important Notes ⚠️

• Current progress: the board has been designed, printed, assembled, and tested on its own. The controller has also been tested on our microscope successfully. Using the SELECT line, one can either use the AION (Confocal.nl) or a FRAP/TIRF module with the AION in bypass mode.
• Floating (digital) inputs should be avoided in a channel that is actively used. I had the case when testing the board, if I only wired one input, the other floating (not wired) input could be HIGH.
• When testing the board, the capacitor value changes depending on how many have been soldered to the board already. If a single capacitor is soldered to the board, a multi-meter should still read 100nF. When two capacitors are soldered to the board, and one measures the value of one capacitor, it indicates 200nF. That is because the capacitors are effectively in parallel and their value adds up. With all four capacitors soldered to the board, they should all read 400nF.

Problem

The AION microscope is controlled by Confocal.nl's own controller. The Confocal.nl controller can also be controlled by the Inscoper Device Controller. The Inscoper Device Controller can control other devices as well. For this reason, the AION can be set into an optical bypass mode, which allows accessing the cameras through the AION without making use of the ReScan, to instead perform other kinds of microscopy. However, the cameras and the laser lines (analog and digital) are normally wired to the Confocal.nl controller. This limits the way in which the cameras and the laser lines can be controlled at the level of the Inscoper Device Controller for use in other applications.

Solution

For two controllers to send signals to the same device, multiplexing is necessary. In the case of the camera trigger (digital) signal, it is not possible to simply use a T-connector because if one controller is high and the other is low a large short-circuit current would flow directly between the two controllers' outputs as they fight each other, potentially damaging the controllers, while the shared node would sit at an undefined voltage that the device could not reliably interpret as high or low.

Requirements

⚠️ Those are for my particular case, please check the documentation of your devices accordingly.

Devices To Multiplex

Oxxius L6Cc With 4 Laser Lines

• 4 digital lines
• 4 analog lines
• Voltage Range (analog and digital): 0-5V
• Input Impedance (analog and digital): 500Ω

2x Kinetix (10Mpix Version)

• 2 digital lines (Trigger In) / 1 per camera
• Voltage Range: 0-5V
• Input Impedance: Unknown
  • For reference, a similar ORCA-Fusion BT has an impedance of 10kΩ

Selected ICs

Digital Multiplexing

SN74HC157N is a 4 x 2:1 digital multiplexer, with a single SELECT bit. Two will be used for a total of 8 (2:1) digital channels. Although only 6 are required.

Analog Multiplexing

CD74HC4053E is 3 x 2:1 analog multiplexer, with three SELECT bits (1 per channel). Two will be used for a total of 6 (2:1) analog channels. Although only 4 are required.

PCB Design Rationale

Assuming up to two lasers are operated simultaneously, the current drawn from 2 digital and 2 analog lines is 40mA. On top of that, the two cameras only contribute 1mA (assuming 10kΩ). Multiplying by a safety factor of 1.2 that is about 50mA. The power will be supplied to the board via the Inscoper Device Controller directly.

All the SELECT bits are common and a single SELECT signal is also taken from the Inscoper Device Controller. This signal effectively switches between AION control and Inscoper control (for the cameras and lasers).

The SELECT line is pulled down by a 10kΩ resistor, defaulting to LOW, which corresponds to AION (for my use case).

Only the SELECT crosses the PCB with minimal length in the back face to keep continuous ground zone.

100nF capacitors are used on each IC for decoupling.

Vertical through-hole SMA connectors will be soldered to the PCB. The PCB will be mounted in a case and a front panel will have SMA (inside facing) to SMB (outside facing) connectors. Minimum distance between SMA connectors optimized for accessible mounting.

Overview

Realization

• Printed by JLCPCB for about 32USD (5xPCBs = 18USD + shipping = 14USD)
• Soldered manually
• We have a local electronics workshop and I'm not sure if the price are truly accurate to the market but here is a small cost breakdown:
  • 32x SMA connectors = 256USD
  • 4x 16-pin sockets = 1USD
  • 2x SN74HC157N = 2.2USD (bought from DigiKey)
  • 2x CD74HC4053E = 1.3USD (bought from DigiKey)
  • 4x 100nF = 0.25USD
  • 1x 10kΩ = 0.05USD

Enclosure

• 1x Enclosure 1401E (Hammond) = 110USD
• 32x SMA-Bulkhead cable 794-2885 (RS PRO) = 640USD

Note: I originally ordered SMA-Bulkhead cables that were thicker (not intentionally) but they sent only a few and the others were on back order. I cancelled the order and got those above that are thinner. The reason I say this is because it is easier to assemble with the thin cables imo. And also, I am sorry for the poor cable management... The enclosure foot print is perhaps a tiny bit too small but it works.

Acknowledgements

We thank Alfonso Fontao Blanco from the Microelectronics Design Center (ETHZ) for kindly reviewing the design and suggesting minor improvements (more distance between SMA connectors, mounting holes, shorter SELECT line on the back face).