# 24V Low Battery Alarm ## Description Low-voltage monitoring circuit for a nominal 24V DC input. The TL431 threshold detector and BC547B driver activate a 5V active buzzer when PLUS drops below about 19.5V. ## Project Details - **Owner:** voron37 - **Created:** 6/24/2026 - **Last Updated:** 6/24/2026 - **Visibility:** Public ## Key Components ### BZ1 — [Buzzer](https://www.flux.ai/jecstronic/buzzer~1j2.md) - Role Details: Active buzzer/zoomer for direct operation from PLUS input. Positive pin to PLUS, negative pin to Q1 collector/BUZZER_NEG; Q1 sinks current when PLUS is below the TL431 threshold. Do not fit a 5V-only BMT-0905XH05 here without a regulator. - Role: Signal Generation - Manufacturer Part Number: Active buzzer / zoomer 12–24V, 2-pin TH - Package: 2-pin through-hole buzzer placeholder - Operating Voltage: 12–24V DCV - Manufacturer Name: FILL IN - Part Type: Buzzer **Pins:** - - - + ### J1 — [691216710002](https://www.flux.ai/adrian95/691216710002~8ea.md) - Connector Type: Terminal Blocks - Role Details: Article: Würth Elektronik 691216710002 2-position 5.00mm terminal block. Pin_1=PLUS, Pin_2=MINUS/GND. - Role: Power Conversion - Part Type: Connector - Manufacturer Name: Würth - Package: 2-position 5.00mm through-hole terminal block - License: https://creativecommons.org/licenses/by/4.0/ - Datasheet URL: https://www.we-online.com/components/products/datasheet/6912167100xx.pdf - Connector Positions: 2 - Manufacturer Part Number: 691216710002 **Pins:** - Pin_1 [pin 1] - Pin_2 [pin 2] ### Q1 — [BC547BTA](https://www.flux.ai/lcsc/bc547bta~cvr.md) - Part Type: Bipolar Transistors - BJT - Role: Switching - Package: TO-92-3 - Role Details: BC547BTA NPN low-side switch. Collector to BUZZER_NEG, emitter to GND, base driven through R4. Suitable if buzzer current stays within BC547B limits; use a small MOSFET if buzzer current is high. - Manufacturer Name: ON Semiconductor - Package or Case Code: TO-92-3_L4.8-W3.7-P2.54-L - Product Info Url: https://lcsc.com/product-detail/Transistors-NPN-PNP_ON-Semicon_BC547BTA_ON-Semicon-ON-BC547BTA_C258144.html - LCSC Part Number: C258144 - Manufacturer Part Number: BC547BTA - JLCPCB Part Class: Extended Part - Datasheet URL: https://storage.googleapis.com/graviton-electric-symbols/document_assets/lcsc/2410010400_onsemi-BC547BTA_C258144.pdf **Pins:** - B [pin 2] - C [pin 1] - E [pin 3] ### U1 — [TL431](https://www.flux.ai/lcsc/tl431~qib4.md) - Datasheet URL: https://www.lcsc.com/datasheet/lcsc_datasheet_1810111320_Shikues-TL431_C111578.pdf - IC Type: Voltage References - Product Info Url: https://lcsc.com/product-detail/Voltage-References_TL431_C111578.html - Role: Voltage Reference - Power Consumtion: 0.001W - Operating Voltage: 2.5V to 36V - Frequency: not specificHz - JLCPCB Part Class: Extended Part - Part Type: Voltage References - Logic Function: Adjustable Shunt Voltage Regulator - Package or Case Code: SOT-23-3_L2.9-W1.6-P1.90-LS2.8-BR - Manufacturer Name: SHIKUES - Package: SOT-23-3 - Manufacturer Part Number: TL431 - LCSC Part Number: C111578 - Role Details: Article: TL431 shunt voltage reference, SOT-23-3, LCSC C111578. Trip ≈ 2.495*(1+68k/10k)=19.46V; above threshold sinks BC547B base control node low. **Pins:** - AN [pin 3] - CAT [pin 2] - REF [pin 1] ### VR1 — [Bourns 3296Y Vertical](https://www.flux.ai/jecstronic/bourns-3296y-vertical~sr3.md) - Part Type: Potentiometer - Package or Case Code: R_POT_PV36Y - Role Details: User-specified Bourns 3329H-1-332LF 0–3.3kΩ trimmer. Wired as fail-safe rheostat: pins 1+2 to R1_TRIM_TOP, pin 3 to THRESHOLD_REF. - Manufacturer Name: Bourns - Manufacturer Part Number: 3329H-1-332LF - License: https://creativecommons.org/licenses/by/4.0/ - Datasheet URL: https://www.bourns.com/docs/product-datasheets/3296.pdf - Resistance: 3.3kΩ - Role: Voltage Division **Pins:** - 1 [pin 1] - 2 [pin 2] - 3 [pin 3] *The full schematic, PCB layout, bill of materials, and interactive design are available at [https://www.flux.ai/voron37/24v-low-battery-alarm~a2](https://www.flux.ai/voron37/24v-low-battery-alarm~a2).* --- *Generated from [https://www.flux.ai/voron37/24v-low-battery-alarm~a2](https://www.flux.ai/voron37/24v-low-battery-alarm~a2)*