ASCOM Mount Dither

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The review package is updated. I’m also refreshing the project specification to match the corrected PhotoMOS/SPST revision.

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The project specification is refreshed to the corrected PhotoMOS version. I’m verifying the final file list and marking the revision complete.

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Final ERC/BOM/schematic checks are clean; no PCB placement/routing was started.

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Implemented the schematic-only V1 revision: replaced the TLP291-4 with four TLP171A(TP,F) PhotoMOS relays, replaced SW1 with a true SPST ON/OFF switch, preserved the V2 archive, and updated the review docs.
Updated files:
Open items:
  • No PCB placement/routing was started.
  • J1 pin-1 marker and “ST-4 - STRAIGHT 1:1 6P6C ONLY” silkscreen note remain layout-stage tasks.
  • Board outline, connector access, battery-holder position, and mounting method still need definition before placement.
Suggested next steps:
  1. Review/approve the corrected PhotoMOS schematic and BOM.
  2. Confirm SW1 = DS04-254-2-01BK-SMT is acceptable as the physical power switch.
  3. Confirm whether J2 should stay as a 1x03 UPDI header or change to test pads before layout.
Approve and prepare layout
Approve PhotoMOS schematic
Keep UPDI header
Define board outline
Verify RJ12 footprint
Review switch choice
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Files

Schematic

Layout

3D View

Project Specification
Project Overview
  • Name: HEM27EC Dither Box
  • Active baseline: V1 minimal autonomous ST-4 dither timer
  • Status: Corrected schematic complete for review; no PCB placement/routing started
  • Purpose: Standalone battery-powered controller that connects directly to the iOptron HEM27EC mount-body ST-4/autoguide port and periodically generates bounded PhotoMOS-isolated guide-pulse dithers without a PC, ASCOM, guiding camera, hand-controller USB, Wi-Fi, display, camera sync, or application.
The previous ESP32-S3 USB-host command-link architecture is preserved only as a conditional V2 archive: V2 USB Host Archive.
Intended Use
  • Used in the field during astrophotography sessions with an iOptron HEM27EC mount.
  • User enables MENU -> Settings -> Set RA Guiding -> Allow RA Guiding on the mount.
  • Device plugs into the mount-body ST-4 port using one dedicated 6P6C/RJ12 ST-4 cable.
  • The 8409 hand controller remains connected normally to HBX.
  • The device may dither during an exposure; the user accepts losing that exposure to avoid walking noise and remove system complexity.
What the Device Should Do
  • Start a selected 60s / 120s / 300s interval timer after power-on.
  • Execute one immediate dither when the interval expires.
  • Restart the interval timer after the dither event completes.
  • Trigger an immediate dither when DITHER NOW is pressed, then restart the interval timer.
  • Use SOFT / NORMAL / STRONG amplitude profiles around one field-calibrated base pulse duration.
  • Generate pseudo-random RA and DEC direction/magnitude.
  • Keep cumulative commanded offset bounded.
  • Guarantee all ST-4 outputs release/open after every pulse, reset, watchdog reset, brownout, crash, or programming event.
Main Features
  • Two internal replaceable AAA cells.
  • ATtiny1616 low-power MCU operating directly from the cell voltage range.
  • P-channel MOSFET reverse-polarity protection.
  • Four Toshiba TLP171A(TP,F) normally-open PhotoMOS relays for RA+, RA-, DEC+, DEC-.
  • 6P6C/RJ12 ST-4 connector using official HEM27 Figure 3 pinout.
  • Physical 3-position DITHER INTENSITY selector: SOFT / NORMAL / STRONG.
  • Physical 3-position INTERVAL selector: 60s / 120s / 300s.
  • DITHER NOW button.
  • Low-duty status LED.
  • 3-pin UPDI programming header.
V1 System Architecture
Diagram



  
    
  
  
    
  






















2x AAA cells
Physical SPST power switch
P-MOS reverse-polarity protection
VCC_PROTECTED approx 1.8V to 3.2V
ATtiny1616
Interval and intensity selectors
DITHER NOW
Status LED
4x TLP171A PhotoMOS relays
6P6C/RJ12 ST-4 port
HEM27EC mount-body ST-4 port
Interfaces and Connections
  • J1 6P6C/RJ12 ST-4 output to HEM27EC mount-body ST-4 port only.
  • J2 UPDI/VCC/GND programming header.
  • No USB host, no USB-A mount port, no 8409 USB connection, no LiPo charger, no sync/shutter connection.
Official ST-4 Pinout
Source: iOptron HEM27 manual, Figure 3, contact numbering shown as 6 5 4 3 2 1.

Table

J1 pinFunctionNet
1NCNo connect
2Common / GroundST4_COMMON
3RA+ / leftST4_RA_PLUS
4DEC+ / upST4_DEC_PLUS
5DEC- / downST4_DEC_MINUS
6RA- / rightST4_RA_MINUS
Power and Runtime Expectations
  • Supply: two AAA cells in series, nominal 3.0 V alkaline, about 1.8 V depleted to 3.2 V fresh.
  • No regulator in V1.
  • ATtiny1616 supports operation down to 1.8 V at reduced clock rates, but firmware must set BODLEVEL2 and guaranteed operation ends at the BOD threshold.
  • Battery voltage is measured in firmware by comparing VCC to the internal reference, avoiding an always-on resistor divider.
  • Practical battery life estimate: roughly 3–18 months depending interval/intensity, temperature, cell chemistry, and LED duty cycle.
Detailed design package: V1 ST-4 Design Review Package.
Firmware-Relevant Hardware Requirements
  • Configure all ST-4 drive pins as input/high-Z immediately after reset.
  • Use MCU sink-drive of TLP171A PhotoMOS LED cathodes through 470 ohm resistors; reset/high-Z means output open/off.
  • Use BODLEVEL2, nominal 2.6 V; continuous BOD while active; sampled BOD may be used during sleep.
  • Use watchdog to recover to all outputs off.
  • Use hard maximum pulse and event duration limits.
  • Use internal pull-ups only while sampling selectors/buttons to reduce current.
  • Use low-duty LED indications only.
Explicitly Not Included in V1
  • ESP32-S3.
  • USB host.
  • USB-A connector.
  • PL2303 or other USB-serial support.
  • 5 V boost or USB VBUS generation.
  • Wi-Fi, Bluetooth, mobile/web app, display.
  • Camera sync or shutter output.
  • Rechargeable LiPo charger/power path.
  • External configuration dependency.
Assumptions and Remaining Review Items
  • ST-4 pinout is based on official HEM27 Figure 3; connector footprint pin numbering must be rechecked during PCB layout against the TE 5555165-1 mechanical drawing.
  • SW1 is now DS04-254-2-01BK-SMT, a true SPST ON/OFF power switch.
  • During layout only, add J1 pin-1 marker and the silkscreen note: ST-4 - STRAIGHT 1:1 6P6C ONLY.
  • Initial pulse duration requires field calibration on the HEM27EC.
  • No PCB placement/routing has been started.
Change Notes
  • 2026-07-21: Pivoted active baseline from ESP32-S3 USB-host command-link design to minimal AAA-powered isolated ST-4 timer. Archived USB-host design as V2 conditional reference.
  • 2026-07-21: Replaced TLP291-4 phototransistor optocoupler with four TLP171A(TP,F) normally-open PhotoMOS relays and replaced ON-OFF-ON power switch with a true SPST ON/OFF switch.

  • Project Overview

  • Intended Use

  • What the Device Should Do

  • Main Features

  • V1 System Architecture

  • Interfaces and Connections

  • Official ST-4 Pinout

  • Power and Runtime Expectations

  • Firmware-Relevant Hardware Requirements

  • Explicitly Not Included in V1

  • Assumptions and Remaining Review Items

  • Change Notes