second and recycle this event every 13 seconds; therefore,
this relay would be engaged for one second for every 13
seconds. However, the entire circuit — relay included — has
to operate from 3 volts at 30 mA. (I’d be happier if it would
operate even at a lower voltage, such as 1.5 volts.) My
problem is that I can’t “amplify” the voltage somehow for
the relay to be actuated just for 1 second.
A. You can’t operate the relay you selected from your
power supply. The relay you specify has a coil resistance of 33 ohms (R = 5/0.15 = 33 ohms), which means
it takes 90 mA to hold the relay in at 3 volts (I = 3/33 =
90 mA). The answer is to use a 3 volt relay and run it at
reduced current, as described in the June 2004 column
(“About Relays”). All Electronics (888-826-5432;
www.allelectronics.com) sells a 3 volt relay (#RLY-
623) that should work. It has a coil resistance of 45
ohms and draws 67 mA at 3 volts. In the reduced current
mode, it draws about 33 mA, which should be within the
capabilities of your power supply. Find the circuit at
The 555 chip has to be a ZSCT1555 from Zetex. Digi-Key (800-344-4539; www.digikey.com) has them in
stock at $3.47 each. Unlike the 555 — which has a minimum operating voltage of 4. 5 volts — the ZSCT1555 operates over a range of 0.9 to 6 volts. The timing resistors and
capacitor are adjusted to your specs, but can be changed
to satisfy any duty cycle. When the output goes low, the
330 µF cap charges through the relay coil and engages the
relay. After the capacitor is charged, the current flows
through the 47 Ω resistor to reduce the relay load. The
1,000 µF cap provides the surge current needed to pull in
Seeking Clock Chip
Q. I have a kit clock that I built in the early 90s that runs
on the MM5314N clock IC from National
Semiconductor. I was wondering if you can help me locate
a few more of these ICs, in case the one in the clock dies?
A. I have one of those kits, too, so I know your concern.
Unfortunately, the MM5314N is almost impossible to
find in the US — and for good reason. An eight-bit MCU like
the 16F628 can do everything it used to — and more — for
less money. Your best bet for a supplier is to look overseas.
The following sites claim to have the MM5314N in stock for
small quantity purchases.
Circle #146 on the Reader Service Card.