Use of TRS Switch Contacts to Turn on Device

[BadElvis]

Hi,

I would like to build a small FM transmitter to transmit an audio signal with my notebook. I will power it with a 9V battery. The plan is to connect notebook and the transmitter with 3.5 mm TRS plugs. Some TRS sockets include 2 normally closed contacts, that are opened when a TRS connector is plugged in. Is it possible to use one of these contacts to switch on the transmitter? That would be a nice feature - it gets turned on as soon as a audio source is connected. But I am unsure how to do the wiring. How can I do it? How can I make sure my audio output (notebook) does not take damage? Do I need a transistor as a switch?

Thanks for your replies!

 

[thylacine1975]

Heya BadElvis,

It's undoubtedly more primitive than you had in mind, but 'Talking Electronics' (a small electronics company here in Aus) published a series of FM transmitters back in the 80's that might fit the bill. They had a whole range of transmitters, each named after 'bugs' (the pun was intentional, methinks for a range of different applications, and one of them was the "Beetle" - for transmitting audio from an electric guitar pickup to an FM receiver. Their design was also TRS switched, where inserting the 6.4mm (mono) guitar patch lead into the transmitter would turn it on.

Here's the schematic:


(Building these things for my muso friends substantially funded my high school electronics hobby

 

[BadElvis]

Hi there

and thanks a lot for your quick replies. I like the simplicity of the Beetle, it also looks simpler than the design I had in mind (Single Chip FM Transmitter), which uses a ready-made VCO instead of the resonant circuit. I reckon that the Beetle might be more sensitiive to supply variations and a little bit tricky to tune? Unfortunately, the VCO is hard to acquire.

Anyway, my problem is, that the switch of my socket is normally closed and only opens when a plug is inserted. I thought about using a NPN BJT like this:


Do you think this could work? Cheers and thanks again!

 

[mister_rf]

Yes, that’s correct solution. Maybe you need to reduce the current consumption in the OFF status, see some improvements in the attached schematics:

 

[BadElvis]

Hi mister_rf,

thanks for your help, will make use of your suggestions!

Cheers!