I have a HC DM303 digital multimeter that works on 9V battery and I wanted to add a buzzer and continuity testing mode but I don't know how. Here are some details and photos of PCB inside
A/D converted IC : NJU9201BD JRC 9012
4 XOR IC : TOSHIBA 8924HB TC4030BP
Can I do it with a passive buzzer or do I need active one?
Is it even possible to add continuity testing mode to this multimeter?
And one more question I used TCRT5000 IR reflective sensor in my previous project and it worked great but it is very sensitive to wrong polarity (I burned 2 of those, luckily they are very cheap). So how could I make it not burn if you swap polarity, I'm not sure if transistor, diode or both burned when polarity is reversed. I know I should add a diode in parallel but parallel to what to transistor or IR diode and what kind of diode rectifying, zener or Schottky
Here is picture of that sensor and schematic how it's connected.
I'd recommend building separate continuity tester. I'm sure you can find a circuit online. Or by a meter with the feature built-in, or buy a separate continuity tester.
If you add circuitry to the meter you'll likely mess-up the other functions. I found the datasheet for the chip in your meter and it doesn't show a continuity/buzzer output.
I'm surprised it was fried by reverse voltage but "reversed" diodes across the LED & phototransistor will allow a current path to bypass the internal components when current flows in the opposite direction. (The existing resistors will protect against excess current through the protection diodes.)
The ohm meter mode is a continuity tester.
In the old days we had
6V lantern battery, buzzer on +, other side of buzzer to test probe, -ve side of battery to an alligator clip. Done.
All the meters I have purchased in the last 20 or more years came with buzzer and continuity mode and cost between $20 and $60.
You may mess up your multimeter and probably interfere with electrial safety.
Today lots of multimeters at decent cost have an integrated continuity testing.
There are many tutorials in the net how to build your own tester. However you (and the intended use) may have to decide if a very simple solution is satifsfying your needs or if you want the device to have a (more or less) well defined resistance threshold.
Buy a better DMM.
The DMM shown in post#1 seems to be bought in the dollar store.
A good DMM starts from about US$25.
You get much more for that price than just a buzzer.
The fuse of your DMM seems to be blown (glass is black).
Leo..
Thanks everybody, I wanted to upgrade this multimeter with buzzer but I will build a separate device just for continuity testing there is bunch of schematics online and I will model a housing and 3D print it. If I understand that continuity mode is just measuring resistance and if the resistance is small enough it detects it as a continuos path and if there is more resistance between two probes it doesn't give sound signal, but my question is where is the limit between continuity and no continuity?
Thanks, I will try and let you guys know if it works.
That's correct, the fuse is blown, gotta get a new one. Just some additional info ratings of that fuse are 250V 2A.
That finally depends on your specific use of the continuity testing. There are applications where you may only accept resistance less than one Ohm, others where it's ok if it's less than e.g. 25 Ohm etc.
This is what the company Fluke says
Understanding Resistance and Beeps: The beep is triggered based on the resistance of the component being tested, influenced by the multimeter's range setting. For instance, at a 400.0 Ω range setting, a multimeter typically beeps if the component has 40 Ω or less resistance.
Optimal Range Setting: Use the lowest range setting for testing components that should have a low-resistance value, such as electrical connections or switch contacts.
