Nuts and Volts - September 2008

Q&A

■ WITH RUSSELL KINCAID

In this column, I answer questions about all aspects of electronics, including computer hardware, software, circuits, electronic theory, troubleshooting, and anything else of interest to the hobbyist. Feel free to participate with your questions, comments, or suggestions.

Send all questions and comments to:

Q&A@nutsvolts.com

WHAT’S UP: Join us as we delve into the basics of electronics as applied to every day problems, like:

✓Wireless TV Converter

●✓Temperature Control Circuit

✓Power Transformers

MULTIMETER BUZZER FREQUENCY

QI need to change the audio frequency of the continuity buzzer function of a DVM890 multimeter. The audible frequency of the buzzer is beyond my hearing range. Having it operate at the standard frequency of 1,000 Hz would be great. Of course, the ideal circuit would be 1,000 Hz

at 0 dB into 600 ohms.

— James P. Brendage

AThe usual buzzer operates at the resonant frequency of the piezo speaker which is 3 kHz. I have the same problem when checking continuity so I hang the meter on my shoulder near my ear. I checked a defunct meter to see how the buzzer was done. The circuit was using a 4093 quad 2 input nand Schmitt-trigger oscillator. I

couldn’t make that work, so I opened my Masteck meter (I figured, I can’t hear it anyway, so what do I have to lose?). This circuit used an LM358 and a transistor, but there was only one 220 pF capacitor in the vicinity, so I increased it to 680 pF. The piezo speaker is undoubtedly less efficient at 1,000 Hz, but I could hear it much better. My recommendation is: open your meter, find the capacitor (it will be in the range of 100 to 1,000 pF), and make it three times larger.

^■^FIGURE ¹ALARM AUTO DIALER

QI would like to interface my alarm system with a phone that has a quick-dial button to call my cell phone. If I receive a call from my home phone and I am not home, I know the alarm has been activated. The alarm has relay contacts that will close for the duration of the alarm (about 15 minutes).

I would like a circuit that takes the contact closure as input and provides two outputs: one to take the phone off-hook and one to activate the redial. There should be a slight delay between going off-hook and redial, and the phone should stay off-hook for several seconds. After the system has timed out, it should not respond to the alarm until reset.