Nuts and Volts - December 2015

made power supplies when the use of an external supply is not possible or desired. These can be found almost anywhere for next to free. Just place the supply in a bench vice right on the seam and apply slight pressure.

Most of the time, you’ll hear a ‘pop’ and the glued seam can easily be pulled apart. You may have to put pressure on a couple of spots, and occasionally you may have to help it out with a flathead and a hammer, but it’s just like shucking an oyster! Figure 5 shows the before and after of this painless operation.

My original schematic called for a higher voltage with lower current for the IR LEDs I was using at the time, so I initially designed this around a 9V power supply fed into Vcc at X3-1 with a 7805 5V regulator to power the rest of the circuit. Pulsing the LEDs at the higher current, however, meant that the 9V supply (a small 200-300 mA type) could no longer supply the necessary power, and the regulator was struggling to keep up — even after adding a heatsink. Both were running way too hot.

Since everything is now powered by 5V anyway, I found a 5 VDC 2. 5 adapter in my wall wart drawer (everybody has one of those, right?) and did away with the regulator. Of course, you are welcome to try the 7805 with a 9V supply if you like, but I highly suggest the use of a 5V supply rated at minimally 1.5 amps (if not higher) fed into Vcc instead. If you are using the printed circuit board (PCB) pattern and go with the 5V input, just add a jumper across pads 1 and 3 of IC3 and leave that component out. Of course, a carefully laid out perfboard works very nicely, too (Figure 6).

WARNING

Before going any further, I would be remiss in this day and age if I didn’t add the obligatory “don’t try this at home” warning, so ... this design is perfectly safe so long as you understand AC line voltage and are comfortable attaching devices directly to the AC line. The only two potentially dangerous points on this circuit

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■ FIGURE 5. A wall wart supply — before and after.

■ FIGURE 6. A well laid out perfboard.

Semiconductors Q1 TIP122 NPN Darlington Q2-5 2N3904 NPN transistor IC1, 2 LM555 timer IC4 74LS109A J-K flip-flop D1 1N4004 rectifier LED1 3 mm Red LED LED2 3 mm Green LED IR_LED1, 2 OP298

IR_Q1, 2 OPB598

• IR_LED and IR_Q sold in matched pairs — Digi-Key OPB100Z IC3 LM7805 (see text)

Resistors (all 1/4 watt)

R1, R2 22 ohm R3 10 ohms R4 200K trimmer R5, R9, R10 10K R6 4.7K R7 150 ohms R8, R11, R13, R15, R18 1K R12 2.2K R14, R17 180 ohm R16 100K R19 6.8K R20 100 ohms Capacitors C1 .001 μf C2, 5 10 μf 16V C3, 4.01 μf Misc K1 5 VDC SPST relay – Digi-Key 255-3442-ND JP1 Three-pin male header X1 Two-position screw terminal — Digi-Key ED2609-ND X3-5 Two-position screw terminal — Digi-Key ED2653-ND