Experiment 8
Making a Flashing Light
When you buy a light string they often add a "flasher." Let's see how that device works.
STEP 1. Find one of the red tipped bulbs on your string of lights. Look carefully at the internal structure. Next to the tungsten filament is another wire. What does it do?
The extra wire acts like a switch. It is made of two kinds of metals that bend when heated. As it cools it returns to its original shape. The wire is connected to the power source. When the tungsten lamp is lit, it heats the wire and it bends. The circuit is broken and the light turns off. When the metal cools, it touches the filament wire again and a connection is re-made. Off and on, off and on. A flasher!
STEP 2. Make an Emergency Flasher!
In this video I connected three flasher bulbs in parallel and connected them to 4.5 Volts.
The bulbs all flash at different rates. One bulb took longer to heat up than the other two so it flashes at a slower rate. I used three 1.5 V AA cells for this experiment. Depending on what you want to do with this flasher, you may want to consider different cells! You can put as many flasher bulbs in parallel as you want. The more you use, the more current will flow.
STEP 3. If you have designed your flasher for an emergency purpose, then you can probably use AA cells. But, if you want to leave it running for a longer time frame, you have to consider the power that your system will consume and then select an appropriate battery.
You may remember a chart from one of the first experiments.
mah drain
The mah column refers to milliampere hours. An AA cell can produce 2400 maHours or 2.4 ampere hours of current at 1.5 volts. My flasher unit uses an average of .30 ampere hours. (When all are on they use about .5 A, when 2 are on about .33 A, and when only one is on it draws about .15 A.)
If the AA cells can only produce a total of 2.4 ampere hours, then my flasher bulbs that use .3 ampere hours might work for 8 hours.... But, that is an oversimplification. AA cells are really only designed to allow an optimal current rate. Note the chart shows 50 in the column for "drain." The manufacture says that this cell is best used with a device that uses only .050 Amperes. So, AA cells are probably not the best choice for this device unless you really are only going to run the emergency light for short times.
A better choice would be D cells. Notice that they have a capacity of 13000 mah or 13 ampere hours. My flasher would probably work for 2 full days before it would start to fail. The manufacture says that we can draw .2 Amperes from this cell, so our average of .3 will probably be OK. So, I will design a portable flasher I can use as an emergency light if my car breaks down.
STEP 4. Design your own. What problem do you want to solve? What sort of flasher and use do you need or want? Will it work outside or in? Will it be on for short times or long? Does it need to be weather proof? How will you turn it on and off? Do you need a switch?
It is best to sketch your plan after you have experimented with the flasher bulbs and are satisfied with how many bulbs you need and what batteries will be required. Remember that if you want bright lights, you will need 4.5 volts. Very bright lights can withstand 6 volts, but they may not last as long. If you use 6 volts or more lamps, more current will be consumed, so plan well!
Discussion. Designing electronic circuits is fun, but also requires some careful thinking. We might have a great circuit, but if we choose the wrong power system it could be rather disappointing. I remember I made a cowboy hat with a rim of lights around the brim for a friend of mine. It was his birthday so I thought I would be clever so he could have a lighted hat. It worked well for the first 15 minutes, then the batteries started to give out. His birthday hat looked dimmer and dimmer as the party progressed!
Here's my design for an emergency flasher. I used a water bottle with 3 D cells and a switch on the side. I think I will use it to mark the campsite at night if we take a walk and come back at dark!
I chose to put my design in a water bottle. I cut off the top and then placed three D cells in series in the bottom. I put a piece of folded paper between the upper and lower cells so the terminals would not touch in the wrong places.
I found a switch from a piece of electronic equipment that was in the recycle bin. I wired it in series with the cells and lights.
And here is my fantastic emergency flasher!