[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 $20 a month on his electric bill. We’ve included [Sal’s] circuit diagram and source code after the break (apologies to our readers on RSS).
First [Sal’s] test program:
1
2
3
4
5
6
7
8
9
dix
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
CLS
SCREEN 12
10 W = INP(&H379)
IF W <= 127 then GOTO 10 R! = TIMER 20 W = INP(&H379) IF W > 127 then GOTO 20
30 W = INP(&H379)
IF W <= 127 then GOTO 30 S! = TIMER T! = S! – R! IF T! = 0 then GOTO 10 KW = 7.2 / T! PRINT KW; ON KEY(1) GOSUB 40 KEY(1) ON GOTO 10 40 END Here is the main application: 1 2 3 4 5 6 7 8 9 dix 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 ans 71 72 73 74 75 76 77 78 79 80 ans 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 CLS REM C:\METER011.BAS 1 screen 12 LOCATE 1, 1: INPUT "INPUT ELECTRICAL METER KWH ", QQ LOCATE 1, 1: FOR X = 1 TO 40: print " "; : SUIVANT DD1$ = 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 "##.