DESIGN BRIEF AND COMPONENTS

A brief description of the minimal performance requirements: a non-stable power supply simple design, no relays an alarm that would switch to ‘on’ like a ‘flip-flop’ rather than turning on slowly.

Low current in the ‘off’ position Common available parts

Design requirements:

Vs (the +ve supply voltage) is both the power supply of the unit and at the same time the voltage being sensed.

With a Vs of 11.6V or higher the alarm must be ‘off’.

When Vs drops to 11.5V or lower, the alarm must be ‘on’. Calculations are done using an 11.5V supply as that is the critical minimal +ve voltage.

A prototype was made first.

Components list:

Vero Board, Mounting box

R1 - 4.7k resistor

R2 - 11k resistor. This value determines the level of the critical switching voltage of Vs at 11.5V. Other resistor values of R2 will create different critical supply voltages. Instead of R2 a choice of resistors can be connected from R3 to Vs through a multi-position switch for different critical voltage choices. R3 - 10k resistor

R4 - 470R resistor

D1 - Zener diode (500mW, 5.6V) D2 & D3 diodes (Schottky 0.4V, 110V, 750mA)

D4 - LED 3mm, red

C1 - electrolytic capacitor (35µF, 25V)

F1 - fuse-S 250mA (in + line)

IC1 - op-amp 741c (DIL8)

VR1 - 10k lin preset potentiometer Q1 - transistor (PNP, 2SB171 or BC557 or other*)

BZ1 - buzzer 12V

Not displayed on diagram:

D5 - Optional 1A diode rectifier for reverse polarity safety between the battery connections + and – in reverse position, ie non-conducting. The marking ring is connected to + but not in the power supply line!