This Old Phone part 3: Ring my bell.

I have the Bluetooth solution and verified operation of the candlestick handset. What was missing was an old timey ringer. Back in a day, a wall mounted ringer assemble was usually installed next to the handset.  This would require a 90V AC to ring the bell.  I want to have an old style ringer but I didn’t want to hassle with the 90 Volt requirements.   I found a surplus bell from an old phone parts supplier.  I removed the larger coil and installed a 5 volt relay with tie-wraps and wire.  I made it work.

The ringing is from a 15 to 20 Hz pulse that duty cycles on for 2 seconds and off for a couple seconds.  I piecemealed this circuit to be triggered from the incoming signal from the speaker. It sets the first 555 for the on off duty cycle while the second 555 is for the 20Hz ringing.

In order to make this a desktop application and something that doesn’t have to be anchored to a wall, I decided to create a “ringer base” which will also act as a charging station.  I found a nice small black cigar box that will serve just right for the ringer base.  The candlestick phone will sit on top and when a call comes in, lifting it off the base will cause the ringer to stop and answer the call.

I created 3 small PCBs from proto material I had hanging around my lab.  The larger one holds the ringer and charger circuit assembly.  I have a single transistor amplifier to act as a simple Schmitt trigger on a smaller PCB ( similar circuit I used for the condenser microphone from last time).  In the center is my connector board which will interface to the phone, charging power, “answer” signal, and speaker from the receiver.

The lid of the cigar box slides out. I cut out an access point to this molex connector, again from my spare parts bin, where at one point I got this assortment of connectors.  I routed out the underside so that the connector can stick up enough to mate with the base of the phone.

So far all things are in place. Next time I will put it all together with the interconnections from the phone.

This old ( Bluetooth connected) phone Part 2:

Since the old carbon microphone was not operational anymore, I needed to up grade to a more modern approach for the BC127 module. I found an inexpensive electret condenser microphone. Working with these can be a little tricky and not quite straightforward depending on the gain of target application  and how much voltage you intend to run it off. Given that I am trying to run this all off of a 3 V source, I didn’t have enough amplification to directly drive the microphone into the input of the BC127.

This this little circuit came in handy:

I built the circuit on a solder-able  breadboard and tested it with an oscilloscope first.  I verified the circuit by  testing it on the prototype platform, currently resembling a mad scientist’s tangled mess of wires.

Success! A call made from my land-line to my cell phone paired to the purpletooth jamboree board connected to this kluge of wires, iron and alligator clips was rather exciting for a geek. I was able to test the microphone’s ability to pick up my voice from about 3 feet away before dropping off.

 

Next step: In part 3, I really want to ring an old time ringer which I purchased. This will requiring hacking into the code a little bit to set a GPIO active to engage the bell. The problem is that the bell will require a higher voltage too and may defeat my dream of being somewhat rechargeable and running off of a single battery and voltage source.  At the very least, I will test the connection to the switch which will engage the “answer” mechanism ( play button on the Jamboree board).

Motivating Students with a Cigar Box Amplifier project

Besides my full-time gig as an engineer, I also teach part-time at a local community college. The circuits class is a continuation from the basic AC/DC fundamentals class and introduces the students to active components like diodes, transistors and op-amps.  I tend to be a bit out of the box and decided that  it’s more enriching to drive the class towards a project based learning experience.  Instead of paying for the typical lab manual and building “static circuits” each week in lab to expand on the theory, I suggested to the class to put the money into the materials required beyond the lab stock of devices to build a stereo  amplifier that they can plug their iPhone or MP3 player into and listen  to.

With only 15 weeks for a semester,  the labs turned into project updates and planning. The class started by learning about power budgets and input and output requirements. The first task was to decide how much power they could afford given the cost of the alternative lab manual.

Understanding diodes turned into a power supply design with a power budget to drive up to a total of 15W of power to two 8 ohm speakers.  Basic transistor circuits turned into figuring out bias and gain for the given inputs and requirements for the next stage of the amplifier. Op-amps turned into learning about a tone controls through active bandpass filters.  The final stage was the power amplifier and because of schedule restraints the class decision was to use a linear Class A design though inefficient, was still able to provide enough output to have acceptable volume to the speakers.

I had the students create their own schematic using ExpressPCB tools and eventually create a PCB that still fell within budget.  They  learned about design reviews and tradeoffs  in a project between scope, costs and time.

I had the class practice soldering on breadboards to build prototypes of sections of the power supply and amp. They had to develop a test plan to verify  the specifications. This way when the final layout was complete on the PCB, they were ready to solder the complete board  and experienced with troubleshooting as well as making measurements against the design specifications.

I found a cigar company willing to sell some empty wooden cigar boxes to house the stereo parts as a nice finish.  At the cost of $2/box, this fit well within the budget.  Another student volunteered to come up with a wooden insert to  mount the speakers and the PCB.

Overall, the project was a big success. The output was delivering only  4.5W per channel but still loud enough to enjoy at a desk. There was no distortion to deal with since all the stages were basically linear class A.  All the students ended up with a  working cigar box stereo amplifier that they learned so much from. A few students wanted to make it more power efficient. They asked about turning the final stage to a class D operation.  Over the summer and with a small Maker Faire grant from the school, they actually went on to build and explore the challenges of designing a Class D amplifier to fit in the same cigar box housing.