[Sal] sent us his digital electric meter monitor, which right away made us nostalgic for some of Forrest Mims’ books. Sal’s schematic and circuit description are similar to Forrest’s style, and we indicate that as a compliment. even in today’s world of CAD and EDS packages, sketching out a circuit by hand is often both much easier and faster. The schematic isn’t the only classic aspect of [Sal’s] design. He’s collecting data using a parallel port on an unused PC: in this case, a Toshiba Libretto running Windows 95. before low-cost flash-based microcontrollers and dev boards were available, the PC parallel port was the go-to hardware hacking interface for lots of of us. plenty of the software running those old hacks was written in basic, and [Sal’s] meter is no exception. His software runs on Microsoft QBasic, which shipped with Windows 95.

The circuit takes advantage of the digital meter’s output: a 10 ms pulse for every 1 Wh of energy used. An IR photo detector from RadioShack detects the meter pulses, which are amplified by an LM324 Op Amp. An NPN transistor then shifts the output to send it to two 74LS73 JK flip flops. The first flip flop uses a transistor to drive an LED for visual output. The second JK flip flop sends the data to the PC. The flip flop has the effect of dividing the number of meter pulses by two, creating a much longer toggled signal that a PC can better detect.

Although using an AVR or photo would consume less power, [Sal’s] setup has already a lot more than paid for its power usage. By monitoring and adapting his electrical usage, [Sal] is saving $20 a month on his electric bill. We’ve included [Sal’s] circuit diagram and source code after the break (apologies to our readers on RSS).

First [Sal’s] test program:

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CLS

SCREEN 12

10 W = INP(&H379)

IF W <= 127 then GOTO 10 R! = TIMER 20 W = INP(&H379) IF W > 127 then GOTO 20

30 W = INP(&H379)

IF W <= 127 then GOTO 30

S! = TIMER

T! = S! – R!

IF T! = 0 then GOTO 10

KW = 7.2 / T!

PRINT KW;

ON KEY(1) GOSUB 40

KEY(1) ON

GOTO 10

40 END

Here is the main application:

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CLS

REM C:\METER011.BAS

1 screen 12

LOCATE 1, 1: INPUT "INPUT ELECTRICAL METER KWH ", QQ

LOCATE 1, 1: FOR X = 1 TO 40: print " "; : URMĂTORUL

DD1$ = MID$(DATE$, 4, 2) ‘DAYS

TH1$ = LEFT$(TIME$, 2) ‘HOURS

TM1$ = MID$(TIME$, 4, 2) ‘MINUTES

TS1$ = RIGHT$(TIME$, 2) ‘SECONDS

4 A = 0: B = 0: C = 0: D = 0: F = 0: G = 0: K = 0: L = 0: M = 0: N = 0

O = 0: P = 0: Q = 0: R = 0: V = 32: LA = 0: MB = 0: NC = 0: OD = 0: NN = 0

DD = 1: U = 1

DIM A(120): dim B(120): dim C(25): dim D(32)

B = VAL(TM1$)

C = VAL(TH1$)

D = VAL(DD1$)

GOSUB 2010

GOSUB 500

8 Q! = TIMER

10 W = INP(&H379)

IF W <= 127 then GOTO 10 R! = TIMER 20 W = INP(&H379) IF W > 127 then GOTO 20

AA& = AA& + 1

BB& = BB& + 1

30 W = INP(&H379)

IF W <= 127 then GOTO 30

S! = TIMER

IF S! < Q! then S! = S! + 86400 T! = S! – R! IF T! >= 0 and T! <= .5 then GOTO 10 K = (7.2 / T!) locate 1, 5: print using "##.