Repairing an old candlestick phone to be used as a headset.
I only did this because I wanted to see if this was possible and I wanted to hear what it would sound like in Discord.
Test recording located at assets/recording.wav (spoiler it just sounds like an intercom). Or listen to it here
A brief summary of what I did in this project.
I jumped into this not knowing anything really. So I had to figure out how the phone worked on a basic level.
To start the research, I think I have what is referred to as a Kellogg F9 Candlestick (according to this telephone collecting site). As far as age, I think it was made in 1935 based on a little signature with the date 2/25/35. My phone also came with a Western Electric oak ringer box magneto, but I won't be using it for this project.
Phones of this era used a carbon microphone. I found a couple good resources for learning how these work:
- How to Build a Simple Carbon Microphone
- The Boys' First Book of Radio and Electronics (1954) pages 200-202
- Principles of Electricity Applied to Telephone and Telegraph Work (1938) pages 66-68 - A VERY in depth book about telegraphy and telephony of this era.
For determining what the rest of the phone was supposed to do, I primarily used telephonecollectors.info. Here is a list of old resources I got to skim through:
- Kellogg Patent Collection 1890-1916
- Kellogg Bulletin 8 - Magneto Telephones 1903
- Kellogg Bulletin 38 - Magneto Telephones 1909
- Transmitter Wiring #229 and Transmitter Wiring #323
- Kellogg Wiring Diagrams
- Telephone Archive - Kellogg
- Supply voltage ended up being two of these 1.5v dry cells in series. I ended up bumping it up to 3.3v for simplicity.
I also downloaded the resources I used and put them in docs/ so they never get lost if that site goes down.
I removed all the magneto connections and old frayed cloth cable. I bought a new cloth cable and re-wired the phone how I needed it.
Surprisingly, the only thing I actually had to repair was a few broken solder joints in the receiver and transmitter. I was able to fairly quickly supply voltage and see my voice show up on my oscilloscope...albeit as a very low signal.
The secondary goal of this project was to use all the original phone hardware. I also wanted to make this easily removable just in case I want to actually hook this phone up to a landline one day.
For some reason I thought I was going to need to build some DAC (digital to analog conversion) and ADC (analog to digital conversion) circuits to get this working. So, I found the following resources:
But duh, of course a computer's sound card/front panel take care of converting signals for you.
Since the phone signal was so low, that calls for an amplifier. I'm still an electronics beginner so I used this page as a basis, which uses an LM386 Low Voltage Audio Power Amplifier.
After implementing that, I just had to play around with various capacitors to eliminate additional noise that was causing insane buzzing/humming. Again, I don't know what I'm doing so I just did trial and error.
To finish the project I finalized my circuit, soldered it to protoboard, and designed/3D printed an enclosure.
To use it, I just plug in the headphone and microphone jack to the front panel of my computer.
I wasn't ready to actually design and order a full PCB for this. But, I put together some basic circuits on EasyEDA. Exported circuit images can be found in eda/.
I'm still not experienced enough to put together proper EDA/CAD with a BOM, but here are the components I used.
- 1 x six position dual row screw terminal block
- 4 x two position screw terminal
- 8 x M2x8 machine screws
- 2 x M4x10 machine screws
- 2 x 3.5mm mini stereo female socket
- 1 x 2.1mm barrel jack
- 1 x red LED
- 1 x toggle switch
- 1 x LM386 IC
- 1 x DIP-8 socket
- 1 x LD1117V33 voltage regulator
- 2 x 10KΩ potentiometers
- 1 x 10KΩ resistor
- 1 x 100Ω resistor
- 1 x 10Ω resistor
- 1 x 0.047μF ceramic capacitor
- 2 x 0.1μF ceramic capacitor
- 1 x 0.01μF ceramic capacitor
- 1 x 1000μF electrolytic capacitor
- 1 x 100μF electrolytic capacitor
- 2 x 10μF electrolytic capacitor
Listed above in individual sections.