On Sat, 2 Aug 2014, Rob wrote:

Stuart Longland wrote:
On 28/07/14 04:18, Michael Black wrote:
IN the old days, endless cheap 100mW walkie talkies would use the
speaker for the microphone on transmit. For that matter, endless
intercom systems used the same speaker as a speaker and as a microphone.

I do remember those, in fact I've got one gutted somewhere. Had a
crystal for 27.145MHz.

I once tried wiring up an electret element, not knowing there was a
difference, and was disappointed when it didn't work.

An earphone may not offer the same level of sound collection that a
speaker with a larger cone allows, you may have to play with things. I
remember taking cheap dynamic earphones and taking the bit that went in
your ear off, and using that as a contact microphone for various things.

In the interest of science, I gave it a shot just then. With a 100nF
capacitor in series to block the DC, I wired it to a DIN5 plug (all my
radios have been set up with adaptors to DIN5 headset jacks) and tried it.

It did work, but without any amplification or impedance matching, the
modulation is well down. I might try winding a small transformer and
see what that does.

In the days when speakers were used as microphones in walkie talkies
and intercoms, it also was quite popular to have transformers between
the final stage transistors and the speaker.

The circuits of such devices were often very cleverly designed, re-using
many components between receive and transmit (using a multipole switch).

They were really complicated switches, for the sake of a few transistors.

It is quite likely that the output transformer was used as a step-up
transformer while the speaker was used as microphone.

That's what I would have thought, but I recall articles about modifying
those cheap walkie talkies and they added transformers to step up the
output from the speaker on transmit.

You're right, in that era, the audio amplifiers were using an output
transformer. But it wasn't just to match impedance, it was part of the
amplifier, and thus needed on transmit too.

Michael