11 KiB
11 KiB
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)
- М205 movement: full-scale current and coil resistance — sets Re. Measure first; guess wrong and the needle pins or barely moves.
- 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).
- 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.
- Sensor sealing/placement: the LM35 must be waterproofed; element-mounted (fast, overshoots) vs water-immersed (accurate, laggy) changes hysteresis feel.
- VREF_PRECISE accuracy now sets setpoint accuracy directly (no ratiometric cancellation) — use a ≤0.5% reference and the master trim RT to null offset.