Use of TRS Switch Contacts to Turn on Device - Page 1
Forum section: electronics / projects

author: Bad_Elvis
author_rank: Newbie
timestamp: February 09, 2012, 06:57:46 am
content: 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 the 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? Can I use a transistor as a switch?Thanks for your replies!

author: electrode
author_rank: Regular Contributor
timestamp: February 09, 2012, 08:13:47 am
content: Is it mono?If so, you can use amono plugwith astereosocket in the device. The mono plug is essentially a stereo plug with ring and sleeve shorted together, thus the ring and sleeve of your socket are your 2 switch contacts. This technique is used a lot in guitar stompboxes if you want to google for an example.

author: Bad_Elvis
author_rank: Newbie
timestamp: February 09, 2012, 11:35:09 am
content: Hi thereand thanks a lot for your quick reply. I use a stereo connector. 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!

author: electrode
author_rank: Regular Contributor
timestamp: February 09, 2012, 12:00:45 pm
content: Let me make sure I understand your diagram correctly:- Pins 2 and 4 are normally closed- Pins 3 and 5 are normally closed- When you plug in your plug, both sets openCorrect?So that means, when unplugged, your base is in-between 220k and 32k, so the voltage at the base will be 13% of Vcc or something. Definitely off. When plugged in, pin 5 is now dangling and the transistor is pulled up via 220k. This is a BJT though, so unless it has some magically high beta, it will barely switch on with such low base-emitter current, let alone go into the saturation region, which is what you want for a switch. An N-channel MOSFET would be more suitable in this case.What's Vcc?Also, could you please elaborate on the purpose of R4 and R5? Are they to do with the rest of your circuit?Hopefully someone else can chime in too.

author: ejeffrey
author_rank: Super Contributor
timestamp: February 09, 2012, 01:31:28 pm
content: I would use a P-channel mosfet on the high side of the load. Then it will draw zero current when off, and only a small extra current when on. You don't need to worry about the transistor beta, and there is no offset on the supply voltage to the transmitter.

author: Bad_Elvis
author_rank: Newbie
timestamp: February 09, 2012, 03:31:05 pm
content: I'm not sure whether I understood you correctly. Do you mean a setup like this:Maybe not, since this draws a little bit of current when powered off. Note that my design is based on this one:http://electronics-diy.com/Single_Chip_FM_Transmitter.php,therefore the 22k resistors for audio input coupling. I'm not sure whether I can increase them (and thereby reduce the standby current).electrode, you understood the switches correctly. Supply voltage will be 9V.Cheers

author: ejeffrey
author_rank: Super Contributor
timestamp: February 09, 2012, 03:59:34 pm
content: Hmm... I misread how the jacks work. I thought the switch connected to ground, not the signal.

author: electrode
author_rank: Regular Contributor
timestamp: February 09, 2012, 09:04:49 pm
content: The P-MOSFET is better than what I came up with last night (basically the same, but with an N-MOSFET pulled high through a resistor and connected to pin 3, with pin 5 to ground).I can't for the life of me figure out a way to do it without the resistors or MOSFET interfering with the rest of the circuit. Sorry. There's probably a really simple solution out there though.