volume attenuator

If you mean replace the 300 Ohm resistor with a potentiometer, you will not get the expected results.

Nope. Keep the 300 ohm resistor where it is (a 270 ohm may be easier to find at Radio Shack -- that's OK). Get a 25 ohm 3 watt rheostat from Radio Shack (Catalog #: 271-265, $3.99 ea.) and hook up the handset speaker like this (view in fixed font or M$ Notepad):

| o-----. | | | | | .-. | | |

That will likely upset the hybrid balance, which will increase the volume of the local talker to his own handset.

Yup, it's ugly. I was thinking a pot on both sides, too, and didn't mention that. It's not pretty, but at least it will kinda work. Replacing the 300 ohm resistor with a pot probably won't.

Thanks for the spot, Mr. Bowey.

Cheers Chris

Why did you choose 25 ohm? Will 100K work?

Thanks.

Chris wrote:

Please write your reply _below_ the quote, not above.

The 25 ohm was chosen because it won't have too high attenuation which would make most of its range useless. So in other words, you will have an useful adjusting range. If you chose 100K, it'd work, but would be pretty useless, as you'd have just the first millimeter of the pot's range useful (after it there would be so high attenuation that u won't hear anything anymore).

If it is a linear pot, isn't the useful range the same regardless of the total resistance? (assuming the pot is the only thing between TIP and RING)

Simoc wrote:

Hi, Mike. Standard Google practice is to click "Show Options", click "reply", and then put your response under the prior responses (which are conveniently shown in Reply to Topic).

The pot isn't just a voltage divider -- it's also a series resistance. Looking at the Radio Shack product, you don't have many choices below

5K (except for a 1K 0.1W micro tweaker pot). If you use that 5K pot, and set it in the middle of the adjustment range, there won't be any power going to the speaker.

The business of using the two phones with the 9V battery and the 300 ohm series resistor is supposed to be a curiosity, and a way to show that it works. It's not supposed to be a practical intercom system. I think the OP might have better results with something like this:

Although it's a little more complex, it is worth it.

Good luck Chris

I bought that 1K 0.1W micro tweaker pot at Radio Shack this morning.

I guess that should work if I put it between tip and ring?