Brusa BC29 Amp Hour Meter
Analog front-end
Circuit discussion
Geo Homsy, 2012 - 2014



M1, IC18 provide regulated 5V from the battery.

Q1 and IC6 provide several clock signals: 1024 Hz and 4Hz

The Q4 output of IC6 drives a charge pump consisting of E5, E4, D5, and D6, to provide a negative supply voltage to op amps IC1 and IC2.

R11, R12, R13, R14, IC2:D form a differential amplifier with -2.5V reference output, and incidentally, 1.25V and -1.25V reference outputs as well.

-2.5V reference is buffered by IC2:B to provide negative integrator reset input to analog mux at IC3:14

Shunt differential inputs enter at R1 and R2.  Integrator inputs are filtered by C3, C4, and the op-amp is protected by D1, D2, D3, D4.

IC1, C1, C2, R4, R7 form the main charge integrator.  The integrator output, at pin 6, triggers one of two comparators:  IC2:A or IC2:C.  

IC2:C is triggered to give a low pulse when the integrator reaches -1.25V, signifying a discharge of 1/900 Ah, or 4 Amp-seconds, or 4 coulombs.

Conversely, IC2:A gives a low pulse when the integrator reaches +1.25V, signifying +4 coulombs of charge.

A "charge" pulse sets flip-flop IC4:? (synchronized on the 1024Hz clock), causing the green LED to blink on the display, and causing 1/9 of a 0.01Ah count up on the display.  It also resets the integrator start value by means of gating +5 volts onto pin 2 of analog mux IC3.

A "discharge" pulse similarly sets flip-flop IC4:?, causing the red LED to blink, and causing 1/9 of a down count.  The pulse also resets the integrator by gating -2.5V onto pin 12 of analog mux IC3.

Either pulse is reset on the next 1024Hz clock by the pulse-forming circuit consisting of IC5:C, C11, and R17.

A "reset" pulse is triggered by a funky network of gates and pulse shapers.  I have a scribbled napkin sketch of this; email geo.homsy@gmail.com for more info