Refer to the schematic in Figure 2.
Let’s start with the power supply for this circuit. A nine
to 12 volt DC unregulated source is fed via connector J4 to
adjustable voltage regulator U4. Related components P1,
R2, and R3 set the voltage to precisely 5. 12 volts. The
reason for this oddball voltage instead of a nice round 5.0
volts is that the PICAXE utilizes its supply voltage as its
reference. With its 10-bit internal ADC (analog-to-digital
converter), this reference voltage will provide a resolution of
exactly five millivolts per step that greatly simplifies
calculations. R1 and D1 provide a voltage of roughly + 6
volts which is used to allow enough input common mode
range for the current-measuring op-amp. The + 5. 12 volts is
also used for most other devices, but it is also converted to
a negative voltage via U3, C3, and C4. This negative voltage
is required by one op-amp section which will monitor the
negative voltage. R0 isolates the power and sense grounds,
but allows a large degree of continuity if one becomes
Since the PICAXE is the heart of the circuit, it is best to
describe the circuit’s operation from its individual pin
functions. Let’s start with the input pins.
Input 0 connects to an external pushbutton switch.
When pressed, a logic high on this pin will cause the
software to toggle the output relay, emit a brief tone by the
piezo buzzer, then update the display accordingly.
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