How the C5's electrics work: the instrument pod and its ULA chip, the control box, the motor protection layers, the battery gauge, and the charger. This page is based mainly on The Unofficial Sinclair C5 Service Manual. It draws on the original engineering paper by P.J. Milner (Sinclair Vehicles) and Perran Newman (Sinclair Research), who designed the system, and on the redrawn schematics (Figures 1–4) published by C5Alive.

• The C5 had to be safe for untrained users aged from 14 upwards (the oldest owner recorded during development was 92), and the designers expected deliberate abuse.
• There is no proportional speed control. The motor is simply switched on or off by a relay. Because the permanent-magnet motor has a fixed no-load speed, the 15 mph legal limit is met by the motor itself.
• So the electronics are almost entirely about protection: the motor against overheating, and the battery against over-discharge and wrong charging. A master switch, fusing and weatherproofing complete the system.

 USER CONTROLS            INSTRUMENT POD                     LAMPS
 motor switch ──┐   [motor-load bar] [buzzer] [battery bar]   front / rear
 lights ────────┤              │                             horn, indicators x4
 indicators ────┤              │                                  │
 horn ──────────┴──────────────┼───────── flasher ────────────────┘
                               │
 BATTERY + CONNECTOR       CONTROL BOX                       MOTOR + PROTECTION
 key master switch    current shunt (1 mΩ)                thermistor in hollow
 manual cut-out  ───► motor relay ─► commutation diode ─► armature shaft;
                      fuses, 5 V regulator, stall          bi-metal thermal cut-out
                      detector (LM358)                     on motor case

The main parts:

1. Instrument pod (IP): the dashboard under the canopy. It holds the two 5-segment LED bar graphs, a piezo buzzer, and the Ferranti ULA chip that does nearly all the logic.
2. Control box (CB): the sealed box at the rear. It holds the motor relay, the current shunt, the 5 V regulator, the stall detector, the fuses and the harness junctions.
3. Motor protection: a thermistor probe inside the armature shaft, plus a bi-metallic trip on the motor case.
4. Battery connector and manual cut-out: the key-operated master security switch on the battery top.

The Sinclair team chose a permanent-magnet motor with high stall torque, for hill climbing and brisk acceleration in traffic. That makes it easy to cook the motor, so it is guarded three ways:

The engineers tried two other approaches first and dropped them. One was an analogue “mathematical model” of motor heating built from integrated circuits. The other was a physical thermal model, a heater on a metal block clamped to the motor. Thermocouple tests showed the shaft-air temperature tracked the deep armature temperature well enough to use in production. Newman later recalled that motor failures were “virtually unknown” before Sinclair Vehicles went into administration.

• The right-hand bar graph shows the battery's compensated terminal voltage under load. The ULA adds back the voltage the battery loses under load (assuming about 18 mΩ of internal resistance) and uses the shunt current to do it.
• The five segments cover 12.0 V down to 10.5 V in equal steps. Battery voltage falls unevenly as it discharges, so the display drops slowly at first and faster near empty. That gives plenty of warning.
• The gauge can only count down. A segment only goes out after the voltage has stayed below its threshold for a 20-second delay, so brief dips on hills don't register. The state is held as long as the battery is connected, because the +5VB rail is always live.
• When the last segment goes, a 20-second audible warning sounds before cut-out, so you can finish a manoeuvre. Wikipedia's sources describe the last light as “about ten minutes left”.
• The cut-out point is set by trimmer VR1 in the pod. See Setting up VR1 below.

• The left-hand bar graph shows motor current, measured as the voltage across the 1 mΩ shunt (1 mV per amp).
• Driving guide from the Service Manual's range chart: cruising in the right-most green segment gives about 13 miles from one battery. Amber means heavy load and red means very heavy load, so pedal to help.

The calibration comes from the Hoover Service Engineer Training Notes (1984). Each segment is worth about 15 A. The yellow segment at each end is always lit when the switch is at D, as a reference.

Key figures from the same notes:

• 30 A is the maximum continuous current the motor can take without a destructive temperature rise. Anything in amber or red means the motor is heating up, and the further right, the faster it heats.
• At 70 A, a cold motor overheats in about 10 minutes.

John Wilson's ammeter retrofit notes agree: about 29–30 A on the flat, about 40 A climbing while pedalling, and under 5 A when coasting.

• IC1 = Ferranti ULA (uncommitted logic array), marked 5G014 MK4 on the redrawn schematic. It is a semi-custom chip in a socket and it cannot be repaired.
• Power: +5VB comes from the battery through regulator TR3 (a 78L05) and is always present, which is why the gauge remembers its state. +5VA comes from the control box and is only present when the C5 is switched on, so the displays only light when powered up.
• The two bar graphs share a 5-line multiplexed bus (SD1–SD5). TR1/TR2 (FST290) switch the anodes. These are selected transistors with V_ce(sat) < 0.5 V at 100 mA.
• There are three internal oscillators:
  • R10/C6: about 0.4 Hz, divided by 8 to give the 20 s fuel-gauge delay.
  • R7/C4: 50 Hz, used for the flash rate and the lock-out timing.
  • R11/C7: about 9.5 kHz, divided by 2 to drive the piezo buzzer and the multiplexer.
• Motor switch path: the handlebar button sends 12 V through a 15 kΩ resistor in the CB to ULA pin 21. The ULA then drives the CB relay transistor from pin 13. This is how the pod can refuse to start the motor.
• Thermistor: a potential divider with R9/R20 across +5VB, feeding comparator pins 5 and 6, which trip at 0.5 × Vcc.

A single 3-bit DAC and comparator are time-shared between the two displays.

1. In motor-current mode, a counter ramps the DAC up until it matches the shunt-derived voltage. The count is the load display, which is sampled and held.
2. In fuel mode, the DAC is set to the current fuel count. If the compensated battery voltage stays below it for the full 20 s delay, the fuel count drops by one.

The compensation equation from the manual is I2 = 0.5 (I1 − I6 + I4). In words: the reading rises with battery voltage (I1), is offset by VR1/R3 (I6), and is pushed up by motor current (I4).

• Connectors:
  • X9 goes pin-for-pin to X1 on the pod (the red pod loom).
  • X6 carries the thermistor (pins 1–2), the motor over-temperature trip (pins 4–5) and a remote cut-out (pins 7–8).
  • The heavy-current connections are on 4–5 mm studs inside the user-accessible part of the box.
• Relay: a specially developed Ital-Amec unit, specified for 10,000 make/break cycles at 140 A. TR3 (ZTX449) drives it, and ZD1 (a BZY88 zener) clamps the coil at turn-off, which speeds contact opening.
• D1, the commutation diode (1N5401 class, across the motor): this is the critical part that suppresses arcing at the relay contacts. See Known problems.
• IC1 is the 5 V regulator for the pod display.
• IC2 (LM358 dual op-amp) does two jobs. It turns the shunt signal into a current sink for the ULA, through R9. It also amplifies the shunt signal (gain 100/15) into the integrator C3, which drives the latching pair TR1 (BC212L) / TR2 (BC182L), i.e. the stall detector.
• Fuses: FS1 and FS2 (1 A and 5 A) on the auxiliary circuits.
• The two plastic halves are ultrasonically welded. One part is a tamper-proof cavity; the other is user-accessible behind a screwed cover. The cable entry points down, with a splash grommet.
• All motor wiring is 6 mm² automotive PVC cable with machine-crimped ring and 6.35 mm spade terminals.

• Battery feed: brown = +12 V, blue = 0 V (original power loom from the key-switch block to the control box).
• Thermistor: purple and grey. Thermal cut-out: orange.
• Indicator/horn harness: Violet/White, Red/White, Blue/White, Black/White, Green/White, Brown/White and Orange/White pairs. The kits use 4-way and 3-way terminal blocks; see Accessories.
• Some C5s have an alternative harness. You can recognise it by a cluster of 6 connectors fixed under the central tunnel near the rear brake arm, and by having no horn connectors taped to the steering bars.

• A two-stage constant-current then constant-voltage profile, made by a hybrid circuit with laser-trimmed resistors driving 2 SCRs as a phase-controlled current source.
• The whole charger is potted in resin for cooling and weatherproofing. Two LEDs show charging and finished.
• It charges the battery in place or removed. Full charge takes about 8 h.

These modifications protect against static damage to the ULA and LM358 failures:

• Pod: fit a 47 kΩ resistor (the “Known problems” section suggests 10 kΩ) in series with the track from X1 pin 6 to IC1 (ULA) pin 3. Add 1N914 catch diodes to Vcc and GND on IC1 pins 3/4.
• Control box: power IC2 (LM358) pin 8 from +5VA instead of +12 V.

Follow these steps on the bench, with the pod out of the vehicle.

1. Connect ULA pin 9 to pin 22 (GND). This forces fuel-gauge-only mode at power-up.
2. Apply the required cut-out voltage, typically 10.5 V, to +BATT at the top of R17.
3. Apply +5 V to Vcc (pin 8).
4. Set I4 to zero, for example by grounding pin 3.
5. Using a meter of at least 1 MΩ impedance, measure between pin 23 (+) and pin 27 (−).
6. Adjust VR1 for +200 mV. If it won't reach that, increase R3 by about 1 kΩ.
7. Remove the pin-9 link and check normal operation. Each segment should drop out after about 20 s, with two hidden segments below the lowest visible one.

• Pod: uprate C12's voltage rating, and double R17 to 94 kΩ so the fuel gauge reads a pair of batteries in series.
• Control box: uprate C2, give IC1 a bigger heatsink, and put a resistor in series with the relay coil.
• Alternative: run the electronics on 12 V and change only the battery-sense input through R17.

Many owners now fit a PWM controller instead. See Modifications.

• The Unofficial Sinclair C5 Service Manual, pp. 4–11, 19–27.
• C5Alive redrawn schematics: Fig. 1 ULA block diagram, Fig. 2 ULA front end, Fig. 3 instrument pod, Fig. 4 control box. These were redrawn by Chris Gavin-Egan from drawings supplied by Roy Woodward.
• Perran Newman, “Prescott Testing & C5 Motor Protection”, c5owners.com.
• John Wilson, “Simplified Relay Controls”, c5owners.com.