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).

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

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

On 03/08/14 11:10, Michael Black wrote:
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.

Yep. This one I gutted, I recall de-soldering the switch and then
reverse-engineering the pinout so I could replace it with a relay, which
I did. A 4-pole double-throw relay IIRC.

I had some hair-brained idea (this was when I was in primary school) to
hook the thing up to the PC-speaker output of the computer (since I knew
how to make tones) and then use some circuit interfaced to the game port
(since I knew how to read the switches on those) and try to send data
using AFSK.

Exactly what data rate I'd achieve, given the whole lot would be
implemented in QBasic I have no idea. I doubt it'd outpace PSK31.

Not that I knew what AFSK was back then. Or that to do what I wanted to
do, I really should have a radio license which I didn't back then.

Stuart Longland wrote:
On 03/08/14 11:10, Michael Black wrote:
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.

Yep. This one I gutted, I recall de-soldering the switch and then
reverse-engineering the pinout so I could replace it with a relay, which
I did. A 4-pole double-throw relay IIRC.

Not only that the switch has many poles, the circuit is often very tricky.
It is not a receiver and a transmitter with a switch to toggle the
power, antenna and speaker/mike to connect to one of them, no it is
a blob of electronics that morphs between being a transmitter and being
a receiver when the PTT switch is switched over.

In those days I sometimes tried drawing the schematic by looking at the
PCB traces and components, and it is very difficult to draw a schematic
that makes any sense...

It is completely contrary to the electronics world today, where one would
prefer having a thousand extra transistors to save a single mechanical
component (like an extra pole on the switch). The times have changed...

On Sun, 3 Aug 2014, Rob wrote:

Stuart Longland wrote:
On 03/08/14 11:10, Michael Black wrote:
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.

Yep. This one I gutted, I recall de-soldering the switch and then
reverse-engineering the pinout so I could replace it with a relay, which
I did. A 4-pole double-throw relay IIRC.

Not only that the switch has many poles, the circuit is often very tricky.
It is not a receiver and a transmitter with a switch to toggle the
power, antenna and speaker/mike to connect to one of them, no it is
a blob of electronics that morphs between being a transmitter and being
a receiver when the PTT switch is switched over.

Yes, the switch would be so much simpler if they were just switching audio
and power.

I get the feeling these were the solid state equivalent of the one tube
transcievers used to homestead the higher bands. They were a modulated
oscillator on transmit, a superregenerative receiver on receive, and a
common audio amplifier. There the space and cost of a tube meant they
switch it between the two functions, but since it was a modulated
oscillator, it was a simpler arrangement than switching between a
superregen and a crystal controlled transmitter.

Those single tube transceivers were certainly simple, and got people onto
the higher bands. ONce a band got busy, there'd be a rule put in that you
had to use crystal control (or have equivalent stability) on that band.
So these rigs would start off at the "UHF" 10metre band, then move to
5metres, then up to 2.5Metres. Even fifty years ago, they were being used
on the 420MHz band. SImple and cheap, you didn't get much range, but they
helped get people on the band.

In those days I sometimes tried drawing the schematic by looking at the
PCB traces and components, and it is very difficult to draw a schematic
that makes any sense...

Expecially when you were a kid without much ability to figure out what the
switch contacts were doing. All these circuit board traces would go into
what amounted to a black box switch, crtainly beyond my skill at the time
to trace out.

It is completely contrary to the electronics world today, where one would
prefer having a thousand extra transistors to save a single mechanical
component (like an extra pole on the switch). The times have changed...

I guess it makes sense at the beginning, but transistor prices dropped
fast, yet the same scheme was used into the seventies. I assume when
cheap walkie talkies moved to 49MHz, they didn't add transistors but still
used that complicated switch (but I've never looked at a superregen 49MHz
walkie talkie). Considering that "transistor radios" at the time were
certain to tell you that they had "X transistors" you'd think the cost of
adding a transistor for transmit (and thus be able to say "four
transistors" or whatever) would increase sales enough that it would offset
th cost of the extra transistor.

It is a lesson worth repeating, adding transistors may nominall make the
circuit more complicated (and expensive), but often results in the overall
design being simpler.

Of course, once ICs came along, that took the idea to the extreme, endless
transistors in the IC, but you never see them.

Michael

"Michael Black" wrote in message
xample.org...

I get the feeling these were the solid state equivalent of the one tube
transcievers used to homestead the higher bands. They were a modulated
oscillator on transmit, a superregenerative receiver on receive, and a
common audio amplifier. There the space and cost of a tube meant they
switch it between the two functions, but since it was a modulated
oscillator, it was a simpler arrangement than switching between a
superregen and a crystal controlled transmitter.

Nowadays when transistors are almost ten-a-penny, it is the switching
that is expensive, so otherwise than as a novelty, there's not much
to be said for single transistor rigs.

I have in my museum pieces a boxed PM2A valve and the conditions
of sale printed on the bottom say that it must not be sold to the public
for less than 8 shillings and 9 pence, which pre-war was about 10%
of the weekly take-home pay, but imagine paying £$40 today for
each active device !