Parts List:
R1 = 100K Led1 = High Brightnes LED, RED, 5mm
R2 = 10K IC1 = LM741, OpAmp
R3 = 1K 9volt Alkaline battery, casing, probes
R4 = 100K
R5 = 500 ohm
How it works:
Occasionally you need a continuity test between two points in an electronic circuit. Unfortunately, most continuity testers are prone to "lie". They don't do that deliberately, but if they see a small resistance, they still tell you that you have continuity. They just don't know any better.
This unit is different. If you have continuity it will tell you so. And if you're reading even a low resistance through a component, the unit will tell you that as well.
The unit uses two 741 op-amps. It offers a short-circuit test current of less than 200uA. It detects resistance values of less than 10 ohms. Nicest of all, it will not break down a PN junction. The device has come in handy in my own shop for debugging electronic circuits.
In building this circuit, use good electronic practice, mounting the 741's in suitable ic sockets on perf-board. While there's nothing critical here, keep the work neat, and leads nice and short. When you're done, mount the unit in a small plastic box. A small dab of silicon rubber adhesive keeps the 9-volt battery in place at the bottom of the case, and will last a long time.
Just in case you're just starting out in electronics, here is how to get the -9, +9 and Gnd. connections.
A small hole with a grommet keeps the leads (probes) together. Another hole with a grommet holds the LED in place on top of the box
where it is plainly visible. This makes a nice one-evening project. Enjoy!
Caution:
There is no easy way to proof the unit against connection to the supply. Please, please be careful if checking AC line wiring and switch off first. In a similar way, if checking electronic apparatus for unwanted bridging between Veroboard tracks, for instance or a suspected crack in a PCB (Printed Circuit Board) track switch off power first also. Always practice good safety and think-before-you-do!
Back to Circuits Menu
by Tony van Roon, CET