ATtiny212 Firmware for Distortion Pedal
Please analyze this schematic carefully. The relay coil now works and K2 clicks, but the distortion effect does not work when the relay switches to the NO / effect position.
The project is a guitar distortion pedal with relay bypass.
Important context:
- U2 is ATtiny212.
- PA1 drives Q2 relay driver.
- K2 is Omron G6S-2F 5VDC relay.
- D2 flyback diode was found reversed during testing and was temporarily removed. Please verify its correct orientation.
- Relay coil works now, but when the relay switches, the distortion path does not produce sound.
- I need you to check whether the schematic routing is correct, especially the relay audio contacts, VREF, analog power, and mute/photorelay circuit.
Please analyze these points:
- Relay K2 audio routing: Check the exact relay pinout and footprint for Omron G6S-2F 5VDC. Verify if the relay contacts are connected correctly.
For bypass / relay OFF:
- JACK_IN_TIP should connect to BYPASS_LINK.
- JACK_OUT_TIP should connect to BYPASS_LINK.
- The signal should bypass the distortion circuit.
For effect ON / relay energized:
- JACK_IN_TIP should connect to EFFECT_IN.
- EFFECT_OUT should connect to JACK_OUT_TIP.
- BYPASS_LINK should be disconnected from the active audio path.
Please explicitly tell me:
- In relay OFF state, which nets are connected?
- In relay ON state, which nets are connected?
- Is COM/NC/NO mapping correct for this relay footprint?
- Is the footprint possibly mirrored or using bottom-view pin numbering incorrectly?
- D2 flyback diode: Verify D2 orientation. Correct expected orientation:
- D2 cathode / stripe side should connect to +5V / relay coil positive.
- D2 anode should connect to RELAY_COIL_NEG / Q2 collector / relay coil negative. If D2 is reversed, explain what happens and confirm how it should be installed.
- Q2 relay driver: Confirm that Q2 is wired correctly:
- PA1 → R4 → Q2 base.
- R5 pulls base to GND.
- Q2 emitter goes to DGND.
- Q2 collector goes to RELAY_COIL_NEG. Expected ON state:
- PA1 = 5V.
- Q2 base = about 0.6–0.8V.
- Q2 collector / RELAY_COIL_NEG = about 0–0.6V.
- Relay coil has about 4.5–5V across it.
- U4 photorelay / mute circuit: Please analyze U4 carefully. It appears connected around EFFECT_OUT and GND/DGND. Check if U4 can mute or short EFFECT_OUT to ground. Tell me:
- Is U4 intended as anti-pop mute?
- Should U4 be ON only briefly during relay switching?
- Should U4 be OFF during normal effect ON operation?
- If PA6 keeps U4 ON, could it completely mute the distortion output?
- What should PA6 logic be during bypass, switching, and effect ON?
- VREF and analog power: Check the analog section. Verify:
- +9V_CLEAN reaches the op-amps.
- VREF is generated correctly by R9/R10 and capacitors.
- VREF should be approximately half of +9V_CLEAN, around 4–4.5V.
- All op-amp reference inputs that need VREF are connected.
- No op-amp input is floating.
- The analog ground and digital ground are connected correctly.
- Distortion signal path: Trace the full effect signal path and confirm it is complete:
- EFFECT_IN
- input buffer
- BUFFER_OUT
- gain / clipping stage
- DRIVE_OUT
- tone section
- volume/output section
- EFFECT_OUT
- relay output to JACK_OUT_TIP
Please identify any possible missing connection, wrong net label, floating node, or incorrect capacitor/potentiometer wiring that would make the distortion path silent.
- Potentiometers: Check the gain, tone, and volume potentiometer connections. Make sure:
- wipers are connected to the correct nets.
- end terminals are not swapped in a way that shorts signal to ground.
- volume output actually reaches EFFECT_OUT.
-
Clipping diodes: Check D3/D4 clipping diodes. Verify they are connected to the correct gain stage nodes and not accidentally shorting the signal.
-
Provide a measurement checklist: Give me a step-by-step list of what to measure with a multimeter:
- +9V_CLEAN to GND
- VREF to GND
- EFFECT_OUT to GND resistance with power off
- JACK_IN_TIP to EFFECT_IN continuity in relay ON
- JACK_OUT_TIP to EFFECT_OUT continuity in relay ON
- JACK_IN_TIP to BYPASS_LINK continuity in relay OFF
- JACK_OUT_TIP to BYPASS_LINK continuity in relay OFF
- PA6 state and whether U4 is muting
- Provide a likely fault list: Based on the schematic, list the most likely reasons why the relay switches but the distortion does not work.
Do not write firmware yet. First analyze the schematic and give me:
- Relay routing truth table.
- VREF/power verification.
- U4 mute analysis.
- Possible schematic/footprint mistakes.
- Measurement checklist.
7:25
Thought for 17 sec
Read K2 datasheet for pin configuration contact arrangement te.
Read U4 datasheet for pin configuration input output photorela.
Read U2 datasheet for pinout pin configuration ATtiny212 SOIC .
Read BOM.
Read schematic.
Read schematic.
Read schematic.
Read schematic.
Read schematic.
Found 1 results for Omron G6S-2F-DC5 datasheet pinout COM NC NO pins 3 4 5 8 9 10 bottom view.
Completed 3 design checks — found 141 issues.
I found two high-probability causes: K2’s COM/NC pin mapping in the schematic appears inconsistent with the Omron G6S-2F pinout, and U4 can mute
EFFECT_OUT if PA6 is held HIGH. D2’s schematic orientation is correct: cathode/stripe to +5V, anode to RELAY_COIL_NEG.Full relay truth table, VREF/power review, U4 mute logic, fault list, and measurement checklist are in Relay Distortion Path Debug Analysis.
Fix K2 mapping
Corrected relay table
Trace effect path
Check U4 mute
Run ERC/DRC
Refine this doc
Ask about this doc
Relay Distortion Path Debug Analysis
Executive summary
The relay coil/driver and flyback diode orientation in the schematic are logically correct now: D2 cathode is on
+5V, D2 anode is on RELAY_COIL_NEG, Q2 collector is on RELAY_COIL_NEG, and Q2 emitter is on ground.The most important schematic issue I found is the relay contact mapping: the Flux symbol for K2 labels COM1 as pin 3 and NC1 as pin 4, but the Omron G6S-2F datasheet pinout shows COM pins are 4 and 9, NC pins are 3 and 10, and NO pins are 5 and 8 for the SMD G6S-2F top view. That means the schematic symbol/footprint pin-function mapping is likely wrong or mirrored for at least the COM/NC contacts. This can absolutely produce the symptom: relay clicks, but the effect path is not actually routed where the schematic appears to say it is.
A second likely issue is the anti-pop photorelay U4: it is wired from
EFFECT_OUT to GND, so if PA6 drives U4 on during normal effect operation, it will short/mute the effect output.1. Relay K2 routing truth table
Datasheet pin mapping for Omron G6S-2F
For Omron G6S-2F-DC5 / G6S-2F-Y-DC5, the contact pinout found from the Omron datasheet/web result is:
Table
| Relay function | Physical pins |
|---|---|
| Coil | 1 and 12 |
| COM | 4 and 9 |
| NC | 3 and 10 |
| NO | 5 and 8 |
Important: Omron shows the G6S-2F SMD terminal arrangement as top view. The through-hole version uses a bottom-view drawing. If a footprint/symbol was created from the wrong view, the contact functions can be mirrored/swapped.
K2 symbol pin mapping currently in Flux schematic
The K2 schematic symbol currently exposes these pins:
Table
| Flux K2 pin name | Pin number in symbol |
|---|---|
1 + | 1 |
12 - | 12 |
COM1 | 3 |
NCl1 | 4 |
NO1 | 5 |
NO2 | 8 |
NCl2 | 9 |
COM2 | 10 |
This conflicts with the Omron datasheet mapping above:
- Flux says
COM1 = 3, but datasheet says pin 3 is NC. - Flux says
NCl1 = 4, but datasheet says pin 4 is COM. - Flux says
NCl2 = 9, but datasheet says pin 9 is COM. - Flux says
COM2 = 10, but datasheet says pin 10 is NC. - NO pins 5 and 8 appear correct.
Intended logical routing from the schematic labels
Based on the schematic labels and adjacent bias resistors, the intended audio relay routing appears to be:
Table
| Logical net | Evidence in schematic |
|---|---|
JACK_IN_TIP | tied to K2 pole 1 common side with R6 1M pulldown |
EFFECT_IN | tied to K2 pole 1 NO side, R11 input resistor, R7 1M pulldown |
BYPASS_LINK | tied to both NC contacts |
EFFECT_OUT | tied to K2 pole 2 NO side, R21 series/output path, U4 mute, R8 1M pulldown |
JACK_OUT_TIP | tied to K2 pole 2 common side |
If the relay symbol were correct, the desired truth table would be:
Table
| Relay state | Pole 1 connection | Pole 2 connection | Result |
|---|---|---|---|
| OFF / de-energized | JACK_IN_TIP → BYPASS_LINK | JACK_OUT_TIP → BYPASS_LINK | Bypass path active |
| ON / energized | JACK_IN_TIP → EFFECT_IN | JACK_OUT_TIP → EFFECT_OUT | Distortion path active |
Actual physical behavior if the footprint follows the Omron datasheet
Because the schematic symbol appears to assign COM/NC incorrectly, the physical relay may behave differently from the schematic view.
Using the Omron physical mapping:
- Pin 3 is actually NC, not COM.
- Pin 4 is actually COM, not NC.
- Pin 9 is actually COM, not NC.
- Pin 10 is actually NC, not COM.
That means a board built from this footprint could have the physical commons on the nets intended to be NC/bypass, while the nets intended to be commons are actually NC terminals. The NO contacts may still go to the effect input/output, but the relay common may not be on
JACK_IN_TIP / JACK_OUT_TIP. This is the top suspect for “relay clicks but no effect signal.”Answer to the requested relay questions
- Relay OFF expected:
JACK_IN_TIPconnects toBYPASS_LINK;JACK_OUT_TIPconnects toBYPASS_LINK. - Relay ON expected:
JACK_IN_TIPconnects toEFFECT_IN;EFFECT_OUTconnects toJACK_OUT_TIP. - Is COM/NC/NO mapping correct? The current Flux K2 symbol mapping does not match the Omron G6S-2F pinout found from the datasheet/web result. NO pins look correct; COM/NC pins appear swapped.
- Could the footprint be mirrored or using bottom-view numbering incorrectly? Yes. The G6S-2F SMD datasheet diagram is top view; using a bottom-view interpretation can mirror/swap contact functions. Also, the symbol itself already appears inconsistent with the datasheet pin functions.
2. D2 flyback diode orientation
Schematic connections:
- D2 cathode
K→+5V - D2 anode
A→RELAY_COIL_NEG - K2 coil positive pin 1 →
+5V - K2 coil negative pin 12 →
RELAY_COIL_NEG - Q2 collector →
RELAY_COIL_NEG
This is the correct flyback orientation for a low-side relay driver.
Correct installed orientation:
- D2 stripe/cathode side:
+5V/ relay coil positive - D2 anode side:
RELAY_COIL_NEG/ Q2 collector / relay coil negative
If D2 is reversed, it becomes forward-biased whenever Q2 turns on, effectively shorting
+5V through D2 and Q2 to ground. Symptoms can include regulator collapse, high current, Q2 overheating, diode damage, or the relay not energizing properly. Temporarily removing D2 allows the relay to click, but it removes inductive kick protection and can stress Q2/U2 over time.3. Q2 relay driver verification
Schematic connections match the expected low-side NPN driver:
- U2
PA1→ R4 4.7k → Q2 base - R5 100k pulls Q2 base to GND
- Q2 emitter → GND / DGND
- Q2 collector →
RELAY_COIL_NEG - Relay coil positive →
+5V
Expected voltages when relay is ON:
Table
| Node | Expected voltage |
|---|---|
| U2 PA1 | about 5 V |
| Q2 base | about 0.6–0.8 V |
| Q2 emitter | 0 V |
Q2 collector / RELAY_COIL_NEG | about 0–0.3/0.6 V |
| Across relay coil | about 4.5–5 V |
This part of the schematic is consistent with the relay clicking.
4. U4 photorelay / mute analysis
U4 is a Toshiba TLP3431 photorelay. In this schematic it is wired as an output mute:
- U4 input LED anode
Ais driven from PA6 through R3 1.2k. - U4 input cathode
Cgoes to GND. - U4 output terminals are between
EFFECT_OUTand GND.
Therefore:
- PA6 HIGH → current flows through R3 and U4 input LED → photorelay output turns ON →
EFFECT_OUTis shorted/muted to GND. - PA6 LOW → U4 input LED off → photorelay output open →
EFFECT_OUTis not muted.
Answers:
- Is U4 intended as anti-pop mute? Yes, based on its placement from
EFFECT_OUTto ground. - Should U4 be ON only briefly during relay switching? Yes. It should normally be ON only around the switching transient, then released.
- Should U4 be OFF during normal effect ON operation? Yes. During normal audio operation, U4 must be OFF or it can mute the output.
- If PA6 keeps U4 ON, could it completely mute the distortion output? Yes. It directly shunts
EFFECT_OUTto GND. - What should PA6 logic be?
- Bypass steady state: normally LOW unless you intentionally mute during switching.
- Switching interval: HIGH briefly to mute
EFFECT_OUTduring relay transition. - Effect ON steady state: LOW.
A quick test is to force PA6 LOW or temporarily disconnect U4/R3 and check whether the effect path returns.
5. VREF and analog power verification
Analog power
The op-amp supply net appears correct:
+9V_CLEANcorresponds to the schematic net containing U5V+, U6V+, C3/C8/C9/C10/C11 decoupling, and R9 top of VREF divider.- U5
V-and U6V-are connected to GND.
Expected measurement:
- U5 pin 8 / U6 pin 8 to GND: about +9 V, depending on adapter/battery and protection drop.
- U5 pin 4 / U6 pin 4 to GND: 0 V.
VREF generation
VREF is generated by R9/R10:
- R9 47k from
+9V_CLEANto VREF - R10 47k from VREF to GND
Expected VREF:
VREF ≈ +9V_CLEAN / 2- With 9.0 V clean supply: about 4.5 V.
- With diode/MOSFET/protection losses and battery variation: roughly 4–4.5 V is normal.
VREF has decoupling/filtering around the analog section; if VREF is missing or near 0 V/9 V, the op-amps will bias incorrectly and the effect path can be silent or badly distorted.
Grounding
The schematic uses portals named DGND, but the actual power portal appears to tie into the global
GND net. Analog and digital grounds appear connected as one net, which is acceptable for this single-supply pedal if layout return currents are handled carefully.6. Distortion signal path review
The intended signal flow appears to be:
JACK_IN_TIP- Relay K2 pole 1, energized state
EFFECT_IN- R11/C14 into U5 buffer/input network
BUFFER_OUT- Gain/clipping stage around U6A with D3/D4
DRIVE_OUT- Tone network / tone pot terminals
- Volume/output network / volume pot terminals
EFFECT_OUT- Relay K2 pole 2, energized state
JACK_OUT_TIP
Potential schematic problems found:
- Relay COM/NC pin mapping is likely wrong relative to Omron’s actual G6S-2F pinout. This is the main suspect.
- U4 can mute
EFFECT_OUTif PA6 is HIGH during normal effect operation. - ERC reports
Net 31is a single-pin net at R21:P2. Because R21 appears to be the output series resistor fromEFFECT_OUT, a single-pin net on R21:P2 is suspicious. It may mean the final output side of the volume/effect output path is not actually connected to where intended. - ERC reports
Net 32is a single-pin net at C20:2. This may be part of the output coupling/volume network; if one side of a coupling capacitor is left floating, the signal path can be open. - ERC reports floating
GAIN_FBportals. This may be a portal/label artifact, but gain-stage feedback should be checked carefully.
7. Potentiometer checks
The potentiometers are represented as separate terminal components, so the schematic relies on correct external wiring/pin numbering. Verify the physical pot wiring against the intended nets:
- Gain pot: terminals associated with gain feedback/inverting node must form a feedback/control path, not leave the op-amp feedback open.
- Tone pot: wiper must connect to
TONE_WIPER; tone network endpoints should not short signal permanently to ground unless at an extreme setting. - Volume pot: the output/wiper must actually reach
EFFECT_OUTthrough the intended coupling/series resistor path.
The single-pin ERC warnings at R21:P2 and C20:2 strongly suggest checking the volume/output section first.
8. Clipping diodes D3/D4
D3 and D4 are connected anti-parallel between the gain-stage nodes:
- One side group: D3:1, D4:2, R14:P2, C16:2, C17:1, U6:OUTA
- Other side group: D3:2, D4:1, R13:P2, C16:1, U6:-INA
That is consistent with symmetrical clipping in the op-amp feedback/gain stage. They do not appear to be accidentally shorting directly to ground. They are unlikely to be the primary cause of total silence unless one diode is physically shorted or the surrounding gain feedback path is open.
9. Measurement checklist
Perform these with the pedal on the bench, using circuit ground as the black probe reference unless stated otherwise.
Power off / continuity checks
-
Relay contact truth table at K2 pins directly
- Relay OFF:
- Pin 3 ↔ pin 4 or pin 4 ↔ pin 3? Determine which physical pins are closed.
- Pin 9 ↔ pin 10 or pin 10 ↔ pin 9? Determine which physical pins are closed.
- Relay ON:
- Pin 4 ↔ pin 5 should close if Omron datasheet mapping is correct.
- Pin 9 ↔ pin 8 should close if Omron datasheet mapping is correct.
- Relay OFF:
-
Expected bypass continuity, relay OFF
JACK_IN_TIP↔BYPASS_LINK: should be near 0 Ω.JACK_OUT_TIP↔BYPASS_LINK: should be near 0 Ω.JACK_IN_TIP↔EFFECT_IN: should be open.JACK_OUT_TIP↔EFFECT_OUT: should be open.
-
Expected effect continuity, relay ON
JACK_IN_TIP↔EFFECT_IN: should be near 0 Ω.JACK_OUT_TIP↔EFFECT_OUT: should be near 0 Ω, possibly through R21 depending where you probe.BYPASS_LINKshould be disconnected from the active audio path.
-
D2 diode orientation
- Stripe/cathode pad continuity to
+5V. - Non-stripe/anode pad continuity to
RELAY_COIL_NEG/ Q2 collector / K2 pin 12.
- Stripe/cathode pad continuity to
-
EFFECT_OUT to GND resistance, power off
- With U4 off/unpowered,
EFFECT_OUTshould not be a hard short to ground. - If it reads near 0 Ω, suspect U4 stuck on/shorted, solder bridge, or wrong net connection.
- With U4 off/unpowered,
-
Check R21 and C20 suspicious open nodes
- R21:P2 should connect to the intended next node/output path, not be isolated.
- C20:2 should connect to the intended output/volume network, not be isolated.
Power on / DC voltage checks
-
+9V_CLEAN to GND
- Expect roughly the adapter/battery voltage after protection, typically near 9 V.
-
+5V to GND
- Expect about 5.0 V.
-
VREF to GND
- Expect about half of
+9V_CLEAN, typically 4–4.5 V.
- Expect about half of
-
Op-amp supplies
- U5/U6 V+ to GND:
+9V_CLEAN. - U5/U6 V- to GND: 0 V.
- Relay driver ON state
- PA1: about 5 V.
- Q2 base: about 0.6–0.8 V.
- Q2 collector /
RELAY_COIL_NEG: about 0–0.6 V. - K2 coil pin 1 to pin 12: about 4.5–5 V.
- U4 mute / PA6
- During normal effect ON steady state, PA6 should be LOW.
- If PA6 is HIGH, U4 is probably muting
EFFECT_OUT. - Measure
EFFECT_OUTDC: it should usually sit around VREF through biasing/output network, not hard 0 V, unless AC-coupled after the final cap.
Audio probe checks
- Inject a small guitar/sine signal and probe in order:
EFFECT_IN- U5 buffer output /
BUFFER_OUT - U6 gain-stage output /
DRIVE_OUT - tone output /
TONE_WIPER - volume output /
EFFECT_OUT JACK_OUT_TIPwith relay ON
The first point where signal disappears identifies the failing block.
10. Likely fault list
Most likely causes, in priority order:
- K2 symbol/footprint COM/NC mapping mismatch with the Omron G6S-2F datasheet. This is the strongest schematic-level suspect.
- Relay footprint mirrored or interpreted from the wrong view. G6S-2F SMD drawing is top view; bottom-view interpretation can swap physical connectivity.
- U4 photorelay mute held ON by PA6, shorting
EFFECT_OUTto ground during normal effect mode. - Output path open near R21 or C20, supported by ERC single-pin-net warnings on
R21:P2andC20:2. - Volume pot wiring/wiper not connected to
EFFECT_OUT, especially because the pot terminals are generic terminals and depend on physical harness/pin numbering. - VREF missing or wrong, which would bias the op-amps incorrectly.
- +9V_CLEAN missing at U5/U6, leaving the analog stages unpowered.
- Gain feedback path open or mislabeled, suggested by floating
GAIN_FBERC warnings. - D2 reinstalled reversed, which would short the relay supply when Q2 turns on; however, with D2 removed and K2 clicking, this is not the current cause of no distortion sound.
- Physical solder/assembly issue on K2, U4, pots, or output coupling components.
Recommended next action
Before changing firmware, verify K2 physical relay contact continuity with a meter directly at the relay pins. If the physical relay follows the Omron pinout, update/fix the K2 part symbol/footprint mapping so pins 4 and 9 are COM, pins 3 and 10 are NC, and pins 5 and 8 are NO. Also force PA6 LOW during steady-state effect ON or temporarily disable U4 to confirm it is not muting
EFFECT_OUT.
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