Home made continuity tester; am I missing something?

Thread Starter

Tonyr1084

Joined Sep 24, 2015
4,370
With boredom deeply setting in with CV19; I have considered building this continuity tester consisting of either a 1632 coin cell or a 2032 connected to an opto-isolator. The opto is connected to a 9V battery with a buzzer and a resistor. What I'm worrying about is the extra leg - the base of the transistor. Does it need to be grounded? Held low through a resistor? Or is it fine the way it is?

And no, I haven't chosen the opto or the buzzer yet.
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Thread Starter

Tonyr1084

Joined Sep 24, 2015
4,370
Reason why I excluded an LED resistor is because I've used CR2032's a number of times to test LED's without any problem. And with the Vf being so close to the battery voltage (assuming new) what value resistor would I use if I used a RED SuperBright LED with a 1.95Vf (call it 2Vf)? A new coin cell is what??? 3V? If so - 1V ÷ 0.02A (20 mA) = 50Ω. No? I get iffy with those low numbers.

[edit] Blue LED Vf (on hand) is 2.82Vf. So 3 - 2.82 = 0.18V. 0.18 ÷ .02 = 9Ω. And like I said - I've lit them for days on end with no ill effects to the LED's. I'm assuming (dangerously) that the opto will behave similarly. But I have a bunch I can scavenge from boards - so I'm not terribly worried about blowing an opto or two out.

As for the 10KΩ on the buzzer - I just threw that in there to show the desire to limit current - dependent on what buzzer I end up with. I may pluck a buzzer from an old Microwave oven control board (have a few laying around). But they may need circuitry to make them buzz. But that I don't know for sure yet.
 

MaxHeadRoom

Joined Jul 18, 2013
20,419
Generally continuity testers indicate different levels similar to the Diode range on a meter, IOW, continuous if below a few ohms, rapid beep if higher, and intermittent beep if fairly high, meters vary.
Or just a simple one that only beeps if below a very low value.
Max.
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
4,370
Like I said - boredom is setting in. I'm not trying to reinvent the continuity meter. Just a quick dirty tester. Ring a wire or check for shorts.
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
4,370
The idea is to have something small and portable for checking fuses in a car or checking a wire or something. One probe can be hard mounted to the tester and the other end connected to a ground clip or something. Of course I'd have to be careful not to apply power to the probes as that would surely blow out the opto.

Again, remember, this is an anti-boredom project. Just some stupid thing to do with my time in isolation.
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Thread Starter

Tonyr1084

Joined Sep 24, 2015
4,370
Like I said - I haven't chosen a buzzer yet. The resistor is just to show the likelihood of needing a resistor. Maybe a 100Ω resistor should go there - I don't yet know. It's possible I might not need a resistor at all. It all depends on the buzzer.
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
4,370
Why do you need isolation?
Rube Goldberg machine maybe. I don't know. Yes, the button cell powers the opto. This way an unintended tap of a higher (or high) voltage doesn't explode the battery. I try to keep in mind how I might screw something up and thus inadvertently blow something up. Just a 9V battery with a buzzer will probably do the job but if I do the wrong thing - which I'm known to have done - can become problematic.

Why the two sources? Well, dividing out the voltages so that I can ring the buzzer with an opto is more of a test of construction rather than an exercise in practicality. I once built a go-cart out of wood and used a self propelled lawn mower to drive it. Why? I couldn't give you a good reason other than that's what I had on hand. I was just a young teen. This is before I built a supercharger for my (later) mini-bike.

I suppose I could blow out the opto. Maybe go short and cause the battery to explode. Maybe my design is unwise. All I was doing was thinking. Thinking is sometimes not such a good activity for me though. Sometimes I come up with some really goofy ideas - as my best friend once said. "He has some Goofy ideas - but damned if they don't work."
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
4,370
Keep in mind - I may test a diode or a FET or something. Low voltage is less likely to damage a component, whereas 9V can smoke a device being tested. That may not be the reason why I came up with this circuit - but it's a potential positive side note to the thing.
 

crutschow

Joined Mar 14, 2008
24,711
A 555 astable timer with the timing resistor as a probe will give you a varying frequency with resistance.
The LMC555 CMOS version can operate down to a 1.5V supply.
 

ElectricSpidey

Joined Dec 2, 2017
1,062
Keep in mind - I may test a diode or a FET or something. Low voltage is less likely to damage a component, whereas 9V can smoke a device being tested. That may not be the reason why I came up with this circuit - but it's a potential positive side note to the thing.
Ahh, see your posts had me believing you weren't going to use this on electronics.
 

Thread Starter

Tonyr1084

Joined Sep 24, 2015
4,370
Remember - this is one of those Corona Virus projects. No - it's not to do anything for or to Corona, it's to stave off boredom. I've sat here thinking about all kinds of projects in including wood working projects. I'm in the process of contracting with a builder (started before CV19) to add a second floor to my garage so I could set up a wood shop up there. The goal of that project is to start up a custom wood working, furniture and cabinets shop. The question is how to get materials up and projects down. I've come up with a lift system for that. The wife keeps saying "Install an elevator" but that would take up a lot of room and cost a lot of money.

We're considering starting to make face masks consisting of two layers of cloth with a copper wool insert to hopefully aid in killing the virus before the person wearing the mask can be infected. We're thinking about a LOT of things. One project is to build an inter-stellar space ship to take family and close friends to another planet and colonize it with healthy people. Like I said - lots of thinking going on. Some of it may actually be reasonable. But these times are stressful. Keeping busy is important. Even if it means making some silly continuity tester. I have plenty of scrap parts laying around that can be used to make stuff out of. Recently (month or two ago) I made a fan speed controller for a centrifugal compressor to pump air into a fire pit - or to extract heat from a fire box. THAT project was purpose built. And it's working. Now I'm looking to spend time making stuff. I'm even considering a power conditioner for my furnace - though the heating season is wrapping up. I have several ferro-resonant transformers I might try and see if they can support the furnace.

In the end, my final comment is that this is not going to be a highly utilized project. More of a "can I do it" thing. Yeah, I can find uses for it. Check fuses, continuity, check a diode or transistor. Maybe a FET or a chip. I don't know. I don't have a dedicated purpose for it, just want to do something besides sit here and watch car crashes on YouTube.
 
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