Just looking back through the thread, I might have spotted it earlier, but you could have used one half of the LM358 for the bridge input circuit from the Everyday Electronics project, and the second half for the bleeper oscillator, lifted as-is from the Not1Xor1 circuit. No need for the TL431 or the CD40106.
You draw it.... I'll build it ;-)
Now it's not that hard, It would be much more beneficial if you drew it and asked for corrections.
Take everything to the left of the output of IC1 from the, again, nicely documented, EE article (you don't need C3 as the LM358 is unity gain stable) and patch on everything to the right of the output of IC2/a (including D4) from Not1Xor1's design.
The problem you have with using this method with a DMM for continuity testing is:
1). Any shunt resistor below, say, 150R is going to continuously sound the DMM continuity bleeper - annoying, and the OP wants audible continuity testing.
2). Energised circuit protection.
One reason field techs love the antique Bach-Simpson 260 is Rx1 test current is almost 130mA at 1.5V which guarantees the reading involves copper - not water, rust etc.
I've had lots of multimeters give bogus ohmmeter readings on wire and cables with their piddly 0.3-0.8mA test current of the 200R range.
Even low voltage automotive, like trailer wiring it's pretty easy to bump 12V.
Now it's not that hard, It would be much more beneficial if you drew it and asked for corrections.
Take everything to the left of the output of IC1 from the, again, nicely documented, EE article (you don't need C3 as the LM358 is unity gain stable) and patch on everything to the right of the output of IC2/a (including D4) from Not1Xor1's design.
Not that hard for you... maybe! I'm just about to crack open BBox 1 and start learning electronics. I can just about read the diagrams, get the bits and wire it together. I'm not yet at the stage where I can understand to the point where I can create my own circuits. I can throw kits together, but don't ask me how or why they work. If something doesn't work when I've built it, I've got no skills to troubleshoot it.
For years, I've been bodging things together and hoping they don't go bang. How I've made it this far without causing a fire, I don't know. Probably by sticking to 5v DC at a guess So yes, some way down the road I'd be happy to take on the challenge. Right now, however, I'm not up to the task.
Sorry for overestimating your position on the learning curve (or maybe deliberately setting you a bit of a stretch goal ).
I made this one a few years ago: http://kripton2035.free.fr/Continuity%20Meters/ed-ultimateconti.html
Here's an improved version of the comparator-based circuit. If a micropower comparator is used for U1, it can be left connected to a couple of AAA cells, without draining the batteries. The tripping voltage is below the forward voltage of a silicon diode. It goes beeps, when Rtest <100R, but doesn't stop beeping, until Rtest > 200R, which provides some noise immunity. The trigger resistance is roughly equal to R4/100 and the resistance, when the buzzer stops sounding is roughly double that. I haven't actually built this circuit, but it should work, as long as a low power piezo buzzer is used, which can work down to 2V.
Now it's not that hard, It would be much more beneficial if you drew it and asked for corrections.
Take everything to the left of the output of IC1 from the, again, nicely documented, EE article (you don't need C3 as the LM358 is unity gain stable) and patch on everything to the right of the output of IC2/a (including D4) from Not1Xor1's design.
Not that hard for you... maybe! I'm just about to crack open BBox 1 and start learning electronics. I can just about read the diagrams, get the bits and wire it together. I'm not yet at the stage where I can understand to the point where I can create my own circuits. I can throw kits together, but don't ask me how or why they work. If something doesn't work when I've built it, I've got no skills to troubleshoot it.
Here's an improved version of the comparator-based circuit. If a micropower comparator is used for U1, it can be left connected to a couple of AAA cells, without draining the batteries. The tripping voltage is below the forward voltage of a silicon diode. It goes beeps, when Rtest <100R, but doesn't stop beeping, until Rtest > 200R, which provides some noise immunity. The trigger resistance is roughly equal to R4/100 and the resistance, when the buzzer stops sounding is roughly double that. I haven't actually built this circuit, but it should work, as long as a low power piezo buzzer is used, which can work down to 2V.
Not wanting to be picky, but isn't that roughly the sort of threshold you get with an ordinary dmm continuity bleeper. If you're making one, it would be nice to have one that was more functional than what you already have. Admittedly the hysteresis might be nice.
P.S. The stumbling block, as far as I can understand it, is that the OP wants to go with what they already have (LM358?) or the TL431s on order, so I guess, no micropower comparators.
The attached (well written) project from an old 1989 copy of Everyday Electronics does the job well. It puts out less than 300mV on the probes and can be set to resolve to below 1R. It also has an isolation mode which checks for insulation above 1M. It has audible indication and decent protection against accidentally applied voltages.
Can I use an LM358L in place of the LM308 on that circuit, do you think? I have some of the former already.
Do you mean the LM358LV, or just the ordinary LM358? The LM358LV is a low power version, which would enable the circuit to be designed so it can be connected continuously, giving a standby battery life of a few years, from a couple of AA cells.
I think it might be LV - I've attached the best picture I can get of the codes...
The design for the ultimate continuity tester calls for a 270nf capacitor. and I need through hole. I can find 220 and 330 in ceramic. I can't find 270 ceramic in anything other than surface mount. Do I use a 400v metallic film? How should I handle this please?
Yes, 220nF or 330nF, 400V metallic film will be fine.
Yes, 220nF or 330nF, 400V metallic film will be fine.
So by that token, I could use a 220nf or 330nf ceramic and not have to get a specific 270nf in metallic then?
If so... which would you recommend... 220 or 330... and why? - (never one to miss a learning opportunity)
Both of those values are close enough. It will still work, but will oscillate at a slightly different frequency.