low-voltage alarm

When a car gets left with a door not closed fully, the door light stays on and drains the battery so the engine won’t start. This project monitors the voltage of a 12 V battery and sounds an alarm if the voltage drops below a configured threshold.

It also has logic-level RS232 ( the 4-pin connector on the right in the photo above) via which it reports the voltage as currently measured.

The schematic was prepared with gschem:

• R1 and R2 serve to divide the 12V down to within the range of the ADC’s 1.1V reference voltage. The actual values ( as opposed to the marked values) of R1 and R2 are measured and inserted into the microcode
• The 100n bypass maintains the voltage level to the AVR during voltage dips as the AVR draws current on clock edges. This helps to make the ADC more accurate
• The buzzer draws 9.6 mA when in operation so it can be driven directly from an AVR pin
• All AVR pins have diodes that clamp to V_cc except RESET, which supports high-voltage programming. Since HV programming is not required, D1 clamps RESET so that static will not damage the AVR
• R3 and C2 serve to pull-up and to stabilise the voltage to protect against the AVR being reset by EMI
• MOSI and MISO are used instead of the traditional TXD and RXD because they’re not so ambiguous. TXD refers to the DTE transmitting, not the DCE. MOSI means “Master Out, Slave In”

Then pcb ( also part of gEDA) was used to design a veroboard:

download

The archive includes full source code, along with gEDA source files for the schematic and a single-sided PCB layout.

• low-voltage-alarm_2009-12-10.tar.gz

parts

• LM78L05 5V voltage regulator
• Atmel ATtiny13
• A piezo-electric buzzer
• capacitors: 100n, 3n3
• headers: 2-pole screw-down, 4-pole keyed
• 1N4148 signal diode
• resistors: 12k, 150k, 4k7