Question about analog voltmeters and antique telephones
Question about analog voltmeters and antique telephones
I recently started setting up an intercom system between my workshop and the kitchen using a pair of antique Leich 901 telephones.
These are old local battery, crank/magneto phones originally intended to be used on small co-op networks (sometimes run over barbed wire!) or other odd uses. Each one has a crank which generates AC to ring all the phones on the circuit, and a talk battery (usually in a separate case) to power it as it was assumed houses didn't have electricity.
I've been able to get them talking following these guides:
https://www.valhallatreefarm.com/magneto%20phone/phonemagneto.htm
Now I'm planning my next steps, which include setting up a nice display panel for the workshop phone (partly to make hanging it on the wall a bit easier).
One of the things I'm thinking of is adding a small voltmeter to the voice line, like in this picture:
From their description of "When you speak, the needle moves rapidly like a VU meter!" I was assuming it's a 0-5 Voltmeter (maybe something like this) but Im worried about the AC ringer power coming down the same line.
According to the best guide I've found: "It should put out about 75 Volts AC when vigorously cranked." While the talk battery is only 4.5 Volts DC.
I have some huge holes in my understanding of electricity (especially considering how often I mess with it) but I'm worried an analog 5v Voltmeter wouldn't like 75 volts. I've read Voltmeters are supposed to be higher resistance than the circuit they're measuring. Does that protect it in a situation like this? Otherwise is there a way to safely set this up? Bonus points: is there a way to rig a light that blinks or flickers using this circuit in case I can't hear the ringer over tool noise and earpro?
Thank you very much!
It's looking like you are going to want to hook the volt meter up to your talk lines. I would test just probing the wires going to the handset, if I understand the schematic correctly the talk circuit is separate from the ringer. That being said, you can get a voltmeter that is tolerant of the voltage. In my experience anything under 400v is fine for most voltmeters, they use a resistor divider that have values in the megohms, the current passed across them is minimal.
As far as blinking on ringing, that one is super simple since the ringer is AC, just use an induction type detector to flash an LED on. Think along the lines of those screwdriver looking line voltage testers that light up when you put them next to a live outlet.
Edit: I should clarify what I mean when I say the ringer is on a separate circuit. There is a coil that energizes when the line goes up to ringer voltage that shunts power to the ringer and away from the talk circuit, so measuring the voltage from the handset or speaker should protect you from seeing the ringer voltage.
That's a really clever answer to the ringer light! I should definitely be able to piece something together from there.
It also hadn't occurred to me to just tap the phone itself for the voltmeter (though it feels a little less authentic than doing the line between them, somehow). It's very good to know about the voltage tolerance - I'll see what I can find!
Thank you very much for your help, I really appreciate it!
It would be how the voltmeter would have been wired back then, you can damage the needle or it's spring by pegging it for too long. Even then though, the only real issue I would forsee would just be the voltmeter spiking all the way to the right while it's ringing, the chance of any real damage is pretty minimal. You can also check the datasheet on the meter, it should tell you what it can safely handle.