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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 11th Nov 2019, 4:14 pm   #1
Ultrasheep
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Default Wave change switch as on-off switch

Good afternoon. I am interested in whether it should be possible to use the wavechange switch in a vintage radio as an on-off switch. In particular I would like to be able to turn on a Bluetooth device when LW is selected and turn it off when MW is selected. My intention is to still use the MW of the radio and then feed the Bluetooth through the radio’s amplifier when LW is selected. Thanks.
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Old 11th Nov 2019, 4:32 pm   #2
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Default Re: Wave change switch as on-off switch

There's no reason why you couldn't use a spare contact on the wavechange switch for low voltage low current switching. I wouldn't advise trying to switch the mains though, as the contacts aren't rated for that and there would be safety issues.
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Old 11th Nov 2019, 4:58 pm   #3
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Default Re: Wave change switch as on-off switch

Thank you for your reply Paul. The radio is a TR82B. I’m looking at building a small circuit to drop the 9V to 5V when the Bluetooth is selected. The wave change switch doesn’t appear to have any “spare” contacts. As you may be able to tell, I’m something of a novice, hence the decision to concentrate on 9V battery radios for now. I am keen to carry out this work without making any non-reversible changes to the radio.
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Old 11th Nov 2019, 5:20 pm   #4
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Default Re: Wave change switch as on-off switch

It may be possible to fix a miniature microswitch to the wavechange switch in such a way that the microswitch actuating 'pip' or roller is moved when the wavechange switch is moved to the longwave position. A cam could be glued onto the switch rotor.

Another method would be to use a light source to shine a narrow light beam through a hole in the wavechange switch to a phototransistor. A FET or relay might be needed to switch power to the Bluetooth device.
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Old 11th Nov 2019, 6:45 pm   #5
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Default Re: Wave change switch as on-off switch

You may also be able to fit a microswitch to operate when the tuning cap is at the extreme end of its range, though obviously you don't want to hack things about too much.
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Old 11th Nov 2019, 9:31 pm   #6
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Default Re: Wave change switch as on-off switch

I have a Pye multiband 1950's radio that uses the wavechange as a power switch, however it uses a piggy-back mains switch on the wafer assembly for that reason.
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Old 11th Nov 2019, 10:07 pm   #7
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Default Re: Wave change switch as on-off switch

Thanks for all of the replies. I’ve got some thinking to do now. As regards microswitches, please could you suggest the type of thing that might be suitable. Thanks again.
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Old 11th Nov 2019, 10:23 pm   #8
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Default Re: Wave change switch as on-off switch

The problem looks very simple and you do not need to modify the radio or add any mechanical switches , optical devices or those sorts of things.

If you look at the schematic you will see that in MW and LW one changeover switch connects the feedback winding of the oscillator coil to one group or another group of trimmer capacitors. Since the other side of that feedback coil is grounded, the signal there, when connected that is, has an average voltage of zero across those capacitors.

When the switch is not closed, the trimmer capacitors are not connected to anything.

So, all you need to do is have a circuit where you connect a 1Meg resistor to the group of trimmer capacitors that are on the LW setting of the switch and the other end of that resistor goes to -9V . That way when the switch is open (in MW mode) the voltage on the trimmer caps will be -9V and it will be an average of zero with the switch closed in LW mode.

The 1Meg resistor is so large the damping effect of it will have no effect on the operation of the radio.

Then you couple this point to the base of a pnp transistor, like a BC557 by another 1Meg resistor, or you could use a p channel mosfet, and have a 0.1uF capacitor to ground that removes the AC component from the osc signal at the base or gate, the emitter of the transistor (or source of the mosfet) is grounded. Then you just have a load resistor in the collector of the transistor, say 47k in the transistor case, or 4.7k in the mosfet case in the drain to -9V, you then have a switching signal you can use to control any other circuitry you like. You can add more switching stages with more transistors if you need to switch something with a substantial current.

If you connect the 1Meg resistors directly onto the switch area, also the alteration in capacitance won't affect the existing tuning or settings for the LW trimmers.

Last edited by Argus25; 11th Nov 2019 at 10:29 pm.
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Old 11th Nov 2019, 11:18 pm   #9
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Default Re: Wave change switch as on-off switch

When I was a callow youth I did try using a wave change switch to switch a mains transformer and soon ended up with welded contacts.
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Old 12th Nov 2019, 4:21 am   #10
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Default Re: Wave change switch as on-off switch

PS: I forgot to mention in my post #8 if you choose to use a small pch mosfet, instead of the pnp transistor, a suitable part is the BS250:

https://www.diodes.com/assets/Datasheets/BS250P.pdf

Also if you use the mosfet, you could increase the two 1M resistor values much higher to 4.7M or 10M if you wanted.
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Old 12th Nov 2019, 9:29 am   #11
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Default Re: Wave change switch as on-off switch

I've attached a picture to help make it clear what I was talking about.

Since you want 5V for your bluetooth device you can shed the extra 3 or 4V across a zener. It pays to allow for the battery going flat, or you could make a regulator, but you want it simple right ? and the bluetooth device is most likely tolerant 4.5 to 5.5V at least.

If you monitor the medium wave part of the switch, in that case the fet remains off, then when you switch to LW the fet goes on, powering the bluetooth device. Sorry about the non standard fet symbol in the diagram, look up the G,D & S connections for the fet on its data sheet.

Somebody else might like to check the idea to see if they think it will work. If it turned out that the 4.7M resistors or the wiring affected the MW band (unlikely if the resistors are close to the switch contacts) you could monitor the LW trimmers instead and then add an inverting stage after the fet to switch the bluetooth device.
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Old 12th Nov 2019, 11:58 am   #12
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Default Re: Wave change switch as on-off switch

Thank you so much for all of your help. I really appreciate it. I will have a closer look at your explanation and diagram this evening. I may be back with more questions!
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