• 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.

• 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.

• 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.

• 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.

Diagram

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.