In my previous article, we took a look at one of the signals generated by the 555 test circuit where the output pin (pin 3) is connected to ground (-) which is a simple series of very brief pulses.

This series of pulses can be used to generate a tone that can be used as an alarm or can be adjusted with a fair amount of precision such that it can be used to tune a musical instrument.

In previous articles, we've used the 555 test circuit as a hot liquid level indicator for the vision impaired and a cable tester as examples of some of the things that can be used as inputs to the 555 test circuit. In this article, we’ll take a look at one of the output signals that can be generated by the 555 test circuit.

This is a simple build of a Michelson Interferometer using the SpyNet Laser Tripwire . About a year ago, I accidentally came across this toy and noticed that it has a red laser and two adjustable mirrors. It also has a receiver that will sound an alarm when the laser beam that shines on it is blocked. Yes, I have reverse engineered the receiver—a rather simple circuit, really—and, of course, improved it and may post an article about it to demonstrate an SCR (Silicon-Controlled Rectifier). I requested a reviewer’s sample from Jakks Pacific but did not receive one, so I had to wait for an after Christmas sale and found a set for 80% off (about $5.00 USD which was less than purchasing a laser pointer and two makeup mirrors).

In my previous article, you learned how to use the 555 test circuit as a cable tester. It can also be used to test whether different materials conduct or don’t conduct electricity such as plastic, glass, cloth, wood, a coin from your pocket, a house key, aluminum foil, or any number of materials from around the house. You can also use it to see how well you conduct electricity to demonstrate one of the measurements, skin conductance, of a polygraph, or lie detector.

In the demonstration video I touch the snaps together to get a base line tone. Next I hold the snaps on the red and black Jumper Wires between my fingertips to demonstrate that your skin does indeed conduct electricity.

In my previous article, you learned how to build a hot liquid level indicator for the vision impaired to help them fill cups with hot liquids such as tea or coffee. The 555 test circuit was used to generate a tone when the liquid in the cup reached the desired level--when the liquid level reached the red and black snaps of the Jumper Wires inside the cup.

The liquid (my coffee) conducts electricity because tapwater contains small amounts of contaminants.

In my previous article, you learned how to build the “The 555 Test Circuit.” We are going to modify the test circuit to create a hot liquid level indicator for the vision impaired to help them fill cups with hot liquids such as tea or coffee. To fill a cup with cold or room temperature liquid, the vision impaired person simply puts his or her index finger inside the cup to monitor when to stop pouring the liquid when it hits his or her finger. When pouring hot liquids from, say, a boiling teakettle, vision impaired persons would not want to monitor the liquid level with a finger since they may scald themselves.