Nuts & Volts - September 2004

Q&A

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.

Dave via Internet

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 Figure 5.

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 the relay.

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?

Benjamin Rappe via Internet

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.