It has been argued that the continuity tester is one of the most essential pieces of equipment for a blind electronics maker. It is a device which provides accessible feedback when current flows in a circuit -- basically an Ohmmeter or resistance meter. A continuity tester with audio or tactile output provides an accessible way of testing and even identifying components, leads, and connections, tracing paths on circuit boards, checking solder joints, and even making other simple measurement tools such as light detectors and thermometers. While the sighted person finds occasional uses for an Ohmmeter, the continuity tester is an indispensable and pervasive tool for the blind maker. I cannot over emphasize the importance of having one.
They come in a variety of flavors, from simple circuits which buzz below a specified resistance threshold to oscillators that vary the pitch of an audible tone depending on the amount of resistance in the circuit. The classic continuity tester described in the following section, as well as many other awesome accessible continuity tester circuits and their uses, are fully documented in the Fall, 1982 issue of the Smith-Kettlewell Technical File. Advanced makers should definitely study these for insight into some old-school accessible test equipment.
The Classic Continuity Tester
The classic Smith-Kettlewell continuity tester is an elegantly simple circuit which makes for an excellent first soldering project (read the Soldering Series in the Smith-Kettlewell Technical File for all the information you need on blind soldering techniques). If you're planning to build electronics, make stuff with Arduino, or otherwise make a habit of messing around with wires and components, I strongly recommend building one. It's reliable, versatile, and extremely nice to have around.
Building the Classic Continuity Tester
The continuity tester's oscillator consists of an audio output transformer and a PNP transistor. An 8 Ohm speaker is connected across the leads of the low-impedance secondary of the transformer. The primary of the transformer has a center tap which connects to the emitter of the transistor. One end of the transformer's primary connects to the Positive terminal of a 9-volt battery. The other primary lead connects through a 0.022 uF capacitor to the base of the transistor. Across the entire primary is another 0.022 uF capacitor. The base of the transistor connects through a 22K resistor to the positive test terminal. The negative test terminal connects to the collector of the transistor and the negative terminal of the battery (ground). That's it!
A Simple Arduino-Based Continuity Tester
While the continuity tester described above is extremely simple and reliable, you may not have these capacitors, transformers, or transistors readily at hand. Possibly you aren't yet confident enough with a soldering iron to build it. Sometimes you may want to slap a continuity tester together using an Arduino and a couple of resistors. The following sketch makes a reasonably handy audible continuity tester, although the classic continuity tester described above is superior in most ways.
Building the Arduino Continuity Tester
Connect a 1 mega Ohm resistor and a 1 kilo Ohm resistor to analog pin 0 of the Arduino board. Connect the other end of the 1 mega Ohm resistor to ground and the other end of the 1 kilo Ohm resistor to +5v. Connect a piezoelectric buzzer to digital pin 9 and ground, (or you can substitute a speaker in series with a 100 Ohm resistor). Connect the positive test lead to analog pin 0 and the negative test lead to ground.
Programming the Arduino Continuity Tester
Copy and paste the following sketch into your development environment, make any desired modifications, and upload it to your Arduino. You can also download the continuity tester sketch here.