Background

When I changed jobs to be an electronic technician, I was given a schematic to build right away. It was titled "Beeper Box".  This task served a few purposes: a quick assessment of a newcomer's construction skills, an introduction to the tools and parts available in the lab, and also, it was an awesome tool to have. A beeper or continuity tester is quite useful, and nowadays this is a standard feature on a DMM (Digital Multi Meter) costing as low as $10 US. 

So, why spend a few hours (or longer) building something that can be bought for the price of a good cheeseburger? The first reason is because this was a long time ago. In 1987, handheld DMMs were an exotic thing to have in a large lab. Sure, we had nice tool boxes full of what was needed, and a machine shop area too. But in 1987 DMMs were quite expensive, and also tended "to grow legs". That is, if left on a bench overnight, it was quite often not there the next day. One of the originals, the Fluke 8020A, was about $169 US when introduced in 1977. In today's dollars it is $863! Thus, although surrounded by a variety of bench multimeters all the way up to a few HP3456A flagship models, there were no handheld DMMs!

Another reason why a beeper box was a good thing to have is it actually significantly better than the continuity function in a DMM. The way a DMM works, if the impedance is less than about 100Ω, the DMM emits a single tone chirp, usually around 1kHz to 2kHz from a piezo buzzer. But the beeper box is different. The beeper box injects some current into the test probes, and amplifies the resulting voltage. The amplified voltage modulates a VCO (Voltage Controlled Oscillator). Thus, the output frequency is a function of the probed impedance. In use, this provides significantly more information to the user. 

USAGE

When the beeper box is used on a blank PCB when checking for shorts and continuity it either beeps 200Hz or nothing. If a short is found where there should bean open, then using the analog capabilities of the beeper box is the best approach. For sake of explanation, let's assume a short from VDD to GND, both planes. To find this short, connect a power supply set to 0.5V and about 200mA current limit to VDD and GND. Connect the positive input of the beeper box to the VDD input, and use a probe on the negative. What happens is the tone increases as the probe moves towards the short circuit. So probing the various VDD pads on the PCB, the short is close to the pad with the highest frequency out of the beeper box. This is faster than using a DMM voltage readout because the user has to look from the PCB to the meter, and track mentally which reading was largest. it is much faster to use one's ear to determine highest frequency than to read a DMM.

Once the PCB is populated with components the beeper box also is quite useful. A short is 200Hz, resistances up to about 3kHz cause a somewhat linear rise in frequency to about 2kHz. Capacitors above about 1µF cause a chirp, with the duration of the chirp proportional to the value of the capacitor.