I believe the opto LED has Vf similar to RED led. So a resistor may be good even with a small coin cell.

 

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.
View attachment 202730

Ground the floating base on the opto-coupler (4N35 ?) with a 300K-Ohm pull-down resistor. That makes it weak enough (30uA) that the probes should over come it, if you connect them, but otherwise it will pull-down and keep the transistor OFF.

Having said that- you don't have any protection other than the opto-isolator. and you don't have anything to stop AC or control the DC level to the opto-isolator. I would perhaps and look online for research with Google on continuity tester circuits.

 

I deleted my wrong comments.

 

The original diagram shows the correct polarity.

 

The IR led needs more current to activate the phototransistor. Please calculate for 20 mA.

original is correct Led needs + at anode

 

Sorry, guys, I was terribly wrong while watching my government on TV talking about the virus.

Without a part number for the opto-isolator then we do not know its LED-transistor current ratio (maybe 1?) and the max allowed current for the LED.
The schematic shows a 10k resistor in series with a buzzer (piezo?) which would have a very low current.

 

Yeah he needs to get rid of that resistor But since no specs we have no idea... I’ve seen buzzers run fine on 12v and 5v direct.

I would put in 100 ohms for the IR side. IR LEDs are anywhere from 1.2-1.5V drop. Generally they are 20-60mA being conservative.

 

With boredom deeply setting in with CV19...

Boredom... no such thing around here.
Luckily for me I got a 3D printer back in January. I built grips for my bench vise and so many other little things.
I am working on a 6-axis robotic arm. Just started regrouting the filthy bathroom tiles.
Finally learned to play the guitar solo of "Hotel California".
Promised myself I will be fluent in French in one year. After that I will be fluent in Spanish after another year.
So much more to do...sorry, gotta go. Time to get some seeds started. Sorry about going off topic.