Re: Building a Better Buzzer System

• From: cooterchekov <no_reply_at_yahoogroups.com>
• Date: Thu, 15 May 2003 09:00:57 -0000
• Message-ID: <b9vl09+a1ah_at_...>

--- In quizbowl_at_yahoogroups.com, ZAMM_Phaedrus <no_reply_at_y...> wrote:

> So, not having expertise in electronics, I toss this out to the 
list.
> Anyone have ideas?
>

I don't know much about software, but the external element can be 
pretty simple. This is how I would build it: 
http://home.gwu.edu/~ers/buzz1.png
That's not a circuit diagram, just an outline sketch. How it works 
is:
Set up a 555 timer or equivalent to put out a square-wave SYNC pulse 
in the range of about 1 kHz (or faster). Every pulse from the timer 
does two things. Set up the 74150 to connect one input to each 
switch, which can be either open or closed (default open). Run 5V 
6mA current to each switch. Connect the 555 SYNC out to the 
INCREMENT in of the 74HC93 and the 74150. Send the output of the 
74HC93 and the 74150 to a buffer and serialize it. Send to the IN 
pin of the serial port.

The 74150 listens to each of sixteen inputs in turn. The 74HC93 
counts in binary from 0000 to 1111. Each SYNC pulse from the 555 
tells the 74150 to listen to the next input, and the 74HC93 to 
increment by 1 (it rolls over after 1111). That gives us five bits 
of data: a four-bit unique identifier, and then an "is-it-on?" bit 
that's either 0 for off or 1 for on.

The part I'm hazy about, but which I'm sure is just a standard 
component, is turning these five bits of parallel data into a byte 
of 8N1 serial data.

Another thing you could certainly do is reset the system from the 
computer. The RS-232 serial standard allows for two-way 
communication; just send a pulse along the computer-to-device pin of 
the serial port to the RESET input of the 74150 and 74HC93.

The most expensive part of the whole thing would be the actual 
buzzer switches. Each of the three integrated circuits I mention 
above are cheap and standard. Also, unless you wanted to add lights 
you wouldn't have to power it very much; you're (supposed to be) 
able to draw I think up to 500mA at 5V off the RS-232 serial port.

Edmund