Friday, September 5, 2008

Big Red, your basic flasher

A basic chore associated with airplanes is making something flash. Sometimes you want them to flash fast, other times you want it to go slow. What's getting flashed may demand a couple of amps whereas other circuits may need only a fraction of an ampere. This can lead to the use of half a dozen different flashers, resulting in a heavy and complicated circuit.

The circuit shown here is a basic One-Size Fits All.

The basic idea is to use an integrated circuit (ie, the Ne-555 chip) to toggle the circuit at a given rate. The output of the flasher is then fed to a small RELAY capable of handling about an amp. If you need to flash a higher amperage you simply wire the existing relay to one having a higher rating.

This isn't a new circuit. If you dig around you can probably find a circuit-board mask for it in one of the archives. The advantage of this circuit is that it's very inexpensive and may be sized to handle anything from a 1A nav light to a 10A strobe by simply selecting a suitable relay.

NOTE (17 SEPT 2008)
One of the comments suggests replacing the fixed resistors R1 & R2 with variable resistors. In fact, that is what I did when bread-boarding this circuit. Once I'd found a setting that gave the approximate flash rate & duration needed for an automotive turn signal, I lifted one leg of the variable pots and measured their resistance. This was matched to the nearest standard value FIXED resistor. The purpose was to make the module easy to fabricate by guys who weren't born with a soldering iron in their hand. Fabricated from all fixed or sealed components, the finished circuit could then be potted with epoxy or similar sealant, rendering it weather-proof. The relay of course may be mounted almost anywhere. -- rsh


Anonymous said...

Is there anything you can't build, Bob? Man, I wouldn't be surprised if you wrote an article about genetic engineering or a guide to programming video games!

For those wishing to implement Bob's flasher circuit, here's a tip: don't bother looking for the bits at Radio Shack. While they USED to stock a wide variety of components and integrated circuits (the TTL7400 series, timers, relays, power supplies, etc.), sadly those days are long gone. They used to be staffed by people who understood radio communication (and often built their own equipment such as antennas, frequency filters, or tube amplifiers) and could "talk shop" with you, but nowadays seem to be staffed by teenagers who are focused solely on selling cell phones and radio control toys.

The few components they DO happen to stock are so incredibly overpriced (e.g. several DOLLARS for a handful of resistors when they should be less than a penny each, $5 for a 2-pack of switches, etc!). IMO it's rarely worth the trip.

For electronics components, I recommend digikey, jameco or mouser. All offer great service & prices a very small fraction of the Radio Shack store. The only drawback is the three vendors above have such an enormous catalog, it might take a bit of work to choose exactly the right part.

(Anyone remember the "Battery Of The Month Club" or the TRS-80 computer? Or even the days when they had a self-serve vacuum tube tester in the store? Ah, the good old days.)


Retro said...

Good to see you back :-)

Anonymous said...

I second that!
Hope your getting better.

Anonymous said...

Let us know how you're doing Bob. Not to get all mushy here but, you're in my and many other's thoughts. Get better.

Anonymous said...

Incidentally Bob as you're probably aware, replacing resistors R1 and R2 with potentiometers whose center values approximate the value of the fixed resistors in the circuit will allow you to change both the repetition rate and the on duration. I think in this application linear taper (as opposed to logarithmic taper) pots would be preferred.

twofoot said...

Handy! Thanks Bob.