YABSPD
YABSPD is a double-redundant Brake System Plausibility Device for Formula Student applications. Two fully independent analog circuits — built from different component manufacturers — monitor throttle position and brake pressure simultaneously, tripping a 35A automotive relay with a user-selectable AND logic between both circuits.
Features
- Double redundancy — Circuit 1 and Circuit 2 operate fully independently with different component manufacturers for enhanced safety
- Circuit 1: LM2903M8 comparators + 74LVC2G08 NAND logic
- Circuit 2: LM393B comparators + NL27WZ08 dedicated AND/OR logic
- Selectable operation: Circuit 1 only, Circuit 2 only, or both (AND of both outputs) via solder bridges SB1/SB2
- Omron G8G-17R 35A automotive relay with N-FET (PMV20XNER) gate drive
- XT60 IP67 input connector for high-current traction path
- 100–300 ms input jitter filter (RC delay, 27kΩ + 10µF) on all four sensor inputs
- 10-second timeout — RC timer (510kΩ + 100µF) latches the fault; requires manual reset
- Manual reset — tactile button (×2, one per circuit) + JST-XH external reset connector
- +100 mV comparator hysteresis to prevent false triggering at threshold
- Adjustable reference voltages via 100kΩ trimpots (VR1–VR6) or fixed resistor networks (pre-calculated, DNP swappable)
- Optional input attenuation — DNP resistor dividers on all four inputs for signals > 5V
- Weak pull-up failsafe on all comparator inputs — open sensor defaults to fault condition
- Status LEDs: OUT1 (red), OUT2 (red), OUT1|OUT2 (green), fault indicators per comparator output
- 5V regulated supply — L78M05 linear regulator from VBATT
- 4× JST-GH 4-pin sensor inputs (Throttle 1, Throttle 2, Brake 1, Brake 2)
Hardware
Comparators
| Circuit | Throttle comparator | Brake comparator | 10s timer comparator |
|---|---|---|---|
| Circuit 1 | LM2903M8-13 (IC1A) | LM2903M8-13 (IC1B) | LM2903M8-13 (IC3A) |
| Circuit 2 | LM393BIDR (IC4A) | LM393BIDR (IC4B) | LM393BIDR (IC6A) |
- Open-collector outputs with 27kΩ pull-ups (+5V)
- ~100mV hysteresis added via positive feedback resistor network (360kΩ feedback, 4.7kΩ / 15kΩ divider)
- Each comparator monitors its input against an independently adjustable reference voltage
Input Conditioning (per channel)
- Jitter filter: 27kΩ series resistor + 10µF capacitor → 270ms RC time constant
- Failsafe pull-up: 27kΩ to +5V — if sensor wire is disconnected, input floats high → fault
- Optional attenuation: DNP resistor divider on RAW input (for sensors with >5V output)
- Input connectors: SM02B-GHS-TB JST-GH (signal + S_REF tie point)
Reference Voltage Networks
Each reference (Throttle_REF_1, Brake_REF_1, Throttle_REF_2, Brake_REF_2, 10s_REF1, 10s_REF2) is adjustable via a 67YR100KLF 100kΩ trimpot between +5V and GND, or can be set to fixed values using the pre-populated resistor divider networks:
| Reference | Default fixed value | Divider ratio |
|---|---|---|
| Throttle_REF_1 | 0.7 V | 6.144 (27kΩ / 3.3kΩ + 390Ω) |
| Brake_REF_1 | 0.5 V | 8.947 (27kΩ / 2.2kΩ + 2.2kΩ) |
| Throttle_REF_2 | Adjustable | Same topology, different values |
| Brake_REF_2 | Adjustable | Same topology, different values |
10-Second Timeout Circuit (per circuit)
- RC time constant: 510kΩ × 100µF = 51 seconds (time to charge beyond 10s_REF threshold ≈ 10s depending on trimpot setting)
- Once the timer comparator (COMP 3 / COMP 6) trips, it latches the output LOW
- Manual reset discharges the capacitor via reset tactile (S1 / S2) or external JST-XH connector (P2)
- 10s_REF trimpot allows fine-tuning of the exact timeout duration
Logic and Output Stage
- Circuit 1 AND: 74LVC2G08GS (IC2) — 2-input AND gate, Throttle_1_OUT AND Brake_1_OUT → AND1_OUT
- Circuit 2 AND: NL27WZ08USG (IC5) — 2-input AND gate, Throttle_2_OUT AND Brake_2_OUT → AND2_OUT
- Final AND: 74LVC2G08GS (IC7) — AND1_OUT AND AND2_OUT → OUT1|OUT2
- Solder bridges: SB1/SB2 allow bypassing either circuit's AND output for single-circuit or dual-circuit operation
- Relay driver: PMV20XNER N-channel MOSFET (Q1), 100Ω gate resistor, 4.7kΩ pull-down
- Relay: Omron G8G-17R_DC12, 35A, COM/NO contacts, coil driven from VBATT via MOSFET
- Flyback diode: B140Q-13-F Schottky across relay coil
Truth Table (OUT̄ = relay coil drive, active-low fault)
| BR_TH1 | TR_TH1 | BR_TH2 | TR_TH2 | OUT̄ | Condition |
|---|---|---|---|---|---|
| 0 | 0 | 0 | 0 | 1 | Normal operation |
| 0 | 0 | 0 | 1 | 1 | Only Throttle 2 high |
| 0 | 0 | 1 | 0 | 1 | Only Brake 2 high |
| 0 | 0 | 1 | 1 | 0 | FAULT — Circuit 2 |
| 0 | 1 | 0 | 0 | 1 | Only Throttle 1 high |
| 0 | 1 | 0 | 1 | 1 | Both Throttles high |
| 0 | 1 | 1 | 0 | 1 | Brake 2 + Throttle 1 |
| 0 | 1 | 1 | 1 | 0 | FAULT — Circuit 2 |
| 1 | 0 | 0 | 0 | 1 | Only Brake 1 high |
| 1 | 0 | 0 | 1 | 1 | Brake 1 + Throttle 2 |
| 1 | 0 | 1 | 0 | 1 | Both Brakes high |
| 1 | 0 | 1 | 1 | 0 | FAULT — Circuit 2 |
| 1 | 1 | 0 | 0 | 0 | FAULT — Circuit 1 |
| 1 | 1 | 0 | 1 | 0 | FAULT — Circuit 1 |
| 1 | 1 | 1 | 0 | 0 | FAULT — Circuit 1 |
| 1 | 1 | 1 | 1 | 0 | FAULT — Both Circuits |
Power Architecture
- Input: VBATT (car accumulator / 12V supply) via XT60 IP67 connector (J5)
- 5V rail: L78M05CDT-TR linear regulator → powers all comparators, logic, and references
- Bulk decoupling: 10µF × 2 on VBATT; 100µF × 3 on signal paths
- JST-XH 2-pin (P1): external +VBATT input solder alternative
Headers & Connectors
| Connector | Type | Signals |
|---|---|---|
| T1 IN | SM02B-GHS-TB (×4-pin) | RAW_Throttle_1, S_REF_1 |
| B1 IN | SM02B-GHS-TB (×4-pin) | RAW_Brake_1, S_REF_2 |
| T2 IN | SM02B-GHS-TB (×4-pin) | RAW_Throttle_2, S_REF_3 |
| B2 IN | SM02B-GHS-TB (×4-pin) | RAW_Brake_2, S_REF_4 |
| OUT (relay NO) | 220AMA04R 4-pin | COM_1, COM_2, N.O._1, N.O._2 |
| OUT1 / OUT2 | Solder pads | Individual circuit outputs |
| XT60 | XT60PW-M (IP67) | +VBATT, NEG |
| Reset | S2B-XH-A (P2) | RST_Button, GND |
| VBATT in | S2B-XH-A (P1) | +VBATT, GND |
Schematics / PCB / 3D
Keybinds
- Select
LMB - Zoom In/Out
Scroll - Pan
RMB Drag - Rotate in 3D
LMB Drag - Reset View
R - Switch to 2D
2 - Switch to 3D
3 - Next / Previous Layer
+/- - Flip Board Top / Bottom
F
Quick start
- Connect VBATT (12V) to the XT60 IP67 connector
- Connect throttle position sensors to T1 IN and T2 IN
- Connect brake pressure sensors to B1 IN and B2 IN
- Adjust the trimpots (VR2, VR3, VR5, VR6) to match your sensor output voltages at the trip threshold, or populate the pre-calculated fixed resistor networks
- Set VR1 / VR4 to configure the 10-second timeout reference
- Select circuit operation using solder bridges SB1 / SB2
- Wire the relay NO contacts (OUT connector) in series with your AIR (Accumulator Isolation Relay) interlock chain
Safety
- This board is a safety-critical device — verify correct operation with known-good sensor signals before installing in the car
- Double-check reference voltages with a multimeter before connecting live sensors — an incorrectly set REF will cause either permanent fault or missed fault conditions
- The 10s timer latches — a sustained simultaneous brake + throttle event requires a manual reset press before the relay will re-engage
- Open-sensor failsafe is active — any disconnected sensor input will pull high via the 27kΩ pull-up, tripping that comparator; do not leave inputs floating during testing
- Verify solder bridge configuration (SB1/SB2) before power-on — incorrect bridge state may disable one or both redundant circuits
- XT60 is rated for the traction circuit current — ensure the relay contacts and wiring are rated for your maximum motor current