Tachometer Output Circuit¶
Purpose: - Many tachometers don't work at idle due to low voltage - Regulator has ultra-sensitive ability to pick up on any signal - Circuit amplifies this to battery voltage for output to any other devices
Circuit Connections¶
1. ESP32 GPIO Input: - ESP32 GPIO pin connects to one end of R78 (2.2kΩ) - This is the 3.3V logic input signal
2. Base Drive Circuit: - Other end of R78 (2.2kΩ) connects to base of Q1 (MMBT5551) - R55 (100kΩ) connects between base of Q1 and GND - R55 ensures base is pulled to ground when ESP32 GPIO is low
3. Transistor Q1 (MMBT5551): - Base: Connected to R78/R55 junction - Emitter: Connected directly to GND - Collector: Connected to one end of R80 (100Ω)
4. Output Circuit: - Other end of R80 (100Ω) connects to TACH_OUT pin - R99 (22kΩ) connects between VIN_2-60-RAW (+Battery) and TACH_OUT - R99 acts as pullup resistor
5. Power Supply: - VIN_2-60-RAW: +Battery voltage (5V to 60V) - GND/GND1: System ground, connected together
6. Output: - TACH_OUT: Goes to dashboard tachometer input
Circuit Operation with ESP32 Square Wave¶
When ESP32 GPIO = HIGH (3.3V): - Base current flows: 3.3V through R78 (2.2kΩ) to base - Base current = (3.3V - 0.7V) / 2.2kΩ ≈ 1.18mA - Q1 turns ON and saturates (VCE_sat ≈ 0.2V) - Current flows: Battery → R99 (22kΩ) → R80 (100Ω) → Q1 → GND - TACH_OUT = 0.2V (essentially 0V)
When ESP32 GPIO = LOW (0V): - R55 (100kΩ) pulls base to ground - No base current flows - Q1 turns OFF (open circuit) - TACH_OUT = Battery Voltage (pulled up through R99) - Only leakage current flows
Frequency Range Analysis (8Hz to 2.64kHz)¶
MMBT5551 Specifications: - fT (transition frequency): ~100MHz - Storage time: ~200ns - Rise/fall times: ~35ns
RC Time Constants: - Base charging: R78 × Cbe ≈ 2.2kΩ × 8pF ≈ 18ns - Output loading: R99 || (Rtach + stray C)
At 2.64kHz (worst case): - Period = 379µs - Rise/fall times (35ns) are 0.009% of period - Switching delays are negligible
Conclusion: The 8Hz to 2.64kHz range is easily achievable with plenty of margin.
Power Consumption Analysis¶
Case 1: 60V Supply, GPIO HIGH (Q1 ON)¶
- Total resistance: R99 + R80 = 22kΩ + 100Ω = 22.1kΩ
- Current: 60V / 22.1kΩ = 2.71mA
- 12V equivalent current: 13.6mA @ 12V
- Power in R99: (2.71mA)² × 22kΩ = 162mW
- Power in R80: (2.71mA)² × 100Ω = 0.7mW
- Power in Q1: 0.2V × 2.71mA = 0.5mW
Case 2: 60V Supply, GPIO LOW (Q1 OFF)¶
- Only leakage current through R99
- Leakage current: ~1µA (MMBT5551 spec)
- 12V equivalent current: 5µA @ 12V (negligible)
Case 3: 5V Supply, GPIO HIGH (Q1 ON)¶
- Current: 5V / 22.1kΩ = 0.226mA
- 12V equivalent current: 0.094mA @ 12V
Case 4: 5V Supply, GPIO LOW (Q1 OFF)¶
- 12V equivalent current: ~2µA @ 12V (negligible leakage)
Case 5: 50% Duty Cycle Square Wave @ 60V¶
- Average current = (2.71mA × 0.5) + (0.001mA × 0.5) = 1.36mA average
- 12V equivalent current: 6.8mA @ 12V average
Power Summary Table¶
| Condition | Supply | GPIO State | Current | 12V Equivalent |
|---|---|---|---|---|
| Extreme High | 60V | HIGH | 2.71mA | 13.6mA @ 12V |
| Extreme Low | 5V | LOW | ~1µA | 5µA @ 12V |
| Typical @ 12V | 12V | HIGH | 0.54mA | 0.54mA @ 12V |
| 50% duty @ 60V | 60V | Square wave | 1.36mA avg | 6.8mA @ 12V avg |
The highest power consumption occurs at 60V with GPIO HIGH, equivalent to 13.6mA at 12V primarily in the 22kΩ pullup resistor.