Theatre Design & Technology - Winter 2007 - 36

Use this pcb artwork to make your own board. You can also download the digital Gerber plots, or order a manufactured board. See the
resources list for more information.
As already noted, the ring voltage is high enough to be dangerous.
This is true of any device that rings a telephone, and is true for telephone
company phone jacks as well. When input power is removed and the red
LED D3 on the ringer goes out, you can safely handle the circuit without
fear-no residual voltages remain.
All resistors can be 1/4W or 1/8W, values are not critical. Capacitors
should be rated for a minimum of 16VDC. I've speciﬁed VNP-series protected mosfet transistors made by STMicroelectronics, but just about any
enhancement-mode power mosfets will work (use the same part number
for Q1 and Q2 to keep the wave symmetrical). You could use smaller
mosfets and a smaller transformer, but avoid letting anything run hot. You
could also use higher power parts if you have them lying around, but be
aware that higher currents also make the output more dangerous.
Circuit protection is provided by fuse F1, a self-resetting device that
will open if anything goes terribly wrong down the line. Diodes D1 and
D2 provide polarity protection, so nothing will happen if you connect the
power supply backwards.
It can be confusing that J4 is a telephone connector-this is NOT
where you connect a telephone to make it ring. Microchip decided to use an
RJ-11 for their ICD interface so I have no choice but to go along with them.
WHERE TO GO FROM HERE
If you choose to not use our pcb, you could leave out connector J5. It
goes to the serial port peripheral on the MCU. With appropriate external
circuitry, this could become an RS232 or RS485 data port. If you change
to a faster crystal (say, 20Mhz) the 16F876A MCU is fast enough to handle
DMX data. With a few extra parts for an RS485 driver, and some additional
software, you could make a DMX-Controlled Telephone Ringer.
The pins that drive the mosfet switches are currently being controlled in a very simple on-off manner. But you could also explore the
pulse-width-modulation module that is built into the MCU. It produces
two separate pwm outputs on the same pins, and could be programmed
to produce a true sine wave. I smell a variable speed ac motor drive in
here somewhere.
See the resources list for Web sites and e-mail addresses where you
can order parts and seek advice. Feedback is very much appreciated-let
me know what you think of this project, and what other electronic projects you'd like to see here. ❖

PROJECT RESOURCES
Technical Support is provided by e-mail only.
Contact James Smith at diy@jamesdavidsmith.com.
To download the ﬁrmware code, schematic, pcb layout, and parts list,
and/or order preprogrammed MCUs and printed circuit boards, visit:
www.jamesdavidsmith.com/diy/ (case sensitive)
The complete bill-of-materials (BOM) list for this project, including
a blank MCU but not the pcb, is databased for easy ordering at:
http://sales.digikey.com/scripts/ru.dll?action=pb_view&pb_
glue=1013581
The Microchip ICD2 In-Circuit Debugger/Programmer is available at:
http://www.microchipdirect.com/ProductDetails.aspx?Catalog=Buy
Microchip&Category=Debuggers&mid=13
The Microchip ICD is bundled with MPLAB development software, which
is also available for free download:
http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE
&nodeId=1406&dDocName=en019469&part=SW007002
PicBasic Pro by Micro Engineering Labs is available at:
http://www.melabs.com/products/pbp.htm
Electronic Component Suppliers:
Digi-Key
http://www.digikey.com/
800-344-4539
Mouser
http://www.mouser.com/
800-346-6873
Newark
http://www.newark.com/
800-639-2758
Low-Volume Circuit-Board Manufacturers:
PCB Express https://www.pcbexpress.com/ 503-829-9108 x555
AP Circuits
http://www.apcircuits.com/ 403-250-3406

James David Smith (call him Jim) has been developing electronic
products for arts, entertainment, and industry for nearly 20 years.
Products include the RC4 Wireless Dimming System, the matrix
of 165 Revolution Scrolling Signs on the Toys 'R' Us ﬂagship store
in Times Square, NYC, and much more. You can reach him at
diy@jamesdavidsmith.com.

THEATRE

2007

DESIGN

TECHNOLOGY

http://www.jamesdavidsmith.com/diy/ http://sales.digikey.com/scripts/ru.dll?action=pb_view&pb_ http://www.microchipdirect.com/ProductDetails.aspx?Catalog=Buy http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE http://www.melabs.com/products/pbp.htm http://www.digikey.com/ http://www.mouser.com/ http://www.newark.com/ https://www.pcbexpress.com/ http://www.apcircuits.com/

Table of Contents for the Digital Edition of Theatre Design & Technology - Winter 2007