Nuts & Volts - September 2004

Q&A

+5V

ANT

Figure 3

1 - 3 ft

1M

10k 100k 33k ₈₄

.001

7

10uH .001

3

Pulse 0.1 _Out

+

270k

2N3904

10pF

₆555

2

Piezo Buzzer

^1uH270k

1

0.1

5

-

.01

Lightning Detector

Figure 3. The receiver is tuned to 300 kHz; the sensitivity is adjusted by the length of the antenna in the range of 1 to 3 feet. The longer the antenna, the more sensitive the receiver. The receiver outputs a pulse to the 555 monostable timer when a lightning strike is detected, which causes the piezo buzzer to beep.

A 5 volt Pulse Out signal is also available to start a timer, which you can stop manually when you hear the thunder (or build your own sonic thunder detector). That will give you the distance to the strike. To triangulate, you’ll need two or more receivers and a software program (like Excel) to do the math.

more often than not, I forget to shut off the light. Since the shed has no windows, I never notice that the light is on until my next trip to the shed. What I’d like is a timer that would automatically turn the light off after, let’s say, 15 minutes.

The switch should work normally if you flip it off before the time expires; otherwise, the timer would take control and turn off the light. Ideally, all the components would fit in a standard electrical utility box, along with the switch. An adjustable time delay would be nice. Any ideas on how I might build such a device?

Dick Baloh via Internet

shed and moving around, the light remains on. Of course, it has its drawbacks. If you working in an area that has you shielded from the sensor, you could find yourself in the dark. So, I can see why you might want to build your own timer circuit with a predictable off time. For this, a single 555 IC and an optoisolator (Figure 4) will do the trick and the circuit will easily fit into the utility box.

This circuit requires that you remove the wall switch from the AC wiring and use it to switch the 9 volt power source instead. This can be a 9 volt battery, but I’d use a wall-wart because the circuit won’t work if the battery goes dead. Turning “on” the light applies voltage to the 555 chip. The 0.1 µF cap on pin 2 forces the trigger input low during power-up and starts the timing period.

The time is adjustable between 2 and 20 minutes using the 1M pot. If you turn off power to the chip before the time expires, the optoisolator’s LED goes out and the light turns off. If you forget and leave the switch on, the 555 timer will turn off the optoisolator LED and do the job for you. When building the circuit, be sure to keep the AC mains as far away as possible from the wall switch and 555 circuitry.

Outbuilding Timer

Q. When I want something from my shed, I flip on the light, get what I need, and leave. Of course,

A. My first suggestion is to install a motion detector wall switch, like the one made by Eagle that’s available at most hardware stores ($19.95). As long as you are in the