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
December 2015 25
■ 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
5 VDC 2A power supply
Speaker wire (to run emitter/receiver pairs)
Small perfboard (for mounting emitter/receiver pairs)
PARTS
LIST