![]() ![]() That output is a square wave that switches Q3 on and off, pulling current through the speaker. The enabled oscillator outputs a frequency of about 500 Hz (to which my hearing responds better than the ~2 kHz of the typical piezo electric beeper). When the probes see continuity, the output of U1D pulls low, and inverter U1C output goes high (thanks to pullup R11), which enables the oscillator. ![]() When the probes see no continuity, the output of U1D is high (thanks to pullup resistor R1), and inverter U1C output at pin 14 goes low, which disables the oscillator. The oscillator consists of U1A, C2, and R7-11. The AUDIO section is an oscillator centered around U1A, followed by an ‘amplifier’ Q3, which is really just an on/off switch pulling current through the speaker. The output (pin 13) of U1D goes low when the probes see continuity (29 ohms or less in my example). The associated resistors, R15-R20, should be no worse than 1%. The CONTINUITY DETECT section is centered around U1D: the LM339 comparator. I find that the time out (with no refresh beeping) is about 2-1/2 minutes, but that will vary depending mainly on the capacitor tolerance. However, each time you beep, the output of U1D goes low, and Q2 turns on briefly to pump fresh charge into C1, which extends the on time. When the voltage on C1 drops below about 2V, Q1 turns off. You need to press the on button only briefly to charge C1, then C1 begins discharging slowly through the two megohm resistor. The tester is now up and running, and draws about 16 mA while idle (considerably more when beeping). When you do press the on button, capacitor C1 charges up very quickly and Q1 turns on, which provides a ground (which is labeled SW_G) for the circuitry above. Until you press the on button, this circuit draws almost zero current from the batteries. It’s being used here as a very low resistance switch to DC ground. The power control section is centered around Q1, which is the N-channel MOSFET. The resistors are all 1% 1/4 watt, and the 100 ♟ capacitor is a simple aluminum electrolytic.įIGURE 1. ![]() The two N-channel MOSFETs may be a bit overkill, but they are fairly inexpensive, and I had several on hand. All through hole parts (well, almost all).Must define continuity somewhere in the 25 ohm range.Must have very low probe voltage (~200 mV).Must turn itself off after a reasonable time (approximately two minutes), but extend the time each time it beeps.(I did cheat on this with two SMT transistors, but they are quite large, SMT-wise, and easy to work with.) I also decided to use only through hole components, to make soldering easier. Most of the time, I go straight for a microcontroller, but this time I decided to use only non-programmable components. That only works when the feedback is ‘instant.’ĭuring a recent vacation, I challenged myself to come up with a design that suits my needs. Frequently, I plant a probe on one pin and ‘rake’ the other pin across IC pins, looking for shorts and opens. There is a slight but unacceptable delay each time it detects continuity.That voltage can damage some devices (not to mention polarity issues). There is a significant voltage between the probes (7V on my Fluke meter).Most multimeters have a built-in continuity tester, but these also have problems: I forget to turn it off, so the batteries die.Half the time, I can’t hear the beeper, or worse, I imagine that I do hear it. I’m getting old, and I work in a noisy environment.It served me well, but had two drawbacks: In other words, if the resistance between my continuity tester probes is less, I get a beep if the resistance is more, there’s no beep.įor years, I used a simple op-amp based circuit with two AAA batteries, some resistors, and a piezo electric beeper. Somewhere in the 25 ohm area has proven to be a convenient trip point for my use. There is plenty of opportunity for error, and a continuity tester is very handy for detecting both shorts and opens. The latter often involve hand-soldering SMT components, such as my favorite 80-pin fine-pitched microcontroller. This involves making all sorts of custom cables and the custom board assemblies. I design and build functional equipment to test PCB assemblies. In my case, it’s usually either for beeping out cables or for beeping out printed circuit board (PCB) assemblies. ![]() What is a continuity test? I say it depends on your particular use. ![]()
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