The buzz option on a multimeter is really useful because it tells you immediately when there is a short, without you having to read the resistance reading on the display.
The following circuits use a similar idea except that they don't check for shorts, but let you listen to digital signals in your circuit.
The main use for this circuit I have at the moment is to listen to the ICSP signal from MPLAB X when programming a chip. MPLAB X can take quite a while before it has re-built the code (C) and then started the programmer (software) and then gets around to do the actual programming (a different piece of software). All the while you are left staring at the screen waiting for the "programming complete" message to appear!
Then you can test the code out - probably finding that you left out a tiny action and therefore have to go through the whole process again!
This listener circuit lets you forget about MPLAB X so you can do another task such as examining the code you are writing or reading the datasheet etc. Since you can listen for the end of programming using this circuit it means you can save some time which is always a good thing.
Figure 1 shows use of a digital logic FET that has, as you would expect, a gate voltage that is sensitive to logic levels. For the 2N7000 Vgs is between 0.8V and 3.0V - if you need 3v3 logic the 2N7002 has Vgs from 1V to 2.5V but for typical values the typical Vgs for both parts is 2.1V
The 2N7000 is very useful as it does not draw current through the gate so it can be attached to any (logic) signal in your system. This is in contrast to the circuit shown in Figure 2 where a darlington pair is used - in this case a small current is drawn through the base via the 1k resistor
The darlington pair (Figure 2) stops too much current being drawn through the base (as is the case when a single transistor is used) since the Beta (current gain) of each transistor multiplies the other. It is similar in operation to the FET except that its trigger voltage is ~1.4V.
If you want to listen to different signals (perhaps they occur at different times) e.g. ISCP and serial output then Figure 3 shows how to use two diodes as a diode-OR gate.
Note: you can use a piezo crystal (mine was attached to the case of an old watch which meant it should be louder - but it was still too quiet!)
Note: you can use any old NPN transistor eg, with Beta 100 or better.
Arduino Battery Charger: A very useful project that lets you charge 'un-rechargeable' alkaline batteries!
The Essential Guide to the 74HC595; What it is and how you can easily use one in any of your projects.
How to use the MCP23017 to increase your I/O by 16 pins (or more) and use its interrupt system.