During a recent vacation, I challenged myself to come
up with a design that suits my needs. Most of the time, I
go straight for a microcontroller, but this time I decided to
use only non-programmable components. I also decided
to use only through hole components, to make soldering
easier. (I did cheat on this with two SMT transistors, but
they are quite large, SMT-wise, and easy to work with.)
Here is my list of requirements:
1. Must be loud enough for me to hear it.
2. Must turn itself off after a reasonable time
(approximately two minutes), but extend the time
each time it beeps.
3. Must have ‘instant’ response.
4. Must have very low probe voltage (~200 mV).
5. Must define continuity somewhere in the 25 ohm
6. No programmable parts.
7. All through hole parts (well, almost all).
The schematic is shown in Figure 1. The only IC is the
LM339 comparator. The two N-channel MOSFETs may be
a bit overkill, but they are fairly inexpensive, and I had
several on hand. The resistors are all 1% 1/4 watt, and the
100 µF capacitor is a simple aluminum electrolytic.
The power control section is centered around Q1,
which is the N-channel MOSFET. It’s being used here as a
very low resistance switch to DC ground. Until you press
the on button, this circuit draws almost zero current from
the batteries. 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. The tester is now up and running, and
draws about 16 mA while idle (considerably more when
beeping). You need to press the on button only briefly to
charge C1, then C1 begins discharging slowly through the
two megohm resistor.
When the voltage on C1 drops below about 2V, Q1
May/June 2018 37
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■ FIGURE 1. Schematic of
the continuity tester.