# kttl — Analog Control Prototype Wiring (LM35 sensor) Fully-analog temperature controller for a kettle, styled after a Soviet radiometer panel. No MCU: a comparator loop reaches/holds a rotary-selected target, a moving-coil meter shows live temperature on a **linear 0–100 °C dial**, and a solenoid-struck keyswitch clicks (Geiger-style) on each heater cycle. Sensor is an **LM35** (10 mV/°C, linear) — chosen so the dial is truly linear and the setpoint ladder is evenly spaced. > **Status:** prototype / paper design. A few values depend on measuring the > actual meter movement and solenoid — see *Open Unknowns*. --- ## 1. Panel controls | Ref | Control | Type | Function | Domain | |-----|--------------------|----------------------|-------------------------------------------|-------------| | SW1 | Rotary selector | 6-position, 1 wafer | Target temp tap (60/70/80/85/90/100 °C) | low-voltage | | SW2 | Rocker (on/off) | SPST | **Heater enable only** (not master power) | low-voltage | | SW3 | Hold toggle | SPDT | After target: HOLD (cycle) vs ONE-SHOT | low-voltage | | M1 | Moving-coil meter | µA/mA movement (М205)| Live temperature, linear 0–100 °C | always-on | Mains is live whenever plugged in. Electronics + meter sit on an **always-on rail**; the heater is gated downstream and switched only by the relay/SSR (mains-last). The rocker just **stops heating** — it is not a master cut-off. --- ## 2. Power tree ``` MAINS (live when plugged in) ├─────────► PSU (mains→5V, always on) ──► +5V rail ─┬─► LM35, comparator, latch, │ │ meter driver, clicker logic │ └─► VREF_PRECISE (LM4040-2.5) │ → setpoint ladder │ └─► HEATER PATH (mains): F1 fuse ─► TC1 thermal cutoff ─► K1/SSR1 ─► ELEMENT ▲ enable = SW2(rocker) AND logic-heat ``` **Note:** the ladder reference is now a **precision reference (not ratiometric)** because the LM35 output is an absolute voltage (10 mV/°C). The 5 V rail no longer needs to be accurate, but VREF_PRECISE does. --- ## 3. Schematic (block + signal level) ``` +5V (always on) VREF_PRECISE = 2.500V (LM4040-2.5) │ │ ┌──────┴──────┐ ┌────────┴────────┐ │ LM35 │ OUT = 10mV/°C │ SETPOINT LADDER │ │ Vs OUT GND│ (0°C=0V, 100°C=1.00V) │ R1..R7 (taps=°C)│ └───┬───┬───┬──┘ └────────┬────────┘ │ │ └─GND │ taps: 0.60/0.70/0.80/ │ │ │ 0.85/0.90/1.00 V │ ├─────────────┬──────────────┐ ┌───────┴───────┐ │ │ │ │ │ SW1 rotary 6-pos│ [Rpd 18k │ │ └───────┬───────┘ to GND] (to meter (to comparator −) │ common = selected Vref │ driver U3) │ R_in 10k │ GND │ │ │ ▼ ▼ │ ┌──────────────────────┐ │ │ U1 COMPARATOR LM393 │ │ Rf 2.4M │ (−) LM35 (+) tap │◄─┴──── hysteresis │ OUT ├──┬──── (to +) ~±1°C └──────────────────────┘ │ OUT HIGH = cold │ on→off edge = "reached" (pull-up 10k to +5V) │ ┌──────────────────────────────┴──┐ │ │ ▼ ▼ ┌──────────────┐ ┌────────────────┐ │ U2 CD4013 │ │ CLICKER one-shot│ │ D-FF latch │ │ → Q1 MOSFET │ │ D=Hi │ │ → L1 solenoid │ │ CLK=reached │ │ (+ D1 flyback)│ │ CLR=POR + SW1│ │ strikes keyswitch│ │ Q' ───┼──┐ └────────────────┘ └──────────────┘ │ ▼ SW3 HOLD ──────► ┌───────────────┐ (HOLD: force Hi; │ ENABLE gate │ 1-SHOT: use Q') │ (AND) │ └──────┬────────┘ │ heater-want SW2 rocker (series) │ ▼ ┌──────────────┐ │ Q2 ─► K1/SSR1│ switches MAINS to ELEMENT └──────┬───────┘ (via F1 + TC1, mains-last) ▼ ELEMENT METER BRANCH: LM35 OUT ─► U3 (op-amp + transistor current sink, Re) ─► M1 I_meter = V_LM35 / Re (no offset: 0°C = 0V) linear 0–100 °C dial ``` --- ## 4. Component values ### 4.1 Sensor — LM35 - **U_LM35:** LM35 (or LM35DZ), Vs = +5 V, OUT = 10 mV/°C. - 0 °C → 0.000 V, 60 °C → 0.600 V, 100 °C → 1.000 V. - **Rpd:** ~18 kΩ from OUT to GND (lets the output pull down toward 0 V; basic LM35 only *sources* current). True 0 °C reading needs a negative rail; with a GND pull-down it reads reliably from ~+2 °C up — fine for a kettle. - Seal the sensor against water/steam (it's a 3-pin active part, not a bead). ### 4.2 Precision reference + setpoint ladder - **VREF_PRECISE:** LM4040-2.5 (2.500 V shunt reference) + bias resistor from +5 V (e.g. ~2.7 kΩ). Absolute accuracy now matters (not ratiometric). - Ladder GND → 2.500 V; SW1 taps land on the LM35 setpoint voltages: | Ref | Segment | Calc. | Use (E96/std) | |-----|----------------------|---------|---------------| | R1 | GND → 60 °C (0.60 V) | 6.00 kΩ | 6.04 kΩ | | R2 | 60 → 70 °C | 1.00 kΩ | 1.00 kΩ | | R3 | 70 → 80 °C | 1.00 kΩ | 1.00 kΩ | | R4 | 80 → 85 °C | 0.50 kΩ | 499 Ω | | R5 | 85 → 90 °C | 0.50 kΩ | 499 Ω | | R6 | 90 → 100 °C | 1.00 kΩ | 1.00 kΩ | | R7 | 100 °C (1.00 V) → ref| 15.0 kΩ | 15.0 kΩ | | RT | master cal trim | — | 500 Ω 10-turn (series w/ R7) | Ladder current ≈ 100 µA. Setpoint voltages = temp × 10 mV/°C — clean and even. ### 4.3 Comparator + hysteresis - **U1:** LM393. LM35 OUT → (−); SW1 tap → (+) via **R_in = 10 kΩ**. OUT HIGH = cold = heater wanted. Pull-up 10 kΩ on OUT to +5 V. - **Rf:** 2.4 MΩ from OUT to (+) → maintain band ≈ ±1 °C (R_in/Rf · Vout_swing). Band varies slightly with selected tap impedance. ### 4.4 Mode latch (one-shot vs hold) - **U2:** CD4013 D flip-flop. D = HIGH. CLK = comparator on→off ("reached") edge (RC-debounce if it chatters). CLR = power-on-reset cap **and** SW1 common (changing temp re-arms). Q′ → enable gate. - **SW3 (Hold):** - HOLD → forces enable gate's 2nd input HIGH (latch bypassed → cycles forever). - ONE-SHOT → enable uses Q′ (heater latches OFF after first reach). - Wire SW3 to also pulse CLR so flipping to HOLD resumes cycling. ### 4.5 Heater drive - Enable gate (heater-want) AND **SW2 rocker** (series) → **Q2** (small MOSFET) → relay coil **K1** (with flyback diode) *or* **SSR1** control input. - K1/SSR1 switches mains to the element, downstream of **F1 (fuse)** and **TC1 (independent thermal cutoff)**. ### 4.6 Clicker - Comparator on→off edge → one-shot pulse → **Q1** (logic-level MOSFET) → **L1** mini push-pull solenoid (5 V, ~3.5 mm stroke, Adafruit 2776 class) + **D1** flyback diode. Plunger strikes a tactile keyswitch (switch gives the click + bump; pick the strike target/resonator for pitch). - HOLD → ticks each maintain cycle; ONE-SHOT → single tick. Add jitter to the pulse for Geiger-style randomness instead of metronomic ticking. ### 4.7 Meter driver - **U3:** op-amp + transistor current sink. LM35 OUT → op-amp (+); meter in the collector/drain; Re in emitter/source. `I_meter = V_LM35 / Re`, independent of coil resistance. **No offset stage** (0 °C already = 0 V). - Re by movement: **1 mA → Re = 1.00 kΩ**; **100 µA → Re = 10.0 kΩ**. - Dial is **linear 0–100 °C** — re-letter the М205 face 0→100. --- ## 5. Safety notes - **SW1/SW2/SW3 are all low-voltage.** No panel switch carries element current; mains is switched only by K1/SSR1 (mains-last). - **F1 fuse + TC1 thermal cutoff in series with the element are mandatory.** The comparator/latch is *control*, not *protection* — it can't save you from a welded relay or a failed/detached sensor. - **Always-on rail = always-live internals.** True off is unplugging; the rocker only stops heating. - Keep the meter/logic rail decoupled from heater switching transients, or the needle twitches and the latch can glitch each cycle. --- ## 6. Open unknowns (measure before committing values) 1. **М205 movement:** full-scale current and coil resistance — sets Re. Measure first; guess wrong and the needle pins or barely moves. 2. **Solenoid force/stroke** vs the chosen keyswitch (~0.5 N, ~2 mm): confirm force *at* the working stroke (not headline force) and intermittent duty at the fastest click rate (near boil). 3. **LM35 low-end:** reads down to ~+2 °C with a GND pull-down; true 0 °C needs a negative supply. Decide if that matters for the dial's bottom mark. 4. **Sensor sealing/placement:** the LM35 must be waterproofed; element-mounted (fast, overshoots) vs water-immersed (accurate, laggy) changes hysteresis feel. 5. **VREF_PRECISE accuracy** now sets setpoint accuracy directly (no ratiometric cancellation) — use a ≤0.5% reference and the master trim RT to null offset. ```