Entries from October 2015 ↓

Light/Dark Activated Switch

These are switches used to turn on lights at dusk and turn them off again at sunrise. Generally, a light sensitive device like a photocell, optical sensor of some sort is ties to an opamp comparator which energizes a relay to switch on the lights. A less sophisticated approach might be a simple mechanical timer. The timer does not need to sense light levels.

An even simpler solution is to use a low voltage mechanical relay energized by a solar panel. This approach assumes that a solar panel system is already in place. Even a 5 watt panel can develop enough power to energize a 5 volt, 150ma relay without significantly effecting its ability to provide useful electrical power.

For the solar panel solution to work reliably the relay needs to be sensitive enough to energise even on a cloudy day or the lights will stay on. Also, the relay needs to provide both normally open and normally closed contacts. The idea is to allow the solar panel energy to turn ON the relay which turns OFF the lights.

To prevent the 18 volts from the panel from over powering the relay a series resistor and 5 volt zener across the relay will keep the relay coil at a maximum of 5 volts. A capacitor across the relay coil would introduce some hysteresis to prevent the relay from cycling on and off at certain light levels.

The rig is so good, it is used by DXpeditions

So goes the sales pitch for a well known amateur radio manufacturer. To be fair, that radio is superior to most. It is sensitive, selective and can be used in crowded RF environments. Why it can even resist blocking interference from a ham next door running full legal limit. Is that really what is needed on DXpeditions?

DXpeditions go to remote locations, are assigned rare call signs, and operate where they are the only radio signal for many miles around. They don’t need special super equipment to make contacts because hundreds of distant hams are going to be falling over themselves for contacts with their rare calls and distant locations. These DX enthusiasts will be using high power and directive antennas to push as much signal as possible toward the DXpedition. The DXpedition is not going to have any trouble hearing them. So what is the real reason DXpeditions prefer that particular radio?

Turns out that the company making the claim offers their radios to DXpeditions free of charge!